Java - Generics

It would be nice if we could write a single sort method that could sort the elements in an Integer array, a String array or an array of any type that supports ordering.

Java Generic methods and generic classes enable programmers to specify, with a single method declaration, a set of related methods or, with a single class declaration, a set of related types, respectively.

Generics also provide compile-time type safety that allows programmers to catch invalid types at compile time.

Using Java Generic concept we might write a generic method for sorting an array of objects, then invoke the generic method with Integer arrays, Double arrays, String arrays and so on, to sort the array elements.

Generic Methods:

You can write a single generic method declaration that can be called with arguments of different types. Based on the types of the arguments passed to the generic method, the compiler handles each method call appropriately. Following are the rules to define Generic Methods:

• All generic method declarations have a type parameter section delimited by angle brackets (<>) that precedes the method's return type ( <> in the next example).

• Each type parameter section contains one or more type parameters separated by commas. A type parameter, also known as a type variable, is an identifier that specifies a generic type name.

• The type parameters can be used to declare the return type and act as placeholders for the types of the arguments passed to the generic method, which are known as actual type arguments.

• A generic method's body is declared like that of any other method. Note that type parameters can represent only reference types not primitive types (like int, double and char).

Example:

Following example illustrate how we can print array of different type using a single Generic method:

public class GenericMethodTest { // generic method printArray public static <> void printArray( E[] inputArray ) { // Display array elements for ( E element : inputArray ){ System.out.printf( "%s ", element ); } System.out.println(); } public static void main( String args[] ) { // Create arrays of Integer, Double and Character Integer[] intArray = { 1, 2, 3, 4, 5 }; Double[] doubleArray = { 1.1, 2.2, 3.3, 4.4 }; Character[] charArray = { 'H', 'E', 'L', 'L', 'O' }; System.out.println( "Array integerArray contains:" ); printArray( integerArray ); // pass an Integer array System.out.println( "\nArray doubleArray contains:" ); printArray( doubleArray ); // pass a Double array System.out.println( "\nArray characterArray contains:" ); printArray( characterArray ); // pass a Character array } }

This would produce following result:

Array integerArray contains: 1 2 3 4 5 6 Array doubleArray contains: 1.1 2.2 3.3 4.4 Array characterArray contains: H E L L O

Bounded Type Parameters:

There may be times when you'll want to restrict the kinds of types that are allowed to be passed to a type parameter. For example, a method that operates on numbers might only want to accept instances of Number or its subclasses. This is what bounded type parameters are for.

To declare a bounded type parameter, list the type parameter's name, followed by the extends keyword, followed by its upper bound.

Example:

Following example illustrate how extends is used in a general sense to mean either "extends" (as in classes) or "implements" (as in interfaces). This example is Generic method to return the largest of three Comparable objects:

public class MaximumTest { // determines the largest of three Comparable objects public static > T maximum(T x, T y, T z) { T max = x; // assume x is initially the largest if ( y.compareTo( max ) > 0 ){ max = y; // y is the largest so far } if ( z.compareTo( max ) > 0 ){ max = z; // z is the largest now } return max; // returns the largest object } public static void main( String args[] ) { System.out.printf( "Max of %d, %d and %d is %d\n\n", 3, 4, 5, maximum( 3, 4, 5 ) ); System.out.printf( "Maxm of %.1f,%.1f and %.1f is %.1f\n\n", 6.6, 8.8, 7.7, maximum( 6.6, 8.8, 7.7 ) ); System.out.printf( "Max of %s, %s and %s is %s\n","pear", "apple", "orange", maximum( "pear", "apple", "orange" ) ); } }

This would produce following result:

Maximum of 3, 4 and 5 is 5 Maximum of 6.6, 8.8 and 7.7 is 8.8 Maximum of pear, apple and orange is pear

Generic Classes:

A generic class declaration looks like a non-generic class declaration, except that the class name is followed by a type parameter section.

As with generic methods, the type parameter section of a generic class can have one or more type parameters separated by commas. These classes are known as parameterized classes or parameterized types because they accept one or more parameters.

Example:

Following example illustrate how we can define a generic class:

public class Box { private T t; public void add(T t) { this.t = t; } public T get() { return t; } public static void main(String[] args) { Box integerBox = new Box(); Box stringBox = new Box(); integerBox.add(new Integer(10)); stringBox.add(new String("Hello World")); System.out.printf("Integer Value :%d\n\n", integerBox.get()); System.out.printf("String Value :%s\n", stringBox.get()); } }

This would produce following result:

Integer Value :10

String Value :Hello World

Java - Collections Framework

Prior to Java 2, Java provided ad hoc classes such as Dictionary, Vector, Stack, and Properties to store and manipulate groups of objects. Although these classes were quite useful, they lacked a central, unifying theme. Thus, the way that you used Vector was different from the way that you used Properties.

The collections framework was designed to meet several goals.

1.The framework had to be high-performance. The implementations for the fundamental collections (dynamic arrays, linked lists, trees, and hash tables) are highly efficient.
2.The framework had to allow different types of collections to work in a similar manner and with a high degree of interoperability.
3.Extending and/or adapting a collection had to be easy.

Toward this end, the entire collections framework is designed around a set of standard interfaces. Several standard implementations such as LinkedList, HashSet, and TreeSet, of these interfaces are provided that you may use as-is and you may also implement your own collection, if you choose.

A collections framework is a unified architecture for representing and manipulating collections. All collections frameworks contain the following:

1.Interfaces: These are abstract data types that represent collections. Interfaces allow collections to be manipulated independently of the details of their representation. In object-oriented languages, interfaces generally form a hierarchy.
2.Implementations i.e. Classes: These are the concrete implementations of the collection interfaces. In essence, they are reusable data structures.
3. Algorithms: These are the methods that perform useful computations, such as searching and sorting, on objects that implement collection interfaces. The algorithms are said to be polymorphic: that is, the same method can be used on many different implementations of the appropriate collection interface.

