Do not call subclass methods from the superclass constructor

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 opinion, there are two ways to avoid such a problem.

Or make class A the 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

Creating the getVar final method

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 is trying to suggest ways to prevent the above problems. However, the decision seems cumbersome because there are some rules that must be followed.

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

Besides creating the final method proposed by the author, are there more ways to prevent the problem described above (not calling subclass methods from the superclass constructor)?

Answer 1

Creating the getVar final method

, .

, .

, , getVar A.

:

// 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();
    }
}

, , ?