I have two classes A and B while B is a subtype of A:

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Asked: May 25, 20262026-05-25T10:51:35+00:00 2026-05-25T10:51:35+00:00

I have two classes A and B while B is a subtype of A:

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I have two classes A and B while B is a subtype of A:

public class A {
    private String stringVar;

    public A()  {
        stringVar = "";
    }

    public String getStringVar() {
        return stringVar;
    }

    public void setStringVar(String str) {
        this.stringVar = str;
    }

    @Override 
    public String toString()  {
        return getStringVar();
    }
}

Class B:

public class B extends A {
    private int intVar;

    public B()  {
        intVar = 0;
    }

    public int getIntVar() {
        return intVar;
    }

    public void setIntVar(int intVar) {
        this.intVar = intVar;
    }

    @Override
    public String toString()  {
        return super.toString() + " " + getIntVar();
    }
}

As you can see in the following main method I assign the b to a. Now “a” can’t invoke b’s methods which is clear, because I’m using an instance of type A now. But it behaves like a B when toString is invoked. Curious, I would have expected toString of a. Why is this so?

public class Main {

    public static void main(String[] args) {
        A a = new A();
        B b = new B();
        b.setIntVar(200);
        b.setStringVar("foo");
        a = b;
        System.out.println(a);
    }
}