In addition to collections, the framework defines several map interfaces and classes. Maps store key/value pairs. Although maps are not collections in the proper use of the term, but they are fully integrated with collections.
The Collection Interfaces:

The collections framework defines several interfaces. This section provides an overview of each interface:
SN Interfaces with Description
1.The Collection Interface
This enables you to work with groups of objects; it is at the top of the collections hierarchy.
2.The List Interface
This extends Collection and an instance of List stores an ordered collection of elements.
3.The Set
This extends Collection to handle sets, which must contain unique elements
4.The SortedSet
This extends Set to handle sorted sets
5.The Map
This maps unique keys to values.
6.The Map.Entry
This describes an element (a key/value pair) in a map. This is an inner class of Map.
7.The SortedMap
This extends Map so that the keys are maintained in ascending order.
8.The Enumeration
This is legacy interface and defines the methods by which you can enumerate (obtain one at a time) the elements in a collection of objects. This legacy interface has been superceded by Iterator.
The Collection Classes:

Java provides a set of standard collection classes that implement Collection interfaces. Some of the classes provide full implementations that can be used as-is and others are abstract class, providing skeletal implementations that are used as starting points for creating concrete collections.

The standard collection classes are summarized in the following table:
SN Classes with Description
1.AbstractCollection
Implements most of the Collection interface.
2.AbstractList
Extends AbstractCollection and implements most of the List interface.
3.AbstractSequentialList
Extends AbstractList for use by a collection that uses sequential rather than random access of its elements.
4.LinkedList
Implements a linked list by extending AbstractSequentialList.
5.ArrayList
Implements a dynamic array by extending AbstractList.
6.AbstractSet
Extends AbstractCollection and implements most of the Set interface.
7.HashSet
Extends AbstractSet for use with a hash table.
8.LinkedHashSet
Extends HashSet to allow insertion-order iterations.
9.TreeSet
Implements a set stored in a tree. Extends AbstractSet.
10.AbstractMap
Implements most of the Map interface.
11.HashMap
Extends AbstractMap to use a hash table.
12.TreeMap
Extends AbstractMap to use a tree.
13.WeakHashMap
Extends AbstractMap to use a hash table with weak keys.
14.LinkedHashMap
Extends HashMap to allow insertion-order iterations.
15.IdentityHashMap
Extends AbstractMap and uses reference equality when comparing documents.

The AbstractCollection, AbstractSet, AbstractList, AbstractSequentialList and AbstractMap classes provide skeletal implementations of the core collection interfaces, to minimize the effort required to implement them.

The following legacy classes defined by java.util has been discussed in previous tutorial:
SN Classes with Description
1.Vector
This implements a dynamic array. It is similar to ArrayList, but with some differences.
2.Stack
Stack is a subclass of Vector that implements a standard last-in, first-out stack.
3.Dictionary
Dictionary is an abstract class that represents a key/value storage repository and operates much like Map.
4.Hashtable
Hashtable was part of the original java.util and is a concrete implementation of a Dictionary.
5.Properties
Properties is a subclass of Hashtable. It is used to maintain lists of values in which the key is a String and the value is also a String.
6.BitSet
A BitSet class creates a special type of array that holds bit values. This array can increase in size as needed.

The Collection Algorithms:

The collections framework defines several algorithms that can be applied to collections and maps. These algorithms are defined as static methods within the Collections class.

Several of the methods can throw a ClassCastException, which occurs when an attempt is made to compare incompatible types, or an UnsupportedOperationException, which occurs when an attempt is made to modify an unmodifiable collection.

Collections defines three static variables: EMPTY_SET, EMPTY_LIST, and EMPTY_MAP. All are immutable.
SN Algorithms with Description
1.The Collection Algorithms
Here is a list of all the algorithm implementation.
How to use an Iterator ?

Often, you will want to cycle through the elements in a collection. For example, you might want to display each element.

The easiest way to do this is to employ an iterator, which is an object that implements either the Iterator or the ListIterator interface.

Iterator enables you to cycle through a collection, obtaining or removing elements. ListIterator extends Iterator to allow bidirectional traversal of a list, and the modification of elements.
SN Iterator Methods with Description
1.Using Java Iterator
Here is a list of all the methods with examples provided by Iterator and ListIterator interfaces.
How to use an Comparator ?

Both TreeSet and TreeMap store elements in sorted order. However, it is the comparator that defines precisely what sorted order means.

This interface lets us sort a given collection any number of different ways. Also this interface can be used to sort any instances of any class.(even classes we cannot modify).
SN Iterator Methods with Description
1.Using Java Comparator
Here is a list of all the methods with examples provided by Comparator Interface.
Summary:

The Java collections framework gives the programmer access to prepackaged data structures as well as to algorithms for manipulating them.

A collection is an object that can hold references to other objects. The collection interfaces declare the operations that can be performed on each type of collection.

The classes and interfaces of the collections framework are in package java.util.

Java - The Hashtable Class

The data structures provided by the Java utility package are very powerful and perform a wide range of functions. These data structures consist of the following interface and classes:

-Enumeration
-BitSet
-Vector
-Stack
-Dictionary
-Hashtable
-Properties

All these classes are now legacy and Java-2 has introcuded a new framework called Collections Framework which is discussed in next tutorial:
The Enumeration:

The Enumeration interface isn't itself a data structure, but it is very important within the context of other data structures. The Enumeration interface defines a means to retrieve successive elements from a data structure.

For example, Enumeration defines a method called nextElement that is used to get the next element in a data structure that contains multiple elements.

