不要从超类构造函数中调用子类方法

Question

Consider the following code

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */

package example0;

/**
 *
 * @author yccheok
 */
public class Main {

    static class A {
        private final String var;

        public A() {
            var = getVar();
            // Null Pointer Exception.
            System.out.println("var string length is " + var.length());
        }

        public String getVar() {
            return "String from A";
        }
    }

    static class B extends A {
        private final String bString;

        // Before B ever constructed, A constructor will be called.
        // A is invoking a overriden getVar, which is trying to return
        // an initialized bString.
        public B() {                
            bString = "String from B";
        }

        @Override
        public String getVar() {
            return bString;
        }
    }

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        B b = new B();
    }

}

Currently, in my mind, there are two ways to avoid such problem.

Either making class A final class.

static final class A {
    private final String var;

    public A() {
        var = getVar();
        // Null Pointer Exception.
        System.out.println("var string length is " + var.length());
    }

    public String getVar() {
        return "String from A";
    }
}

Or

Making getVar method final

static class A {
    private final String var;

    public A() {
        var = getVar();
        // Null Pointer Exception.
        System.out.println("var string length is " + var.length());
    }

    public final String getVar() {
        return "String from A";
    }
}

The author trying to suggest ways to prevent the above problem. However, the solution seems cumbersome as there are some rules to be followed.

http://benpryor.com/blog/2008/01/02/dont-call-subclass-methods-from-a-superclass-constructor/

Beside making final and the author suggested way, is there more ways to prevent the above problem (Don't call subclass methods from a superclass constructor) from happen?

Answer 1

Making getVar method final

This is definitely what you need to do.

If you're relaying on the functionality of a method to initialize an object, you shouldn't let subclasses broke that method.

Answering your question, other way to prevent it is to make getVar private in A .

See this simplified version of your code:

// A.java
class A {
    private final String var;
    public A(){
        var = getVar();
        var.length();
    }
    private String getVar(){
        return "This is the value";
    }
}
class B extends A {
    private final String other;
    public B(){
        other = "Other string";
    }
    public String getVar(){
        return other;
    }
}
class Main{
    public static void main( String [] args ) {
        new B();
    }
}

BTW, why did you put those as static nested classes, just to create confusion?