为什么我的孩子 class 不能在 java 中正确初始化变量

Question

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() {

    }
}

Answer 1

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 (ie 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.

Answer 2

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

Answer 3

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