Generics provides a way to communicate the type of a collection to the compiler.  By this compiler can maintain the consistency of  collection at the time insertion and retrieval of an object.

Before Generics	After Generics
ArrayList l = new ArrayList (); l.add("One"); l.add("Two"); //Type cast when you get element from Collection String str = (String) l.get(2); System.out.println(str); Iterator it = l.iterator(); while (it.hasNext()) {    String val = (String) it.next();//Required type cast }	ArrayList<String> l = new ArrayList<String>(); l.add("One"); l.add("Two"); //Do not type cast when get element from Collection String str = l.get(2); System.out.println(str); Iterator<String> it = l.iterator(); //specify type while (it.hasNext()) {    String val = it.next(); //No type cast }
HashMap map = new HashMap (); map.put(“1”,”One”); //Correct	HashMap<String,Integer> map = new HashMap<String,Integer>(); map.put(“1”,”One”); //Compilation Error

Contents

1. 1 Program :
   1. 1.1 Step 1 - Coding
   2. 1.2 Step 2 - Deployment
   3. 1.3 Step 3 - Testing
   4. 1.4 Output

Program :

Step 1 - Coding

Create a text file c:\sunilos\TestGenerics.java  and copy below contents.


import java.util.*;

/*
 * A program of Generics.
 * copyright (c) sunilos Technologies Indore
 * @url : www.sunilos.co.in
 *
 */

public class TestGenerics {

    public static void main(String[] args) {

                //Define a List and communicate its elements' data type to compiler

        ArrayList<String> l = new ArrayList<String>();
        l.add("One");
        l.add("Two");
        l.add("Three");
        l.add("Four");
        l.add("Five");

        String str = l.get(2); //No Type cast
        System.out.println(str);

        Iterator<String> it = l.iterator();

                System.out.println("List elements ");

        while (it.hasNext()) {
            System.out.println(it.next());

        }

                //Define a Map and communicate its Key and Value data types to compiler
       
        HashMap<String,Integer> map  = new HashMap<String,Integer>();
        map.put("One", new Integer(1));

    }
  
}

Step 2 - Deployment

1. Create a folder 'c:\sunilos'. 
2. Create or copy TestGenerics.java  into 'c:\sunilos' folder.
3. Open your command prompt and go to  'c:\sunilos'
   
4. Compile TestGenerics.java with help of 
• javac TestGenerics.java command. Compiled class file will be created in the same folder
  
5. Congratulations!! your Control Statement Java program is ready to serve.

Step 3 - Testing

1. Make sure you are on Command Prompt under c:\sunilos directory
   
2. Now start your Control Statements java program from command prompt with help of 
• java TestGenerics command.

Output

Three
List elements
One
Two
Three
Four
Five

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