CODEWITHSUNDEEP
javasubclass
deltanovember
deltanovember

Reputation: 44041

Why does my child class not initialize variables properly in java

In the code below, myString is always initialized to null. I have to manually initialize in an init() or similar. As far as I can tell it is related to superclass/subclass but I don't understand the exact mechanism

public class A extends B {

    private String myString = "test";


    public static void main(String[] args) {
        new A();
    }

    public A() {
        super();

    }

    public void c() {
        System.out.println(myString);
    }

}

public class B {

    public B() {
        c();
    }

    public void c() {

    }
}

Upvotes: 0

Views: 552

Answers (3)

Sergey Kulagin
Sergey Kulagin

Reputation: 81

Thinking in Java Second Edition by Bruce Eckel, Behavior of polymorphic methods inside constructors (p. 337-339).

Upvotes: 0

Saurabh Gokhale
Saurabh Gokhale

Reputation: 46395

Add a call to c(); overidden method right after the object has been fully created and call to superclass constructor is done.

Change your code to this .. 

public class A extends B {

    private String myString = "test";


    public static void main(String[] args) {
        new A();
    }

    public A() {
        super();
         c();     // Include the call to c(); here ...

    }

    public void c() {
        System.out.println(myString);
    }

}

public class B {

    public B() {

    }

    public void c() {

    }
} 

 // Output : test

Upvotes: 0

Howard
Howard

Reputation: 39177

The issue with your code is, that myString is initialized at the begin of the constructor of class A but right after the super constructor (i.e. class B). Since you access the variable before from the constructor of class B (indirectly via call to overriden methode c) your get this behaviour.

As a rule of thumb: if you want to avoid unexpected behavior do not call overriden methods before the constructor has been executed.

Upvotes: 3

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