To have more detail about this interface, check The Enumeration.
The BitSet

The BitSet class implements a group of bits, or flags, that can be set and cleared individually.

This class is very useful in cases where you need to keep up with a set of boolean values; you just assign a bit to each value and set or clear it as appropriate.

To have more detail about this class, check The BitSet.
The Vector

The Vector class is similar to a traditional Java array, except that it can grow as necessary to accommodate new elements.

Like an array, elements of a Vector object can be accessed via an index into the vector.

The nice thing about using the Vector class is that you don't have to worry about setting it to a specific size upon creation; it shrinks and grows automatically when necessary.

To have more detail about this class, check The Vector.
The Stack

The Stack class implements a last-in-first-out (LIFO) stack of elements.

You can think of a stack literally as a vertical stack of objects; when you add a new element, it gets stacked on top of the others.

When you pull an element off the stack, it comes off the top. In other words, the last element you added to the stack is the first one to come back off.

To have more detail about this class, check The Stack.
The Dictionary

The Dictionary class is an abstract class that defines a data structure for mapping keys to values.

This is useful in cases where you want to be able to access data via a particular key rather than an integer index.

Since the Dictionary class is abstract, it provides only the framework for a key-mapped data structure rather than a specific implementation.

To have more detail about this class, check The Dictionary.
The Hashtable

The Hashtable class provides a means of organizing data based on some user-defined key structure.

For example, in an address list hash table you could store and sort data based on a key such as ZIP code rather than on a person's name.

The specific meaning of keys in regard to hash tables is totally dependent on the usage of the hash table and the data it contains.

To have more detail about this class, check The Hashtable.
The Properties

Properties is a subclass of Hashtable. It is used to maintain lists of values in which the key is a String and the value is also a String.

The Properties class is used by many other Java classes. For example, it is the type of object returned by System.getProperties( ) when obtaining environmental values.

Java - Packages

Packages are used in Java in-order to prevent naming conflicts, to control access, to make searching/locating and usage of classes, interfaces, enumerations and annotations easier etc.

A Package can be defined as a grouping of related types(classes, interfaces, enumerations and annotations ) providing access protection and name space management.

Some of the existing packages in Java are:

-java.lang - bundles the fundamental classes
-java.io - classes for input , output functions are bundled in this package

Programmers can define their own packages to bundle group of classes/interfaces etc. It is a good practice to group related classes implemented by you so that a programmers can easily determine that the classes, interfaces, enumerations, annotations are related.

Since the package creates a new namespace there won't be any name conflicts with names in other packages. Using packages, it is easier to provide access control and it is also easier to locate the related classed.
Creating a package:

When creating a package, you should choose a name for the package and put a package statement with that name at the top of every source file that contains the classes, interfaces, enumerations, and annotation types that you want to include in the package.

The package statement should be the first line in the source file. There can be only one package statement in each source file, and it applies to all types in the file.

If a package statement is not used then the class, interfaces, enumerations, and annotation types will be put into an unnamed package.
Example:

Let us look at an example that creates a package called animals. It is common practice to use lowercased names of packages to avoid any conflicts with the names of classes, interfaces.

Put an interface in the package animals:

/* File name : Animal.java */
package animals;

interface Animal {
public void eat();
public void travel();
}

Now put an implementation in the same package animals:

package animals;

/* File name : MammalInt.java */
public class MammalInt implements Animal{

public void eat(){
System.out.println("Mammal eats");
}

public void travel(){
System.out.println("Mammal travels");
}

public int noOfLegs(){
return 0;
}

public static void main(String args[]){
MammalInt m = new MammalInt();
m.eat();
m.travel();
}
}

Now you compile these two files and put them in a sub-directory called animals and try to run as follows:

$ mkdir animals
$ cp Animal.class MammalInt.class animals
$ java animals/MammalInt
Mammal eats
Mammal travels

The import Keyword:

If a class wants to use another class in the same package, the package name does not need to be used. Classes in the same package find each other without any special syntax.
Example:

Here a class named Boss is added to the payroll package that already contains Employee. The Boss can then refer to the Employee class without using the payroll prefix, as demonstrated by the following Boss class.

package payroll;

public class Boss
{
public void payEmployee(Employee e)
{
e.mailCheck();
}
}

What happens if Boss is not in the payroll package? The Boss class must then use one of the following techniques for referring to a class in a different package.

-The fully qualified name of the class can be used. For example:

payroll.Employee

-The package can be imported using the import keyword and the wild card (*). For example:

import payroll.*;

-The class itself can be imported using the import keyword. For example:

import payroll.Employee;

Note: A class file can contain any number of import statements. The import statements must appear after the package statement and before the class declaration.
The Directory Structure of Packages:

Two major results occur when a class is placed in a package:

-The name of the package becomes a part of the name of the class, as we just discussed in the previous section.
-The name of the package must match the directory structure where the corresponding bytecode resides.

Here is simple way of managing your files in java:

Put the source code for a class, interface, enumeration, or annotation type in a text file whose name is the simple name of the type and whose extension is .java. For example:

// File Name : Car.java

package vehicle;

public class Car {
// Class implementation.
}

Now put the source file in a directory whose name reflects the name of the package to which the class belongs:

....\vehicle\Car.java

Now the qualified class name and pathname would be as below:

-Class name -> vehicle.Car
-Path name -> vehicle\Car.java (in windows)

In general a company uses its reversed Internet domain name for its package names. Example: A company's Internet domain name is apple.com, then all its package names would start with com.apple. Each component of the package name corresponds to a subdirectory.

