[Java] Calling a method in an extended class

[79wjd]

So I'm making a game of chess and I've created a Piece class and then I have multiple extensions of the piece class for each individual piece. So say I have someting like this, how would I go about calling the method in the extended class? 

 

public abstract class Piece {

   public abstract boolean validMove(.......);

}

 

public class Rook extends Piece{

      public boolean validMove(.....){

            do stuff

      }

}

Piece newPiece = new Rook("white", 0, 0); //creates a rook at position 0,0 

if(_______.isValidMove(newPiece, destX, destY){ //should call Rook's validMove function if newPiece is a rook, or King's validMove if newPiece is a king, etc... 

    do stuff

}

[Mr_KoKa]

I think super is what you need, https://docs.oracle.com/javase/tutorial/java/IandI/super.html

[79wjd]

3 minutes ago, Mr_KoKa said:

I think super is what you need, https://docs.oracle.com/javase/tutorial/java/IandI/super.html

So like this: (and leave everything else the same)

 

public class Rook extends Piece{

      public boolean validMove(.....){
            super.validMove();
            do stuff

      }

}

[Mr_KoKa]

Yes, but I guess you cannot forget to pass arguments as the class method that you inherit from has some marked by you with ....., so I guess it would look like this:

public class Rook extends Piece{

      public boolean validMove(.....){
            super.validMove(.....);
            do stuff

      }

}

P.S. Variables count doesn't need to match.

[79wjd]

7 minutes ago, Mr_KoKa said:

Yes, but I guess you cannot forget to pass arguments as the class method that you inherit from has some marked by you with ....., so I guess it would look like this:

 

Okay, so I'm a bit unsure since it's supposed to return....

 

Is this how I would want it? Am I supposed to do anything with the isValid variable? 

public class Piece {

//should never be called
public boolean validMove(int destX, int destY, Piece[][] pieces, Piece activePiece) {

       return false; 

}
}

//in Rook extension of Piece

public boolean validMove(int destX, int destY, Piece[][] pieces, Piece activePiece){

          boolean isValid = super.validMove(destY, destY, pieces, activePiece);

          int deltaX = destX - this.x; 

          int deltaY = destY - this.y; 

          if(deltaX == 0 && deltaY == 0) //start = destination 

                   return false; 

          if(deltaX == 0 || deltaY == 0) {

                  if(checkPath(deltaX, deltaY, pieces))

                  return true;

          }

          return false; 

}

[fizzlesticks]

If the isValid method of Piece should never be called and always returns false, the line 

boolean isValid = super.validMove(destY, destY, pieces, activePiece);

isn't needed, it doesn't do anything especially since you never use the return value.

[79wjd]

1 minute ago, fizzlesticks said:

If the isValid method of Piece should never be called and always returns false, the line 

boolean isValid = super.validMove(destY, destY, pieces, activePiece);

isn't needed, it doesn't do anything especially since you never use the return value.

It doesn't always return false. I just set the return value there to false since it's never called anyway (and if it is accidentally called, I definitely don't want it to return true). 

[79wjd]

44 minutes ago, Mr_KoKa said:

I think super is what you need, https://docs.oracle.com/javase/tutorial/java/IandI/super.html

I read that page again, and if I call super.validMove(.....) from within the extended Rook class, then it will execute validMove in the Piece class -- that's not what I want? 

 

I want to call validMove from main, but I want to call the validMove that is associated with the class extension of the currently selected piece, not the generic validMove function that is in the Piece class.  

[fizzlesticks]

2 minutes ago, djdwosk97 said:

I read that page again, and if I call super.validMove(.....) from within the extended Rook class, then it will execute validMove in the Piece class -- that's not what I want? 

 

I want to call validMove from main, but I want to call the validMove that is associated with the class extension of the currently selected piece, not the generic validMove function that is in the Piece class.  

Just call somePiece.validMove(), if it's a rook object it will call the rook's method, if it's a pawn it will call the pawn's method etc.

[79wjd]

7 minutes ago, fizzlesticks said:

Just call somePiece.validMove(), if it's a rook object it will call the rook's method, if it's a pawn it will call the pawn's method etc.

Well, I'm creating an array of Pieces and assigning a type of piece to specific points in the array. So if I were to call 

//MAIN

Piece[][] pieces = new Piece[8][8]; 

pieces[0][0] = new Rook(0,0); 

if(pieces[0][0].validMove(.....))

Then it tries to call the validMove function that's in the Piece class, and if there is no validMove function in the Piece class then it throws an error. 

[Mr_KoKa]

I don't know how, but when I'm trying this, it says "Extended class...".

