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javacasting
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lots_of_questions

Reputation: 1149

Casting child to a different child Java

I am trying to cast a child to a sibling in Java (though I'm not sure if this is allowed). Basically what is written below:

public interface BaseInterface {
    public int a = 5;
}

public class ClassA implements BaseInterface {
    public int a = 3;
}

public class ClassB implements BaseInterface {}

public static void main(String[] args) {
    BaseInterface a = new ClassA();
    ClassB b = (ClassB) a;
}

I keep getting a ClassCastException. I am trying to copy all member variables from the BaseInterface object a to b. Can someone provide a solution on how to do this? Thanks!

Upvotes: 1

Views: 4249

Answers (3)

Alex DiCarlo
Alex DiCarlo

Reputation: 4891

public interface BaseInterface {
    public int getA();
}

public class ClassA implements BaseInterface {
    private final int a;

    public ClassA(int a) {
        this.a = a;
    }

    public int getA() {
        return a;
    }
}

public class ClassB implements BaseInterface {
    private final int a;

    public ClassB(BaseInterface baseInterface) {
        this.a = baseInterface.getA();
    }

    public int getA() {
        return a;
    }
}

public static void main(String[] args) {
    BaseInterface a = new ClassA(5);
    ClassB b = new ClassB(a);
}

Will do what you want. As others noted, casting will always give a ClassCastException.

Upvotes: -1

Paul Bellora
Paul Bellora

Reputation: 55213

This is not possible. Instead, you should give ClassA and ClassB copy constructors taking a BaseInterface:

public class ClassB implements BaseInterface {

    public ClassB(BaseInterface other) {
        //copy state from other instance
    }
}

public static void main(String[] args) {
    BaseInterface a = new ClassA();
    ClassB b = new ClassB(a);
}

Of course this means you're copying to a new object instead of converting, but it's the only option if you want to go from a ClassA to a ClassB.

Upvotes: 3

Matt Ball
Matt Ball

Reputation: 359796

A ClassA is not a ClassB, so of course this is not allowed. Even if you suppress all warnings/errors and get the code to compile, at runtime the cast will cause a ClassCastException.

http://docs.oracle.com/javase/tutorial/java/IandI/subclasses.html

Upvotes: 0

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