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.





Sar Maroof is a professional software development teacher, gives master classes and publishes technical articles. He is also an expert software developer and worked for several big as well as small companies and later as a freelancer.

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