A reference to an object can be cast into a reference to an object of its implemented interface : Reference Type Casting « Type Casting « SCJP

1. SCJP
2. Type Casting
3. Reference Type Casting

public class MainClass{
    public static void main(String[] argv){
        MyInterface my = new MyClass();
        System.out.println();
    }
}

interface MyInterface{
    public void aMethod();

}    
class MyClass implements MyInterface{
    public void aMethod(){
    }
}