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Thread: Class heirarchy

  1. #1
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    Default Class heirarchy

    Hello

    So i'm here again posting some questions about a simple 2-player chess engine that I'm making.
    I have a question regarding classes and sub-classes. It requires some explanation of the workings so please bear with me.

    My structure is as follows:

    I have a class called ChessGame in which resides the main array of the piece positions and the computations of the game like if the move of the piece is legitimate etc.
    I have a class which is called ChessPiece , and I have a class for each of the pieces : King , Queen etc... which extend ChessPiece.

    The main array which holds the positions of the pieces is of type : ChessPiece . Basically what I want is for that array to hold the INSTANCES of each piece that is on the board . And 'null' if the square is
    empty.

        public ChessPiece[][] ChessBoard = {
     
            {l_rookBlack,l_bishopBlack,l_knightBlack,queenBlack,kingBlack,r_knightBlack,r_bishopBlack,r_rookBlack},
            {pawn1Black,pawn2Black,pawn3Black,pawn4Black,pawn5Black,pawn6Black,pawn7Black,pawn8Black,},
            {null,null,null,null,null,null,null,null},
            {null,null,null,null,null,null,null,null},
            {null,null,null,null,null,null,null,null},  
            {null,null,null,null,null,null,null,null},
            {l_rookWhite,l_bishopWhite,l_knightWhite,queenWhite,kingWhite,r_knightWhite,r_bishopWhite,r_rookWhite},
            {pawn1White,pawn2White,pawn3White,pawn4White,pawn5White,pawn6White,pawn7White,pawn8White,},
     
        };

    The array shows the initial position of the pieces as generated at the beginning of the game.

    My question is : How can I access the methods and variables of the instances of the pieces?
    At the beginning of the program I create a new instance of the ChessGame class

    ChessGame gameState;  // Global declaration in the Interface class
    gameState = new ChessGame();  // initialisation in the constructor of the Interface class

    Then in the ChessGame class I declare and initialise all of the instances of each piece.

    Why can't I do something like :

    gameState.ChessBoard[2][4].getColor();
    ?

    I've tried creating an instance of ChessPiece in the ChessGame class and declaring the pieces themselves in the ChessPiece class and then trying something like

    gameState.chessP.kingWhite.getColor();

    but that gives me a stack overflow error because the contructor in ChessPiece is called everytime an instance of the Piece is created and it basically loops.

    Can someone advise me how I can structure this?
    What I basically want to do is have an array of Instances of all the pieces in the ChessGame class.
    The reason I'm having this issue is because every diffrent piece type (king , queen etc) is in it's own class
    that's why I made the Chesspiece Class so I could put all the pieces under one 'type' .
    So how does one go about working with this kind of thing?


    Thanks in advance
    Tim


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    Default Re: Class heirarchy

    Could you explain where the recursion is? If a piece, say a King, extends ChessPiece and an instance of a King is created, where is the loop?
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Class heirarchy

    There is no recursion. I don't know why it loops , but I know that everytime an instance of a King for example is called , then the constructor of the ChessPiece class is called.
    I know this from adding a println statement to the constructor of ChessPiece and seeing it be called 32 times.
    If I Just create an instance of ChessGame and from there create the instances of the pieces themselves then it's fine but i can't access the methods/variables through ChessGame -> ChessPiece -> king.getColor() for example.
    If I create an stance of ChessGame which creates an instance of ChessPiece which create the instances of the pieces themselves then I get a stack overflow error.
    The reason I want to create the instances of the pieces through the ChessPiece umbrella class is so that I can put instances of those pieces in a "ChessPiece" type array and use them as my main board.
    If there is a better way of doing it then I'm all ears.

    If you do not understand then please do ask , I will try to make clearer my purpose as I'm pretty stumped here and would like some consultation.

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    Default Re: Class heirarchy

    everytime an instance of a King for example is called , then the constructor of the ChessPiece class is called.
    If King extends ChessPiece then that is expected.

