Generic method java 6 <T> before return type

Question

What is the difference between:

public <T> void createArray(T sample){
    ArrayList<T> list = new ArrayList<T>();
    list.add(sample);
}

and

public void createArray(T sample){
    ArrayList<T> list = new ArrayList<T>();
    list.add(sample);
}

I read that the method signature for using types should have <T> before the return type but how come I am still able to create the method without the <T>? What is the implication if I do or do not put it?

Answer 1

In the second method, the type parameter would be typically defined in the class declaration to which the method belongs:

class MyClass<T> {
   public void createArray(T sample){
      ArrayList<T> list = new ArrayList<T>();
      list.add(sample);
   }
   ...
}

So the second method belongs to a generic type. The first method is a generic method because it defines its own type parameter.

Answer 2

In the first case, the generic parameter T is defined for the method. Other methods may have a different T.

In the second case, the generic parameter T is defined for the class or interface. All methods within that class or interface must have the same T.

Defining a class-wide generic allows you to enforce the same type parameter on many methods. You can also have fields of the generic type. See ArrayList<t> for an example.