cast an object to an interface in java?

Question

if we cast an object to an interface, won\'t this object be able to call its own methods? in the following example, myObj will only be able to call MyInterface meth

Answer 1

Only the methods in the interface are visible, but all methods in the object may still be invoked, as long as they're made accessible otherwise. For example:

public interface MyInterface {
    String getName();
}

public class Obj implements MyInterface {
    private String getFirstName() {
        return "John";
    }

    private String getLastName() {
        return "Doe";
    }

    @Override
    public String getName() {
        return getFirstName() + " " + getLastName();
    }
}

public class Sec implements MyInterface {
    private String getDate() {
        return new Date().toString();
    }

    @Override
    public String getName() {
        return getDate();
    }
}

In the above cases, both Obj and Sec can call their private members, even though they will not be visible in other classes. So when you say...

MyInterface myObj = new Obj();
MyInterface mySec = new Sec();

...as you've asked in the question, while it is true that in myObj and mySec the only method visible is getName(), their underlying implementation can be different.