    Can you copy a section of the stack trace, enough to show the looping and paste it here so we can see how the calls are being made.

    create the instances of the pieces through the ChessPiece
    That does not make sense. A super class should not be creating instances of its sub class. That will cause an infinite loop.

    A Game class creating an array of ChessPieces and filling it with pieces makes sense.
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Class heirarchy

    A Game class creating an array of ChessPieces and filling it with pieces makes sense.
    Well yes , but how do I access functions and variables inside the classes?
    Lets say we have the following instances:
    ChessGame newGame;
    ChessPiece newSet;
    King whiteKing;

    and my array is of type ChessPiece.
    I want to access variables/methods from whiteKing.
    For some reason it doesn't work if I do ChessPiece[0][4].getColor();
    Am I doing it wrong?
    A colleague of mine suggest I use an abstract class type but I don't know how to implement that

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    Default Re: Class heirarchy

    it doesn't work
    If you get errors, copy the full text and paste it here.
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Class heirarchy

    Your colleague is suggesting that ChessPiece should be abstract. Abstract classes cannot be initialized, so you cannot create any "ChessPiece" objects. This would be correct, since you don't want "ChessPiece" objects, you want "King" objects, "Queen" objects, ect.

    Well yes , but how do I access functions and variables inside the classes?
    Lets say we have the following instances:
    ChessGame newGame;
    ChessPiece newSet;
    King whiteKing;
    and my array is of type ChessPiece.
    I want to access variables/methods from whiteKing.
    For some reason it doesn't work if I do ChessPiece[0][4].getColor();
    Have you initialized the whiteKing object? If you are getting a null pointer exception, it would probably be because you haven't initialized the object yet.
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    Default Re: Class heirarchy

    Ok here is part of my code:

    The Rook Class:

    public class Rook extends ChessPiece {
     
     
        private boolean isAlive;  // whether the piece is on the board
        private int piecePositionX;
        private int piecePositionY;
     
        private String pieceColor;
        private String pieceRank;
        private String pieceIcon;
        public Rook(String pieceColor, String pieceRank, int piecePositionX , int piecePositionY ) 
        {
     
           //super();
           isAlive = true; // Whether the piece is still on the board
           this.pieceColor = pieceColor; // Color of the piece
           this.pieceRank = pieceRank;  // Type of piece
           this.piecePositionX = piecePositionX;
           this.piecePositionY = piecePositionY;
     
        }
     
        // getters
     
     
        public String getColor()
        {
            return pieceColor;
        }
     
     
        public String getRank()
        {
            return pieceRank;
        }
     
         public int getPositionX()
        {
              return piecePositionX;       
        }
     
        public int getPositionY()
        {
              return piecePositionY;       
        }
     
        // setters
     
        public void setPositionXY(int piecePositionX , int piecePositionY )
        {
            this.piecePositionX =  piecePositionX;
            this.piecePositionY =  piecePositionY;
     
        }
     
     
        public void setRank(String pieceRank)
     
        {
            this.pieceRank = pieceRank;
        }
     
       public void setDead(boolean isAlive)
       {
           this.isAlive = false;
       }
     
    }


    The ChessPiece Class that doesn't really have anything Inside it but just serves as an umbrella class so I can put all the pieces into one array of type "ChessPiece" :

    package Chess_contents;
     
    /**
     *
     * @author Shagas
     */
     
    public class ChessPiece {
     
       // nothing here
     
        public ChessPiece()
     
         {
     
                // nothing here    
         }
     
    }

    and here is the ChessGame class:

    import javax.swing.JFrame;
     
    /**
     *
     * @author Shagas
     */
     
     
    public class ChessGame  {
     
        // Declaring objects of the pieces
     
        // White
     
     
     
        King kingWhite;
        Queen queenWhite;
     
