Java Variables of Interface type

Question

I know that object which is assigned to the reference variable whose type is an interface could be an instance of a class that implements the interface. But for the following code blocks:

public interface foo {
    public abstract void method_1();
}

class bar implements foo {  
    @Overide
    public void method_1() { //Implementation... }

    public void method_2() { //Do some thing... } 
}
.....

foo variable = new bar();
variable.method_1(); // OK;
variable.method_2(); // Is it legal?

Is it possible to make the variable (whose declared type is foo but actual type is bar) call the method _2 which is not declared in the interface ? Thanks in advance!

Answer 1

variable.method_2() won't compile as variable is of type foo . foo does not have a method method_2() .

Answer 2

Yes, you can cast:

((bar)variable).method_2();

But you probably shouldn't. The whole point of an interface is to only use the methods it provides. If they're not sufficient, then don't use the interface.

Answer 3

No, it is not. If you want access to method_2 , you have to declare the type of variable to be bar .

Answer 4

Is it possible to make the variable (whose declared type is foo but actual type is bar) call the method_2 which is not declared in the interface ?

Not its not possible. It will be a compile time error.

There is other standard deviation also in your code

1. Interface and class names should be in upper camel case (this is UpperCamelCase).

Answer 5

No, it is not legal. However, you can inspect the type at runtime and cast to the correct type:

if (variable instanceof bar) ((bar)variable).method_2();

(Strictly speaking you can cast without the instanceof check if you know for sure the type is correct, or are happy to get an exception thrown if you are wrong.)