Example: The company had a com.apple.computers package that contained a Dell.java source file, it would be contained in a series of subdirectories like this:

....\com\apple\computers\Dell.java

At the time of compilation, the compiler creates a different output file for each class, interface and enumeration defined in it. The base name of the output file is the name of the type, and its extension is .class

For example:

// File Name: Dell.java

package com.apple.computers;
public class Dell{

}
class Ups{

}

Now compile this file as follows using -d option:

$javac -d . Dell.java

This would put compiled files as follows:

.\com\apple\computers\Dell.class
.\com\apple\computers\Ups.class

You can import all the classes or interfaces defined in \com\apple\computers\ as follows:

import com.apple.computers.*;

Like the .java source files, the compiled .class files should be in a series of directories that reflect the package name. However, the path to the .class files does not have to be the same as the path to the .java source files. You can arrange your source and class directories separately, as:

\sources\com\apple\computers\Dell.java

\classes\com\apple\computers\Dell.class

By doing this, it is possible to give the classes directory to other programmers without revealing your sources. You also need to manage source and class files in this manner so that the compiler and the Java Virtual Machine (JVM) can find all the types your program uses.

The full path to the classes directory, \classes, is called the class path, and is set with the CLASSPATH system variable. Both the compiler and the JVM construct the path to your .class files by adding the package name to the class path.

Say \classes is the class path, and the package name is com.apple.computers, then the compiler and JVM will look for .class files in \classes\com\apple\comptuers.

A class path may include several paths. Multiple paths should be separated by a semicolon (Windows) or colon (Unix). By default, the compiler and the JVM search the current directory and the JAR file containing the Java platform classes so that these directories are automatically in the class path.
Set CLASSPATH System Variable:

To display the current CLASSPATH variable, use the following commands in Windows and Unix (Bourne shell):

-In Windows -> C:\> set CLASSPATH
-In Unix -> % echo $CLASSPATH

To delete the current contents of the CLASSPATH variable, use :

-In Windows -> C:\> set CLASSPATH=
-In Unix -> % unset CLASSPATH; export CLASSPATH

To set the CLASSPATH variable:

-In Windows -> set CLASSPATH=C:\users\jack\java\classes
-In Unix -> % CLASSPATH=/home/jack/java/classes; export CLASSPATH

Java - Interfaces

An interface is a collection of abstract methods. A class implements an interface, thereby inheriting the abstract methods of the interface.

An interface is not a class. Writing an interface is similar to writing a class, but they are two different concepts. A class describes the attributes and behaviors of an object. An interface contains behaviors that a class implements.

Unless the class that implements the interface is abstract, all the methods of the interface need to be defined in the class.

An interface is similar to a class in the following ways:

-An interface can contain any number of methods.
-An interface is written in a file with a .java extension, with the name of the interface matching the name of the file.
-The bytecode of an interface appears in a .class file.

Interfaces appear in packages, and their corresponding bytecode file must be in a directory structure that matches the package name.

However, an interface is different from a class in several ways, including:

-You cannot instantiate an interface.
-An interface does not contain any constructors.
-All of the methods in an interface are abstract.
-An interface cannot contain instance fields. The only fields that can appear in an interface must be declared both static and final.
-An interface is not extended by a class; it is implemented by a class.
-An interface can extend multiple interfaces.

Declaring Interfaces:

The interface keyword is used to declare an interface. Here is a simple example to declare an interface:

Encapsulation can be described as a protective barrier that prevents the code and data being randomly accessed by other code defined outside the class. Access to the data and code is tightly controlled by an interface.

The main benefit of encapsulation is the ability to modify our implemented code without breaking the code of others who use our code. With this feature Encapsulation gives maintainability, flexibility and extensibility to our code.
Example:

Let us look at an example that depicts encapsulation:

/* File name : NameOfInterface.java */
import java.lang.*;
//Any number of import statements

public interface NameOfInterface
{
//Any number of final, static fields
//Any number of abstract method declarations\
}

Interfaces have the following properties:

-An interface is implicitly abstract. You do not need to use the abstract keyword when declaring an interface.
-Each method in an interface is also implicitly abstract, so the abstract keyword is not needed.
-Methods in an interface are implicitly public.

Example:

/* File name : Animal.java */
interface Animal {

public void eat();
public void travel();
}

Implementing Interfaces:

When a class implements an interface, you can think of the class as signing a contract, agreeing to perform the specific behaviors of the interface. If a class does not perform all the behaviors of the interface, the class must declare itself as abstract.

Aclass uses the implements keyword to implement an interface. The implements keyword appears in the class declaration following the extends portion of the declaration.

/* File name : MammalInt.java */
public class MammalInt implements Animal{

public void eat(){
System.out.println("Mammal eats");
}

public void travel(){
System.out.println("Mammal travels");
}

public int noOfLegs(){
return 0;
}

public static void main(String args[]){
MammalInt m = new MammalInt();
m.eat();
m.travel();
}
}

This would produce following result:

Mammal eats
Mammal travels

When overriding methods defined in interfaces there are several rules to be followed:

-Checked exceptions should not be declared on implementation methods other than the ones declared by the interface method or subclasses of those declared by the interface method.
-The signature of the interface method and the same return type or subtype should be maintained when overriding the methods.
-An implementation class itself can be abstract and if so interface methods need not be implemented.

When implementation interfaces there are several rules:

-A class can implement more than one interface at a time.
-A class can extend only one class, but implement many interface.
-An interface itself can extend another interface. An interface cannot extend another interface.

Extending Interfaces:

An interface can extend another interface, similarly to the way that a class can extend another class. The extends keyword is used to extend an interface, and the child interface inherits the methods of the parent interface.

The following Sports interface is extended by Hockey and Football interfaces.

