Understanding inheritance of class variables

Question

I'm building a Challenge24Solver class in Java. The logic itself works and finds the solutions as would be expected (with an arbitrary number of arguments). Anyway that part of the projects is works as I expect.

The question comes from problems with the representation of the solutions. It's fair to say that I've completed this project in Python and decided to attempt in Java as a sort of introduction, which may be the problem, that I'm trying to do this too like Python.

Here are some of my classes:

abstract class Operation { \\ Basic operation class
  static String token;
  abstract public double operate (double x, double y);
}
class AddOp extends Operation {
  static String token = "+";
  public double operate (double x, double y) {
    return x+y;
  }
}
//other operation classes SubOp, MulOp, DivOp
class Challenge24Step { // represents one step in a solution
  Operation operator;
  double x, y; //operands
  double value; //the value of the step (x operator y)
  public Challenge24Step (Operation operator, double x, double y) {
    this.operator = operator;
    // constructor code;
    }
  public String toString () {
    return String.format("%s %s %s = %s", this.x, this.operator.token, this.y,
                         this.value);
  }
}

The problem is that it still gets token from the Operation class: "null"

I understand that this is probably because operator is declared as Operation, but I don't understand why it doesn't go through standard inheritance: instance, class, each superclass

How would I rearrange the script so that the more specific operation classes' tokens are used?

Sergey Kalinichenko · Accepted Answer · 2013-06-17 02:44:29Z

3

You cannot make token static in the base class, because then there would be a single token per all inherited classes. You need to make it an instance variable. You cannot even put it in a static method, because static methods in Java are not overridable.

In Java, variables are not overridable. If you inherit a class and add a variable by the same name, the variable in the derived class will hide, not override the one in the base class.

To fix this, make token an abstract method in the base, provide an implementation in the derived class, and return the desired value. If you do this, you could replace the abstract class with an interface, because there would be no variables or implementations in it:

interface Operation { // Basic operation class
    String getToken();
    double operate (double x, double y);
}
class AddOp implements Operation {
    public String getToken() {return "+"; }
    public double operate (double x, double y) {
        return x+y;
    }
}

Alternatively, leave it unassigned in the base, add a constructor that takes the value of the token, and assign it there:

abstract class Operation { // Basic operation class
    public final String token;
    abstract public double operate (double x, double y);
    protected Operation(String token) {this.token = token; }
}
class AddOp extends Operation {
    public AddOp() { super("+"); }
    public double operate (double x, double y) {
        return x+y;
    }
}

edited Jun 17, 2013 at 2:44

answered Jun 17, 2013 at 2:08

Sergey Kalinichenko

729k85 gold badges1.2k silver badges1.6k bronze badges

Sign up to request clarification or add additional context in comments.

6 Comments

nachokk Over a year ago

hey i did class Base{ x="s" } class Child extends Base{ x="x"} and main Base b = new Child(); Sysout(b.x) and prints "s"

Prasad Kharkar Over a year ago

@nachokk The same reason is mentioned. variables are not overridden. Only methods are overridden

DoubleMx2 Over a year ago

Thanks for the explanation and suggestions. I'll probably use the second fix. At the current time I just decided to define public String toString () to return clsName.token keeping token static. Also should it be abstract class Operation {}

nachokk Over a year ago

@PrasadKharkar yeah i know but i think it was hidden, so it's not make me clear hide concept, if i had Child child = new Child() , sysout(child.x) obvious prints x

Prasad Kharkar Over a year ago

What do you mean by hide concept? Can you be more specific and ask a new question here? I would definitely try my best to solve your query :). Requesting you to ask a different question because hiding concept is a little different than what is asked in this question

|

Bohemian · Accepted Answer · 2013-06-17 02:04:51Z

3

Static variables are referenced via the class. You have defined operator as being of type Operation, so operator.token refers to Operation.token.

I recommend using a getter for this purpose:

abstract Operation { \\ Basic operation class
  abstract public double operate (double x, double y);
  abstract public String getToken();
}

class AddOp extends Operation {
  public double operate (double x, double y) {
    return x+y;
  }
  public String getToken() {
      return "+";
  }
}

answered Jun 17, 2013 at 2:04

Bohemian♦

427k103 gold badges604 silver badges750 bronze badges

Collectives™ on Stack Overflow

Understanding inheritance of class variables

2 Answers 2

6 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

6 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related