Tutorials :Flexible Object Operation with Java Generics :

The Big Bang of Java Generics

1: import java.util.List;
2: import java.util.LinkedList;

3: class WithoutGenerics {    
4:    public static void main(String[] args) {   
5:    List floatList=new LinkedList();    
6:    floatList.add(new Float(4.5));    
7:    floatList.add(new Float(1.5));
8:    Float nr=(Float)(floatList.iterator().next());
9:    
10:    System.out.println("First float number: "+nr);
11:    }
12: }

9: String str=(String)(floatList.iterator().next());

Now you have a dilemma, because it's obvious that the casts from lines 8 and 9 are annoying. Firstly, you know what types of objects you have stored in the list, so why should you write an explicit cast when accessing the list s elements? Secondly, when you accidentally/intentionally mistake the cast type, why you do not get a compile-time error or at least a warning about this problem? Both questions have the same answer: Because Java Collection classes internally treat elements in the collection as Objects.

This problem is responsible for the birth of the Java Generics core idea, which goed like this: What if a developer could indicate the particular data type to be stored in a list? The practical application of this theoretical concept is to use angle brackets, shown in line 5 in the below example (which is the Java Generics version of the previous code):

1: import java.util.List;
2: import java.util.LinkedList;

3: class WithGenerics {    
4: public static void main(String[] args) {
5:    List floatList = new LinkedList();
6:    floatList.add(new Float(4.5));    
7:    floatList.add(new Float(1.5));
8:    Float nr=floatList.iterator().next();
9:    
10:   System.out.println("First float number: "+nr);
11:   }
12: } 

Now, modify the example by replacing line 9:

9: String str=(String)(floatList.iterator().next());

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