//Filename: Sports.java
public interface Sports
{
public void setHomeTeam(String name);
public void setVisitingTeam(String name);
}

//Filename: Football.java
public interface Football extends Sports
{
public void homeTeamScored(int points);
public void visitingTeamScored(int points);
public void endOfQuarter(int quarter);
}

//Filename: Hockey.java
public interface Hockey extends Sports
{
public void homeGoalScored();
public void visitingGoalScored();
public void endOfPeriod(int period);
public void overtimePeriod(int ot);
}

The Hockey interface has four methods, but it inherits two from Sports; thus, a class that implements Hockey needs to implement all six methods. Similarly, a class that implements Football needs to define the three methods from Football and the two methods from Sports.
Extending Multiple Interfaces:

A Java class can only extend one parent class. Multiple inheritance is not allowed. Interfaces are not classes, however, and an interface can extend more than one parent interface.

The extends keyword is used once, and the parent interfaces are declared in a comma-separated list.

For example, if the Hockey interface extended both Sports and Event, it would be declared as:

public interface Hockey extends Sports, Event

Tagging Interfaces:

The most common use of extending interfaces occurs when the parent interface does not contain any methods. For example, the MouseListener interface in the java.awt.event package extended java.util.EventListener, which is defined as:

package java.util;
public interface EventListener
{}

An interface with no methods in it is referred to as a tagging interface. There are two basic design purposes of tagging interfaces:

Creates a common parent: As with the EventListener interface, which is extended by dozens of other interfaces in the Java API, you can use a tagging interface to create a common parent among a group of interfaces. For example, when an interface extends EventListener, the JVM knows that this particular interface is going to be used in an event delegation scenario.

Adds a data type to a class: This situation is where the term tagging comes from. A class that implements a tagging interface does not need to define any methods (since the interface does not have any), but the class becomes an interface type through polymorphism.

Java - Abstraction

Abstraction refers to the ability to make a class abstract in OOP. An abstract class is one that cannot be instantiated. All other functionality of the class still exists, and its fields, methods, and constructors are all accessed in the same manner. You just cannot create an instance of the abstract class.

If a class is abstract and cannot be instantiated, the class does not have much use unless it is subclassed. This is typically how abstract classes come about during the design phase. A parent class contains the common functionality of a collection of child classes, but the parent class itself is too abstract to be used on its own.

Abstract Class:

Use the abstract keyword to declare a class abstract. The keyword appears in the class declaration somewhere before the class keyword.

/* File name : Employee.java */ public abstract class Employee { private String name; private String address; private int number; public Employee(String name, String address, int number) { System.out.println("Constructing an Employee"); this.name = name; this.address = address; this.number = number; } public double computePay() { System.out.println("Inside Employee computePay"); return 0.0; } public void mailCheck() { System.out.println("Mailing a check to " + this.name + " " + this.address); } public String toString() { return name + " " + address + " " + number; } public String getName() { return name; } public String getAddress() { return address; } public void setAddress(String newAddress) { address = newAddress; } public int getNumber() { return number; } }

Notice that nothing is different in this Employee class. The class is now abstract, but it still has three fields, seven methods, and one constructor.

Now if you would try as follows:

/* File name : AbstractDemo.java */ public class AbstractDemo { public static void main(String [] args) { /* Following is not allowed and would raise error */ Employee e = new Employee("George W.", "Houston, TX", 43); System.out.println("\n Call mailCheck using Employee reference--"); e.mailCheck(); } }

When you would compile above class then you would get following error:

Employee.java:46: Employee is abstract; cannot be instantiated Employee e = new Employee("George W.", "Houston, TX", 43); ^ 1 error1

Extending Abstract Class:

We can extend Employee class in normal way as follows:

/* File name : Salary.java */ public class Salary extends Employee { private double salary; //Annual salary public Salary(String name, String address, int number, double salary) { super(name, address, number); setSalary(salary); } public void mailCheck() { System.out.println("Within mailCheck of Salary class "); System.out.println("Mailing check to " + getName() + " with salary " + salary); } public double getSalary() { return salary; } public void setSalary(double newSalary) { if(newSalary >= 0.0) { salary = newSalary; } } public double computePay() { System.out.println("Computing salary pay for " + getName()); return salary/52; } }

Here we cannot instantiate a new Employee, but if we instantiate a new Salary object, the Salary object will inherit the three fields and seven methods from Employee.

/* File name : AbstractDemo.java */ public class AbstractDemo { public static void main(String [] args) { Salary s = new Salary("Mohd Mohtashim", "Ambehta, UP", 3, 3600.00); Salary e = new Salary("John Adams", "Boston, MA", 2, 2400.00); System.out.println("Call mailCheck using Salary reference --"); s.mailCheck(); System.out.println("\n Call mailCheck using Employee reference--"); e.mailCheck(); } }

This would produce following result:

Constructing an Employee Constructing an Employee Call mailCheck using Salary reference -- Within mailCheck of Salary class Mailing check to Mohd Mohtashim with salary 3600.0 Call mailCheck using Employee reference-- Within mailCheck of Salary class Mailing check to John Adams with salary 2400.

Abstract Methods:

If you want a class to contain a particular method but you want the actual implementation of that method to be determined by child classes, you can declare the method in the parent class as abstract.

The abstract keyword is also used to declare a method as abstract.An abstract methods consist of a method signature, but no method body.

Abstract method would have no definition, and its signature is followed by a semicolon, not curly braces as follows:

public abstract class Employee { private String name; private String address; private int number; public abstract double computePay(); //Remainder of class definition }

Declaring a method as abstract has two results:

• The class must also be declared abstract. If a class contains an abstract method, the class must be abstract as well.

• Any child class must either override the abstract method or declare itself abstract.

A child class that inherits an abstract method must override it. If they do not, they must be abstract,and any of their children must override it.

Eventually, a descendant class has to implement the abstract method; otherwise, you would have a hierarchy of abstract classes that cannot be instantiated.

