inheritence in java inherit variables?

Question

As I understand the inherited class should also inherit variables, so why doesn't this code work?

public class a {
    private int num;

    public static void main(String[] args) {
        b d = new b();
    }
}

class b extends a {
    public b() {
        num = 5;
        System.out.println(num);
    }
}

Answer 1

num variable's access modifier is private and private members are not accessible out of own class so make it protected it will accessible from subclass.

public class a {
     protected int num;
     ...
}

Reference of Controlling Access to Members of a Class

Answer 2

As i understand the inherited class should inherit also variables,

you got it wrong, instance variables are not overriden in sub-class. inheritence and polymorphism doesnt apply for instance fields. they are only visible in your sub-class if they are marked protected or public. currently you have super class variable marked private. no other class can access it. mark it either protected or public in-order for other class's to access it.

Answer 3

public class A{
    public int num=5;
    public static void main(String[] args) {
        b d = new b();
        d.c();
    }
}

class b extends A
{
    public void c() 
    {
        System.out.println(num);
    }
}

definitely this is what you need i think

Answer 4

private scope can only be accessed by the containing class.

For this to work num would need to be declared protected scope.

However this would also make it accessible to other classes in the same package. My recommedation would be to create a get / set method in order to maintain proper encapsulation.

you could then access num in class b by calling getNum()