        Knight l_knightWhite;
        Bishop l_bishopWhite;
        Rook l_rookWhite;
     
        Knight r_knightWhite;
        Bishop r_bishopWhite;
        Rook r_rookWhite;
     
        Pawn pawn1White;
        Pawn pawn2White;
        Pawn pawn3White;
        Pawn pawn4White;
        Pawn pawn5White;
        Pawn pawn6White;
        Pawn pawn7White;
        Pawn pawn8White;
     
        // Black
     
     
     
        King kingBlack;
        Queen queenBlack;
     
        Knight l_knightBlack;
        Bishop l_bishopBlack;
        Rook l_rookBlack;
     
        Knight r_knightBlack;
        Bishop r_bishopBlack;
        Rook r_rookBlack;
     
        Pawn pawn1Black;
        Pawn pawn2Black;
        Pawn pawn3Black;
        Pawn pawn4Black;
        Pawn pawn5Black;
        Pawn pawn6Black;
        Pawn pawn7Black;
        Pawn pawn8Black;
     
     
     
     
     
         private int chosenSquareXCoordinate; 
         private int chosenSquareYCoordinate;
     
     
         private String pieceBuffer; // chosen piece
     
     
     
        public ChessPiece[][] currentBoard = {
     
            {l_rookBlack,l_bishopBlack,l_knightBlack,queenBlack,kingBlack,r_knightBlack,r_bishopBlack,r_rookBlack},
            {pawn1Black,pawn2Black,pawn3Black,pawn4Black,pawn5Black,pawn6Black,pawn7Black,pawn8Black,},
            {null,null,null,null,null,null,null,null},
            {null,null,null,null,null,null,null,null},
            {null,null,null,null,null,null,null,null},  
            {null,null,null,null,null,null,null,null},
            {l_rookWhite,l_bishopWhite,l_knightWhite,queenWhite,kingWhite,r_knightWhite,r_bishopWhite,r_rookWhite},
            {pawn1White,pawn2White,pawn3White,pawn4White,pawn5White,pawn6White,pawn7White,pawn8White,},
     
        };
     
     
     
     
     
     
     
       // public GraphicalInterface ui;
     
        public ChessGame()
        {
     
     
            // White 
     
            l_rookWhite = new Rook("WHITE", "ROOK" , 7 , 0);
            l_bishopWhite = new Bishop("WHITE", "BISHOP" , 7 , 1);
            l_knightWhite = new Knight("WHITE", "KNIGHT" , 7 , 2);
            queenWhite = new Queen( "WHITE" , "QUEEN" , 7, 3 );
            kingWhite = new King( "WHITE" , "KING" , 7 , 4  );
            r_knightWhite = new Knight("WHITE", "KNIGHT" , 7 , 5);
            r_bishopWhite = new Bishop("WHITE", "BISHOP" , 7 , 6);
            r_rookWhite = new Rook ("WHITE", "ROOK" , 7 , 7);
     
            pawn1White = new Pawn("WHITE", "PAWN" , 6 , 0);
            pawn2White = new Pawn("WHITE", "PAWN" , 6 , 1);
            pawn3White = new Pawn("WHITE", "PAWN" , 6 , 2);
            pawn4White = new Pawn("WHITE", "PAWN" , 6 , 3);
            pawn5White = new Pawn("WHITE", "PAWN" , 6 , 4);
            pawn6White = new Pawn("WHITE", "PAWN" , 6 , 5);
            pawn7White = new Pawn("WHITE", "PAWN" , 6 , 6);
            pawn8White = new Pawn("WHITE", "PAWN" , 6 , 7);
     