//Base.class
public class Base {
    public boolean isValid(){
        System.out.println("Base class...");
        
        return false;
    }
}

//Extended.class
public class Extended extends Base {    
    public boolean isValid(){
        System.out.println("Extended class...");
        
        return true;
    }    
}

//Test.class
public class Test {
    public static void main(String[] args) {
        Base[][] array = new Base[2][2];
        array[0][0] = new Extended();
        array[0][0].isValid();
    }    
}

If it would work like you're saying I should get "Base class..."

[79wjd]

7 minutes ago, Mr_KoKa said:

I don't know how, but when I'm trying this, it says "Extended class...".

//Base.class
public class Base {
    public boolean isValid(){
        System.out.println("Base class...");
        
        return false;
    }
}

//Extended.class
public class Extended extends Base {    
    public boolean isValid(){
        System.out.println("Extended class...");
        
        return true;
    }    
}

//Test.class
public class Test {
    public static void main(String[] args) throws InterruptedException {
        Base[][] array = new Base[2][2];
        array[0][0] = new Extended();
        array[0][0].isValid();
    }    
}

If it would work like you're saying I should get "Base class..."

Could it be because I'm creating my Piece array in a different function call? The function that builds the board (and places the pieces) will be called by main and then the move function (where the call to validMove resides) will be called from main. (I technically don't have a main method yet since I'm just setting up all my required classes).

 

 

 

 

*sigh* 

I missed the second ending parenthesis. I think I'm going to cry. Well, there goes like an hour of my life I'll never get back. 

[fizzlesticks]

1 minute ago, djdwosk97 said:

Could it be because I'm creating my Piece array in a different function call? The function that builds the board (and places the pieces) will be called by main and then the move function (where the call to validMove resides) will be called from main. (I technically don't have a main method yet since I'm just setting up all my required classes).

That wouldn't make a difference. The only thing that would matter is if your functions have different parameters or return types which would prevent it from overriding the parent's function. Hard to help without seeing you're real code.

[79wjd]

Just now, fizzlesticks said:

That wouldn't make a difference. The only thing that would matter is if your functions have different parameters or return types which would prevent it from overriding the parent's function. Hard to help without seeing you're real code.

yeah, no, I just missed a second ending parenthesis and Eclipse decided to complain about everything except that (and since the whole line was marked with an error I didn't see the missing parenthesis). 

[79wjd]

@Mr_KoKa @fizzlesticks

 

Say I have a pieces array with an extended class object at each index. 

Piece[][] pieces = new Piece[8][8];

pieces[0][0] = new Rook(0,0);

Is there a way for me to quickly check to which class the piece at (0,0) belongs? (other than adding an identifying parameter to the rook class)

[fizzlesticks]

You can call getClass().getSimpleName() on the object to get a string for the class name. Or you can use the "instanceof" operator. But using inheritance is meant to avoid this kind of thing, why do you need to know what class it is?

[Nineshadow]

Oh, I remember making a chess game a year or so ago. But I'm not sure how I did it...pretty sure I just used an array to store the table state and each value represented a different piece. Actually, let me find it. I'm curious.

[Yamoto42]

@djdwosk97

7 hours ago, fizzlesticks said:

You can call getClass().getSimpleName() on the object to get a string for the class name. Or you can use the "instanceof" operator. But using inheritance is meant to avoid this kind of thing, why do you need to know what class it is?

This.  And it goes not just for Java, but for OOP in general.

 

Consider the following classes:

class A

class B extends A

class C extends A

 

 

If you declare your variable of type A, and anywhere (other than calling a constructor) care whether it is actually A, B, C, or really anything else that might inherent A (or implement A if it is an interface), then there is a very high chance your approach is fundamentally flawed.

There are occasional cases in some languages, for example if something (due to library design), must except type Object which then must be cast...but as a rule of thumb, if you care what the actual (as opposed to declared) class is you're probably doing it wrong.

[sukhvir41]

On 09/08/2016 at 6:24 AM, djdwosk97 said:

So I'm making a game of chess and I've created a Piece class and then I have multiple extensions of the piece class for each individual piece. So say I have someting like this, how would I go about calling the method in the extended class? 

 

public abstract class Piece {

   public abstract boolean validMove(.......);

}

 

public class Rook extends Piece{

      public boolean validMove(.....){

            do stuff

      }

}

Piece newPiece = new Rook("white", 0, 0); //creates a rook at position 0,0 

if(_______.isValidMove(newPiece, destX, destY){ //should call Rook's validMove function if newPiece is a rook, or King's validMove if newPiece is a king, etc... 

    do stuff

}

Your super method is abstract so when you inherit the that class you have to give a body to the abstract method or call that class also abstract. When you call the isValid method be the reference be of the super class or base class it will call the base class method as your super class method is abstract. You can't not create an object of an abstract class.