Java Tutorials : Java Generics And References For Java

6.Generics

The feature Generics in java allows a programmer to create classes and objects that can operate on any defined types. It allows clients to substitute a suitable java type at the compile time preventing un necessary type casting being done in application code.

Generic Methods:

public class GenericMethod

{

   //This an generic method for printing array of what ever type it gets                        

   public static < E > void ArrayPrint( E[] ArrayInput )

   {

      // Display array elements              

         for ( E element : ArrayInput ){        

            System.out.printf( "%s ", element );

         }

         System.out.println();

    } 

    public static void main( String args[] )

    {

        // Create arrays of Charatcters,Integers

        Integer[] Arrayint = { 1, 2, 3, 4, 5 };

        Character[] Arraychar = { 'J', 'A', 'V', 'A'}; 

        System.out.println( "Array Of Integers:" );

        printArray( Arrayint  );  

        System.out.println( "\nArray Of Characters:" );

        printArray( Arraychar );    } 

}

Generic methods are defined using type parameters.
Every generic method has a type parameters section that precedes the return type of the method
The type parameters are separated by commas and enclosed in angle brackets < and >
Type parameters also known as type variables are identifiers that specify generic type names
In generic method type parameters are used for:
1. Declare the return type of method
2. Declare formal parameters
3. Declare local variables

Generic Classes:

Just as Generic methos you can define Generic classes. In this case, the type parameters are enclosed in angle brackets < and > and placed after name of the class and a type parameter is used to declare variables and generic methods with in the class

public class GenericsSnippet {

 private E obj; 

 //Generic constructor

 GenericsSnippet(){} 

 //Getting the Generic Value

 public E getVal(){

  return obj;

 } 

 //Setting the Generic object value.

 public void setVal(E obj){

  this.obj = obj;

 }

}

Generic classes are used to write a single definition for a set of related classes.
Generic classes are also known as parametric classes
In generic classes you can declare reference variables using type parameter within the definition of the class
Objects cannot be instantiated using type parameters

Bounded Type parameters

Some times you will need to restrict the kind 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 (Integer, float etc.).

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

public class FindMax
{   
   public static <T extends Comparable <T>> T maximum(T x, T y, T z)
   {                      
      T max = x;
      if ( y.compareTo( max ) > 0 )
    {
         max = y;
      }
      if ( z.compareTo( max ) > 0 )
     {
         max = z;
      }
      return max; 
  }
   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( "Maximum 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" ) );
   }
}

Wildcard types

Wild card types are used for putting restrictions on the types that are used in generic programming. These are used as type parameters in the declarations of variables and of formal parameters in method definitions.

Difference between Wildcard bound and type parameter bound

A wildcard can have only one bound, while a type parameter can have several bounds
A wildcard can have a lower or an upper bound, while there is no such thing as a lower bound for a type parameter.

7. Types of Java Applications

There are mainly 4 types of applications that can be created using Java programming:

1) Standalone Application: Standalone applications are also known as desktop applications or window-based applications. These are traditional software that we need to install on every machine. Examples of standalone application are Media player, antivirus, etc. AWT and Swing are used in Java for creating standalone applications.

2) Web Application : An application that runs on the server side and creates a dynamic page is called a web application. Currently, Servlet, JSP, Struts, Spring, Hibernate, JSF, etc. technologies are used for creating web applications in Java.

3) Enterprise Application: An application that is distributed in nature, such as banking applications, etc. is called an enterprise application. It has advantages like high-level security, load balancing, and clustering. In Java, EJB is used for creating enterprise applications.

4) Mobile Application: An application which is created for mobile devices is called a mobile application. Currently, Android and Java ME are used for creating mobile applications.