     
            // Black
     
     
            l_rookBlack = new Rook("BLACK", "ROOK" , 0 , 0);
            l_bishopBlack = new Bishop("BLACK", "BISHOP" , 0 , 1);
            l_knightBlack = new Knight("BLACK", "KNIGHT" , 0 , 2);
            queenBlack = new Queen( "BLACK" , "QUEEN" , 0, 3);
            kingBlack = new King( "BLACK" , "KING" , 0 , 4);
            r_knightBlack = new Knight("BLACK", "KNIGHT" , 0 , 5);
            r_bishopBlack = new Bishop("BLACK", "BISHOP" , 0 , 6);
            r_rookBlack = new Rook ("BLACK", "ROOK" , 0 , 7);
     
            pawn1Black = new Pawn("BLACK", "PAWN" , 1 , 0);
            pawn2Black = new Pawn("BLACK", "PAWN" , 1 , 1);
            pawn3Black = new Pawn("BLACK", "PAWN" , 1 , 2);
            pawn4Black = new Pawn("BLACK", "PAWN" , 1 , 3);
            pawn5Black = new Pawn("BLACK", "PAWN" , 1 , 4);
            pawn6Black = new Pawn("BLACK", "PAWN" , 1 , 5);
            pawn7Black = new Pawn("BLACK", "PAWN" , 1 , 6);
            pawn8Black = new Pawn("BLACK", "PAWN" , 1 , 7);
     
     
     
            this.currentBoard = currentBoard;
     
     
     
        }

    So my question is how can I access the methods and variables of the instances of the pieces in the currentBoard array?

    I tried something like currentBoard[0][0].getColor(); but it says symbol not found , location chessPiece.
    Basically it doesn't see the Rook class which extends the chesspiece class.

    If I make my ChessPiece class abstract and declare a method that is in all the classes that extend it like this :

    public abstract class ChessPiece {
     
       // nothing here
     
        public abstract int getPositionX();
    }

    then I have access to the method of the Rook class for example :

     public void getInfo(){
            System.out.println(currentBoard[0][0].getPositionX());
        }

    but when I run it it gives me a Nullpointer exception. I don't know why , I initialised the pieces as you can see in my code above.

    Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at Chess_contents.ChessGame.getInfo(ChessGame.java:17 0)
    at Chess_contents.ChessGame.Square_pressed(ChessGame. java:184)
    at Chess_contents.Interface.mousePressed(Interface.ja va:214)
    at java.awt.Component.processMouseEvent(Component.jav a:6502)
    at javax.swing.JComponent.processMouseEvent(JComponen t.java:3320)
    at java.awt.Component.processEvent(Component.java:627 0)
    at java.awt.Container.processEvent(Container.java:222 9)
    at java.awt.Component.dispatchEventImpl(Component.jav a:4861)
    at java.awt.Container.dispatchEventImpl(Container.jav a:2287)
    at java.awt.Component.dispatchEvent(Component.java:46 87)
    at java.awt.LightweightDispatcher.retargetMouseEvent( Container.java:4832)
    at java.awt.LightweightDispatcher.processMouseEvent(C ontainer.java:4489)
    at java.awt.LightweightDispatcher.dispatchEvent(Conta iner.java:4422)
    at java.awt.Container.dispatchEventImpl(Container.jav a:2273)
    at java.awt.Window.dispatchEventImpl(Window.java:2719 )
    at java.awt.Component.dispatchEvent(Component.java:46 87)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.j ava:735)
    at java.awt.EventQueue.access$200(EventQueue.java:103 )
    at java.awt.EventQueue$3.run(EventQueue.java:694)
    at java.awt.EventQueue$3.run(EventQueue.java:692)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPri vilege(ProtectionDomain.java:76)
    at java.security.ProtectionDomain$1.doIntersectionPri vilege(ProtectionDomain.java:87)
    at java.awt.EventQueue$4.run(EventQueue.java:708)
    at java.awt.EventQueue$4.run(EventQueue.java:706)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPri vilege(ProtectionDomain.java:76)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java: 705)
    at java.awt.EventDispatchThread.pumpOneEventForFilter s(EventDispatchThread.java:242)
    at java.awt.EventDispatchThread.pumpEventsForFilter(E ventDispatchThread.java:161)
    at java.awt.EventDispatchThread.pumpEventsForHierarch y(EventDispatchThread.java:150)
    at java.awt.EventDispatchThread.pumpEvents(EventDispa tchThread.java:146)
    at java.awt.EventDispatchThread.pumpEvents(EventDispa tchThread.java:138)
    at java.awt.EventDispatchThread.run(EventDispatchThre ad.java:91)