If Salary is extending Employee class then it is required to implement computePay() method as follows:

/* File name : Salary.java */
public class Salary extends Employee
{
  private double salary; //Annual salary

  public double computePay()
  {
     System.out.println("Computing salary pay for " + getName());
     return salary/52;
  }

  //Remainder of class definition
}

Java - Polymorphism

Polymorphism is the ability of an object to take on many forms. The most common use of polymorphism in OOP occurs when a parent class reference is used to refer to a child class object.

Any java object that can pass more than on IS-A test is considered to be polymorphic. In Java, all java objects are polymorphic since any object will pass the IS-A test for their own type and for the class Object.

It is important to know that the only possible way to access an object is through a reference variable. A reference variable can be of only one type. Once declared the type of a reference variable cannot be changed.

The reference variable can be reassigned to other objects provided that it is not declared final. The type of the reference variable would determine the methods that it can invoke on the object.

A reference variable can refer to any object of its declared type or any subtype of its declared type. A reference variable can be declared as a class or interface type.
Example:

Let us look at an example.

public interface Vegetarian{}
public class Animal{}
public class Deer extends Animal implements Vegetarian{}

Now the Deer class is considered to be polymorphic since this has multiple inheritance. Following are true for the above example:

-A Deer IS-A aAnimal
-A Deer IS-A Vegetarian
-A Deer IS-A Deer
-A Deer IS-A Object

When we apply the reference variable facts to a Deer object reference, the following declarations are legal:

Deer d = new Deer();
Animal a = d;
Vegetarian v = d;
Object o = d;

All the reference variables d,a,v,o refer to the same Deer object in the heap.
Virtual Methods:

In this section, I will show you how the behavior of overridden methods in Java allows you to take advantage of polymorphism when designing your classes.

We already have discussed method overriding, where a child class can override a method in its parent. An overridden method is essentially hidden in the parent class, and is not invoked unless the child class uses the super keyword within the overriding method.

/* File name : Employee.java */
public class Employee
{
private String name;
private String address;
private int number;
public Employee(String name, String address, int number)
{
System.out.println("Constructing an Employee");
this.name = name;
this.address = address;
this.number = number;
}
public void mailCheck()
{
System.out.println("Mailing a check to " + this.name
+ " " + this.address);
}
public String toString()
{
return name + " " + address + " " + number;
}
public String getName()
{
return name;
}
public String getAddress()
{
return address;
}
public void setAddress(String newAddress)
{
address = newAddress;
}
public int getNumber()
{
return number;
}
}

Now suppose we extend Employee class as follows:

/* File name : Salary.java */
public class Salary extends Employee
{
private double salary; //Annual salary
public Salary(String name, String address, int number, double
salary)
{
super(name, address, number);
setSalary(salary);
}
public void mailCheck()
{
System.out.println("Within mailCheck of Salary class ");
System.out.println("Mailing check to " + getName()
+ " with salary " + salary);
}
public double getSalary()
{
return salary;
}
public void setSalary(double newSalary)
{
if(newSalary >= 0.0)
{
salary = newSalary;
}
}
public double computePay()
{
System.out.println("Computing salary pay for " + getName());
return salary/52;
}
}

Now you study the following program carefully and try to determine its output:

/* File name : VirtualDemo.java */
public class VirtualDemo
{
public static void main(String [] args)
{
Salary s = new Salary("Mohd Mohtashim", "Ambehta, UP",
3, 3600.00);
Employee e = new Salary("John Adams", "Boston, MA",
2, 2400.00);
System.out.println("Call mailCheck using
Salary reference --");
s.mailCheck();
System.out.println("\n Call mailCheck using
Employee reference--");
e.mailCheck();
}
}

This would produce following result:

Constructing an Employee
Constructing an Employee
Call mailCheck using Salary reference --
Within mailCheck of Salary class
Mailing check to Mohd Mohtashim with salary 3600.0

Call mailCheck using Employee reference--
Within mailCheck of Salary class
Mailing check to John Adams with salary 2400.0

Here we instantiate two Salary objects . one using a Salary reference s, and the other using an Employee reference e.

While invoking s.mailCheck() the compiler sees mailCheck() in the Salary class at compile time, and the JVM invokes mailCheck() in the Salary class at run time.

Invoking mailCheck() on e is quite different because e is an Employee reference. When the compiler seese.mailCheck(), the compiler sees the mailCheck() method in the Employee class.

Here, at compile time, the compiler used mailCheck() in Employee to validate this statement. At run time, however, the JVM invokes mailCheck() in the Salary class.

This behavior is referred to as virtual method invocation, and the methods are referred to as virtual methods. All methods in Java behave in this manner, whereby an overridden method is invoked at run time, no matter what data type the reference is that was used in the source code at compile time.

Java - Overriding

In the previous chapter we talked about super classes and sub classes. If a class inherits a method from its super class, then there is a chance to override the method provided that it is not marked final.

The benefit of overriding is: ability to define a behavior that's specific to the sub class type. Which means a subclass can implement a parent calss method based on its requirement.

In object oriented terms, overriding means to override the functionality of any existing method.
Example:

Let us look at an example.

class Animal{

public void move(){
System.out.println("Animals can move");
}
}

class Dog extends Animal{

public void move(){
System.out.println("Dogs can walk and run");
}
}

public class TestDog{

public static void main(String args[]){
Animal a = new Animal(); // Animal reference and object
Animal b = new Dog(); // Animal reference but Dog object

a.move();// runs the method in Animal class

b.move();//Runs the method in Dog class
}
}

This would produce following result:

Animals can move
Dogs can walk and run

In the above example you can see that the even though b is a type of Animal it runs the move method in the Dog class. The reason for this is : In compile time the check is made on the reference type. However in the runtime JVM figures out the object type and would run the method that belongs to that particular object.

