CODEWITHSUNDEEP
java
Alf
Alf

Reputation: 1295

Access one object from another?

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;
}

Upvotes: 2

Views: 176

Answers (3)

MD Sayem Ahmed
MD Sayem Ahmed

Reputation: 29166

You didn't create an instance of Test02, so you'll get a NullPointerException whenever you try to access test02 from Test01's constructor.

To remedy this, redefine your Test01 class's constructor like this - 

class Test01
{
    int x;
    Test02 test02; // need to assign an instance to this reference.

    public Test01()
    {
        test02 = new Test02();    // create instance.
        x = test02.x;
    }
}

Or, you can pass an instance of Test02 from Main - 

class Main
{
    public static void main(String[] args)
    {
        Test02 test02 = new Test02();
        Test01 test01 = new Test01(test02); //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(Test02 test02)
    {
        this.test02 = test02;
        x = test02.x;
    }
}

class Test02
{
    int x = 10;
}

The reason is that whenever you try to create an instance of Test01, it tries to access the test02 variable in its constructor. But the test02 variable doesn't point to any valid object at this moment. That's why you will get NullPointerException.

Upvotes: 1

Bela Vizer
Bela Vizer

Reputation: 2537

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;
    }
}

Upvotes: 2

invariant
invariant

Reputation: 8900

Test02 test02 = new Test02();

test02 object created have scope for main method only .

x = test02.x; it will give null pointer because no object created !

Upvotes: 1

Related Questions