Java puzzle 03: Inheritance

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This Java puzzle is a mystery!

By creating an object of the subclass MySub in the main method, the constructor of the superclass MySuper is called.

The constructor of MySuper invokes the method myMethod. The statement System.out.print(“-x” + m); ¬†should write -x3, because the value of m is 3.
The statement System.out.print(“-x” + n); should write -x2, because the value of n is 2.

The mystery is that this program writes -x0 to the standard output.
What is your explanation for that?

Note: The classes MySuper and MySub are two separate files in one package.

Java puzzle 3: Inheritance

Level: Hard

MySuper.java

public class MySuper
{
  int m = 3;

  public MySuper()
  {
    myMethod();
  }
  void myMethod()
  {
    System.out.print("-x" + m);
  }
}

MySub.java

public class MySub extends MySuper
{
  int n = 2;

  void myMethod()
  {
    System.out.print("-x" + n);
  }
  public static void main(String[] args)
  {
    MySub mySub = new MySub();
  }
}
Author: Sar Maroof

Answer explanation

  1. The constructor of the superclass invokes the method myMethod, which is overridden
    in the subclass MySub.
  2. The statement System.out.print(“-x” + n); in the method myMethod of
    the subclass MySub writes the string “-x” and the value of the variable n
    to the standard output.
  3. The constructor of the superclass invokes the method myMethod, which is overridden in the subclass and the method tries to access the variable n inside the subclass before it is initialzied. Therefore the program writes -x0 instead of -x2 to the standard output.

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Sar Maroof is graduated from HBO Amsterdam "higher professional education". He is a professional computer programming teacher, gives master classes and publishes technical articles. His experience with developing software started from 2001 and he has worked as a software developer for several big as well as small companies and later as a freelancer. He is the author of Java quizmaster and Build a Java application in 7 days.

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