Why does the code print 0 instead of 5?

Question

Why does this code print 0 instead of 5?

class B {
    private int b;
    public int getB() { return b; }
    public void setB(int b) { b=b; }
}

class A {
    public static void main (String[] args) {
        B b = new B();
        b.setB(5);
        System.out.println(b.getB());
    }
}

Run it via Ideone.

Answer 1

Because the code is assigning the parameter b for the parameter itself b and the attribute b class is not being changed. When there is ambiguity, the local symbol wins. If the parameter had another name, it would work. But since it’s not legal to change a name just to get around it, after all this could affect readability, you should make it explicit that you want to move the class variable, indicating with this.b. Thus:

class B {
    private int b;
    public int getB() { return b; }
    public void setB(int b) { this.b = b; }
}

class A {
    public static void main (String[] args) {
        B b = new B();
        b.setB(5);
        System.out.println(b.getB());
    }
}

Behold working in the ideone. And in the repl it.. Also put on the Github for future reference.

Answer 2

You must use this.b to reference the attribute of your class. Thus, your class B code should be:

class B {
  private int b;
    public int getB() { return this.b; }
    public void setB(int b) { this.b=b; }
}

Answer 3

The code b=b; has no effect on the method public void setB(int b) class B.
When a parameter exists with the same field name (field) it is necessary to use the this to access the field and the reason you are printing 0 is because this is the initial value assigned to variables and fields of the type int.