Question

I know it’s absolutely dull question (so newbie) but i’m stuck. How can access fields of one object from another object?
The question => how to avoid creating Test02 object twice? (1st time => from main() loop, 2nd time => from constructor of Test01)?

class Main
{
    public static void main(String[] args)
    {
        Test02 test02 = new Test02();
        Test01 test01 = new Test01(); //NullPointerException => can't access test02.x from test01
        System.out.println(test01.x); 

    }
}

class Test01
{
    int x;
    Test02 test02; //can't access this, why? I creat test02 object in main() loop before test01

    public Test01()
    {
        x = test02.x;
    }
}

class Test02
{
    int x = 10;
}

Answer 1

Either you have to instantiate test02 in Test01 (eg. in constructor) or pass the instantiated object in main to Test01

so either:

public Test01()
    {
        x = new Test02();
//...
    }

or

class Test01
{
    int x;
    Test02 test02; //the default value for objects is null you have to instantiate with new operator or pass it as a reference variable

    public Test01(Test02 test02)
    {
        this.test02 = test02; // this case test02 shadows the field variable test02 that is why 'this' is used
        x = test02.x;
    }
}