Therefore in the above example, the program will compile properly since Animal class has the method move. Then at the runtime it runs the method specific for that object.

Consider the following example :

class Animal{

public void move(){
System.out.println("Animals can move");
}
}

class Dog extends Animal{

public void move(){
System.out.println("Dogs can walk and run");
}
public void bark(){
System.out.println("Dogs can bark");
}
}

public class TestDog{

public static void main(String args[]){
Animal a = new Animal(); // Animal reference and object
Animal b = new Dog(); // Animal reference but Dog object

a.move();// runs the method in Animal class
b.move();//Runs the method in Dog class
b.bark();
}
}

This would produce following result:

TestDog.java:30: cannot find symbol
symbol : method bark()
location: class Animal
b.bark();
^

This program will throw a compile time error since b's reference type Animal doesn't have a method by the name of bark.
Rules for method overriding:

- The argument list should be exactly the same as that of the overridden method.
- The return type should be the same or a subtype of the return type declared in the original overridden method in the super class.
- The access level cannot be more restrictive than the overridden method's access level. For example: if the super class method is declared public then the overridding method in the sub class cannot be either private or public. However the access level can be less restrictive than the overridden method's access level.
- Instance methods can be overridden only if they are inherited by the subclass.
- A method declared final cannot be overridden.
- A method declared static cannot be overridden but can be re-declared.
- If a method cannot be inherited then it cannot be overridden.
- A subclass within the same package as the instance's superclass can override any superclass method that is not declared private or final.
- A subclass in a different package can only override the non-final methods declared public or protected.
- An overriding method can throw any uncheck exceptions, regardless of whether the overridden method throws exceptions or not. However the overridden method should not throw checked exceptions that are new or broader than the ones declared by the overridden method. The overriding method can throw narrower or fewer exceptions than the overridden method.
- Constructors cannot be overridden.

Using the super keyword:

When invoking a superclass version of an overridden method the super keyword is used.

class Animal{

public void move(){
System.out.println("Animals can move");
}
}

class Dog extends Animal{

public void move(){
super.move(); // invokes the super class method
System.out.println("Dogs can walk and run");
}

}

public class TestDog{

public static void main(String args[]){

Animal b = new Dog(); // Animal reference but Dog object
b.move();//Runs the method in Dog class

}
}

This would produce following result:

Animals can move
Dogs can walk and run

Java - Inheritance

Inheritance can be defined as the process where one object acquires the properties of another. With the use of inheritance the information is made manageable in a hierarchical order.

When we talk about inheritance the most commonly used key words would be extends and implements. These words would determine whether one object IS-A type of another. By using these keywords we can make one object acquire the properties of another object.
IS-A Relationship:

IS-A is a way of saying : This object is a type of that object. Let us see how the extends keyword is used to achieve inheritance.

public class Animal{
}

public class Mammal extends Animal{
}

public class Reptile extends Animal{
}

public class Dog extends Mammal{
}

Now based on the above example, In Object Oriented terms following are true:

*

Animal is the superclass of Mammal class.
*

Animal is the superclass of Reptile class.
*

Mammal and Reptile are sub classes of Animal class.
*

Dog is the subclass of both Mammal and Animal classes.

Now if we consider the IS-A relationship we can say:

*

Mammal IS-A Animal
*

Reptile IS-A Animal
*

Dog IS-A Mammal
*

Hence : Dog IS-A Animal as well

With use of the extends keyword the subclasses will be able to inherit all the properties of the superclass except for the private properties of the superclass.

We can assure that Mammal is actually an Animal with the use of the instance operator.
Example:

public class Dog extends Mammal{
public static void main(String args[]){

Animal a = new Animal();
Mammal m = new Mammal();
Dog d = new Dog();

System.out.println(m instanceof Animal);
System.out.println(d instanceof Mammal);
System.out.println(d instanceof Animal);
}
}

This would produce following result:

true
true
true

Since we have a good understanding of the extends keyword let us look into how the implements keyword is used to get the IS-A relationship.

The implements keyword is used by classes by inherit from interfaces. Interfaces can never be extended.
Example:

public interface Animal {}

public class Mammal implements Animal{
}

public class Dog extends Mammal{
}

The instanceof Keyword:

Let us use the instanceof operator to check determine whether Mammal is actually an Animal, and dog is actually an Animal

interface Animal{}

class Mammal implements Animal{}

class Dog extends Mammal{
public static void main(String args[]){

Mammal m = new Mammal();
Dog d = new Dog();

System.out.println(m instanceof Animal);
System.out.println(d instanceof Mammal);
System.out.println(d instanceof Animal);
}
}

This would produce following result:

true
true
true

HAS-A relationship:

These relationships are mainly based on the usage. This determines whether a certain class HAS-A certain thing. This relationship helps to reduce duplication of code as well as bugs.

Lets us look into an example:

public class Vehicle{}
public class Speed{}
public class Van extends Vehicle{
private Speed sp;
}

This shows that class Van HAS-A Speed. By having a separate class for Speed we do not have to put the entire code that belongs to speed inside the Van class., which makes it possible to reuse the Speed class in multiple applications.

In Object Oriented feature the users do not need to bother about which object is doing the real work. To achieve this, the Van class hides the implementation details from the users of the Van class. SO basically what happens is the users would ask the Van class to do a certain action and the Vann class will either do the work by itself or ask another class to perform the action.

A very important fact to remember is that Java only supports only single inheritance. This means that a class cannot extend more than one class. Therefore following is illegal:

public class extends Animal, Mammal{}

However a class can implement one or more interfaces. This has made Java get rid of the impossibility of multiple inheritance

Java Encapsulation

Encapsulation is the concept of hiding the implementation details of a class and allowing access to the class through a public interface. For this, we need to declare the instance variables of the class as private or protected.

