Use of variables from other class

Question

hallo,
i have two classes A and B
i would like to access a variable of the class B out of A class
without class A extends B or
A classA = new A();
to use
i attempted it with a constructor
class A{
    B klb;
    public A(B b1){
    this.klb = b1;
   int a;
   a= klb.varausB;
}with that I get, however, a NullPointerException
in a similar form i saw it already once and it functioned
somebody can help me?

Answer 1

that is it what I mean
public class ClassA {
ClassB clb;
public ClassA (ClassB cb){
this.clb = cb;
public static void main(String[] args) {
System.out.println("Var from class B:
class B: "+clb.varb1);// there is an error, i know
but how it is right?
public class ClassB {
public String varb1 ="var b1";
}nevertheless so similarly it should function, or?So if you create an instance of ClassA, clb will be initialized. But you don't do that in main. Hence your errors:
1) You're not calling the constructor, so clb is null
2) You're trying to access an instance variable without having an instance, so clb is not only null, it actually doesn't exist.
public static void main(String[] args) {
  Class A myA = new ClassA();
  System.out.println("Var from class B: "+ myA.clb.varb1);
}You're outside of any object, ehnce you need to address clb through an instance of ClassA.