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Generics in Java

The Java Generics programming is introduced in J2SE 5 to deal with type-safe objects. It makes the code stable by detecting the bugs at compile time.

Before generics, we can store any type of objects in the collection, i.e., non-generic. Now generics force the java programmer to store a specific type of objects.

Fig: Generics In Java 

Advantage of Java Generics

There are mainly 3 advantages of generics. They are as follows:

1) Type-safety: We can hold only a single type of objects in generics. It doesn?t allow to store other objects.

Without Generics, we can store any type of objects.

      List list = new ArrayList();    

      list.add(10);                         

      list.add("10");  

     With Generics, it is required to specify the type of object we need to store.

    List<Integer> list = new ArrayList<Integer>();    

    list.add(10);  

    list.add("10");// compile-time error  

2) Type casting is not required: There is no need to typecast the object.

Before Generics, we need to type cast.

   List list = new ArrayList();    

    list.add("hello");    

    String s = (String) list.get(0);//typecasting    

    After Generics, we don't need to typecast the object.  

    List<String> list = new ArrayList<String>();    

    list.add("hello");    

    String s = list.get(0);    

3) Compile-Time Checking: It is checked at compile time so problem will not occur at runtime. The good programming strategy says it is far better to handle the problem at compile time than runtime.

    List<String> list = new ArrayList<String>();    

    list.add("hello");    

    list.add(32);//Compile Time Error 

Syntax

 To use generic collection

ClassOrInterface<Type>    

Example 

To use Generics in java

ArrayList<String> 

Example of Generic

    import java.util.*;  

    class TestGenerics2{  

    public static void main(String args[]){  

    Map<Integer,String> map=new HashMap<Integer,String>();     

    map.put(1,"vijay");   

    map.put(4,"umesh");  

    map.put(2,"ankit");  

    //Now use Map.Entry for Set and Iterator  

    Set<Map.Entry<Integer,String>> set=map.entrySet();  

I  terator<Map.Entry<Integer,String>> itr=set.iterator();  

   while(itr.hasNext()){  

   Map.Entry e=itr.next();//no need to typecast  

    System.out.println(e.getKey()+" "+e.getValue());   

}    }

     Output:

                             vijay

                        ankit

                        umesh