  9. #9
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    Default Re: Class heirarchy

    it says symbol not found
    You need to copy the full text of the error message and paste it here. What you posted left out what symbol was not found.
    Please copy full text of the compiler's error message and paste it here.
    The message should show the source with a ^ under the location of the error.
    Here is a sample from the javac compiler:
    TestSorts.java:138: cannot find symbol
    symbol  : variable var
    location: class TestSorts
             var = 2;
             ^

    The ChessPiece class has no methods. The code you posted uses the currentBoard which is type ChessPiece. currentBoard has no methods that can be used.

    If ChessPiece were abstract and had all the common methods as abstract then that code should work.
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Class heirarchy

    Ok, I understand your problem.
    Your ChessPiece array contains implemented subclasses of ChessPiece, but the "type" of the array is ultimately: ChessPiece. This means that the only methods and variables you can access for the subclasses you pull from the ChessPiece array (without casting) are the ones declared in the ChessPiece class. The availability of the methods and variables are determined at compile-time. However, the implementation of the method will be determined at runtime, based on which subclass of ChessPiece the object really is (the compiler has no idea what type of ChessPiece it is, it just knows that it's a ChessPiece or a subclass of ChessPiece).
    Every ChessPiece should have a color, correct? So that method should exist in the ChessPiece class. But what if we want to implement the getColor() method differently for each subclass of ChessPiece? The subclasses can override the method and implement it their way. It will all get sorted out at runtime.
    Do you understand that much?
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    Default Re: Class heirarchy

    The ChessPiece class has no methods. The code you posted uses the currentBoard which is type ChessPiece. currentBoard has no methods that can be used.

    If ChessPiece were abstract and had all the common methods as abstract then that code should work.
    Yes I understand this , please read the last edit to my last post where I try to make the ChessPiece class abstract and add the methods which all the subclasses contain into it. It gives me a nullpointer exception at runtime . I pasted the error code.

    Ok, I understand your problem.
    Your ChessPiece array contains implemented subclasses of ChessPiece, but the "type" of the array is ultimately: ChessPiece. This means that the only methods and variables you can access for the subclasses you pull from the ChessPiece array (without casting) are the ones declared in the ChessPiece class. The availability of the methods and variables are determined at compile-time. However, the implementation of the method will be determined at runtime, based on which subclass of ChessPiece the object really is (the compiler has no idea what type of ChessPiece it is, it just knows that it's a ChessPiece or a subclass of ChessPiece).
    Every ChessPiece should have a color, correct? So that method should exist in the ChessPiece class. But what if we want to implement the getColor() method differently for each subclass of ChessPiece? The subclasses can override the method and implement it their way. It will all get sorted out at runtime.
    Do you understand that much?
    I do understand almost all of what you are saying .
    So basically what you are saying is that I should copy all the methods that are in the subclasses into the ChessPiece class and when I call them the methods in the subclasses will override them
    because even though the array is of type ChessPiece , the objects inside it are of another class type ?

    --- Update ---

    aussiemcgr I tried to implement the method getColor() for example in the ChessPiece class but I still get that nullpointerexception that I posted above.