The client code should access only the public methods rather than accessing the data directly. Also, the methods should follow the Java Bean's naming convention of set and get.

Encapsulation makes it easy to maintain and modify code. The client code is not affected when the internal implementation of the code changes as long as the public method signatures are unchanged. For instance:

public class Employee
{
private float salary;
public float getSalary()
{
return salary;
}
public void setSalary(float salary)
{
this.salary = salary;
}

OOPs Concept

Introduction: In this section, we will discuss the OOPs concepts along with fundamentals used to develop the java applications and programs.

OOP means Object Oriented Programming. This is a technique used to create programs around the real world entities. In OOPs programming model, programs are developed around objects and data rather than actions and logics. In OOPs, every real life object has properties and behavior. This feature is achieved in java through the class and object creation. They contains properties (variables of some type) and behavior (methods). OOPs provides better flexibility and compatibility for developing large applications.

Class: A class defines the properties and behavior (variables and methods) that is shared by all its objects. It is a blue print for the creation of objects.

Object: Object is the basic entity of object oriented programming language. Class itself does nothing but the real functionality is achieved through their objects. Object is an instance of the class. It takes the properties (variables) and uses the behavior (methods) defined in the class.

Encapsulation, Inheritance and Polymorphism are main pillars of OOPs. These have been described below :

Encapsulation: Encapsulation is the process of binding together the methods and data variables as a single entity. This keeps both the data and functionality code safe from the outside world. It hides the data within the class and makes it available only through the methods. Java provides different accessibility scopes (public, protected, private ,default) to hide the data from outside. For example, we can create a class "Check" which has a variable "amount" to store the current amount. Now to manipulate this variable we can create methods. For example to set the value of amount create setAmount() method and to get the value of amount create getAmount() method . Here is the code for "Check" class :
class Check{
private int amount=0;
public int getAmount(){
return amount;
}
public void setAmount(int amt){
amount=amt;
}
}

public class Mainclass{
public static void main(String[] args){
int amt=0;
Check obj= new Check();
obj.setAmount(200);
amt=obj.getAmount();
System.out.println("Your current amount is :"+amt);
}
}

Here the data variable "amount" and methods setAmount() and getAmount() are enclosed together with in a single entity called the "Check" class. These two methods are used to manipulate this variable i.e. set and get the current value of amount.

Inheritance: Inheritance allows a class (subclass) to acquire the properties and behavior of another class (superclass). In java, a class can inherit only one class(superclass) at a time but a class can have any number of subclasses. It helps to reuse, customize and enhance the existing code. So it helps to write a code accurately and reduce the development time. Java uses extends keyword to extend a class.
class A{
public void fun1(int x){
System.out.println("int in A");
}

}

class B extends A{
public void fun2(int x,int y){
fun1(6); // prints "int in A"
System.out.println("int in B");
}
}

public class C{
public static void main(String[] args){
B obj= new B();
obj.fun2(2);
}
}

In the above example, class B extends class A and so acquires properties and behavior of class A. So we can call method of A in class B.

Polymorphism : Polymorphism allows one interface to be used for a set of actions i.e. one name may refer to different functionality. Polymorphism allows a object to accept different requests of a client (it then properly interprets the request like choosing appropriate method) and responds according to the current state of the runtime system, all without bothering the user.

There are two types of polymorphism :

1. Compile-time polymorphism
2. Runtime Polymorphism

In compiletime Polymorphism, method to be invoked is determined at the compile time. Compile time polymorphism is supported through the method overloading concept in java.

Method overloading means having multiple methods with same name but with different signature (number, type and order of parameters).
class A{
public void fun1(int x){
System.out.println("int");
}
public void fun1(int x,int y){
System.out.println("int and int");
}
}
public class B{
public static void main(String[] args){
A obj= A();

// Here compiler decides that fun1(int)
is to be called and "int" will be printed.
obj.fun1(2);

// Here compiler decides that fun1(int,int)
is to be called and "int and int" will be printed.
obj.fun1(2,3);
}
}

In rumtime polymorphism, the method to be invoked is determined at the run time. The example of run time polymorphism is method overriding. When a subclass contains a method with the same name and signature as in the super class then it is called as method overriding.

class A{
public void fun1(int x){
System.out.println("int in A");
}
public void fun1(int x,int y){
System.out.println("int and int");
}
}

class C extends A{
public void fun1(int x){
System.out.println("int in C");
}
}

public class D{
public static void main(String[] args){
A obj;

obj= new A(); // line 1
obj.fun1(2); // line 2 (prints "int in A")

obj=new C(); // line 3
obj.fun1(2); // line 4 (prints "int in C")
}
}

In the above program, obj has been declared as A type. In line 1, object of class A is assigned. Now in the next line, fun1(int) of class A will be called. In line 3, obj has been assigned the object of class C so fun1(int) of class C will be invoked in line 4. Now we can understand that same name of the method invokes different functions, defined in different classes, according to the current type of variable "obj". This binding of method code to the method call is decided at run time.

OOPs in Java

Java is a object oriented programming and to understand the functionality of OOP in Java, we first need to understand several fundamentals related to objects. These include class, method, inheritance, encapsulation, abstraction, polymorphism etc.

Class - It is the central point of OOP and that contains data and codes with behavior. In Java everything happens within class and it describes a set of objects with common behavior. The class definition describes all the properties, behavior, and identity of objects present within that class. As far as types of classes are concerned, there are predefined classes in languages like C++ and Pascal. But in Java one can define his/her own types with data and code.

Object - Objects are the basic unit of object orientation with behavior, identity. As we mentioned above, these are part of a class but are not the same. An object is expressed by the variable and methods within the objects. Again these variables and methods are distinguished from each other as instant variables, instant methods and class variable and class methods.