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    Default Re: Class heirarchy

    I do understand almost all of what you are saying .
    So basically what you are saying is that I should copy all the methods that are in the subclasses into the ChessPiece class and when I call them the methods in the subclasses will override them
    because even though the array is of type ChessPiece , the objects inside it are of another class type ?
    Pretty much. If you wanted to make ChessPiece abstract, you could do this without actually implementing the methods in the ChessPiece class.
    For example of how abstract and overriding methods work:
    public abstract class A {
     
    	public abstract int getNumber();
    }
     
    public class B extends A {
    	@Override
    	public int getNumber() {
    		return 1;
    	}
    }
     
    public class C extends A {
    	@Override
    	public int getNumber() {
    		return 2;
    	}
    }
     
    public class Runner {
    	public static void main(String[] args) {
    		A[] objects = new A[3];
    		objects[0] = new B();
    		objects[1] = new C();
    		objects[2] = new B();
     
    		System.out.println(objects[0].getNumber()+"");
    		System.out.println(objects[1].getNumber()+"");
    		System.out.println(objects[2].getNumber()+"");
    	}
    }
    This code would print out:
    1
    2
    1
    The objects array holds objects of type: A. But this includes all subclasses of A too. So since the getNumber() method is declared in the A class, we can call objects[i].getNumber() without getting a compile error. However, the value that method returns is determined by which subclass of A is in that index of the array, which is not determined until runtime.

    --- Update ---

    aussiemcgr I tried to implement the method getColor() for example in the ChessPiece class but I still get that nullpointerexception that I posted above.
    Before calling the getColor() method, check if the item in the index is null. Keep in mind that you do have null indexes in your array. You cannot call the getColor() method (or any method) on a null reference.
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    Default Re: Class heirarchy

    aussiemcgr:

    Thanks for the help.
    I once again understood most of it . If I was going to do it this way would I have to declare my pieces Like this:

    ChessPiece pawn1Black;
    pawn1Black = new Pawn("BLACK", "PAWN" , 1 , 0);

    instead of like this ?

    Pawn pawn1Black;
    pawn1Black = new Pawn("BLACK", "PAWN" , 1 , 0);

    Aha yes , I checked and it is indeed null on the currentBoard[0][0] (an instance of Rook is supposed to be there) but I can't understand why It's null .
    I initialised it before that . Could you check my code above (class ChessGame) and see what I'm doing wrong?
    I'd be very gratefull.

    --- Update ---

    Aha ..... I found the problem.

    I fixed it by separating the declaration :

    private ChessPiece[][] currentBoard;
    in the ChessGame class


    and moving the

    currentBoard = new ChessPiece[][] {
     
            {l_rookBlack,l_bishopBlack,l_knightBlack,queenBlack,kingBlack,r_knightBlack,r_bishopBlack,r_rookBlack},
            {pawn1Black,pawn2Black,pawn3Black,pawn4Black,pawn5Black,pawn6Black,pawn7Black,pawn8Black,},
            {null,null,null,null,null,null,null,null},
            {null,null,null,null,null,null,null,null},
            {null,null,null,null,null,null,null,null},  
            {null,null,null,null,null,null,null,null},
            {l_rookWhite,l_bishopWhite,l_knightWhite,queenWhite,kingWhite,r_knightWhite,r_bishopWhite,r_rookWhite},
            {pawn1White,pawn2White,pawn3White,pawn4White,pawn5White,pawn6White,pawn7White,pawn8White,},
     
        };

    to the constructor of the ChessGame class.
    So it seems to be finally working now Thanks alot for everyones input .

  14. #14
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    Default Re: Class heirarchy

    To answer your question from earlier:
    ChessPiece pawn1Black;
    pawn1Black = new Pawn("BLACK", "PAWN" , 1 , 0);
    OR:
    Pawn pawn1Black;
    pawn1Black = new Pawn("BLACK", "PAWN" , 1 , 0);
    If you are putting it directly into your ChessPiece array, it makes no difference.

    The second one is useful if you plan on directly using the pawn1Black object, and you are wanting to call methods which are only found in the Pawn class (and not found in the ChessPiece class). But if you are adding it to the ChessPiece array and then pulling the object out of the array before using it, the object which comes out of the array will always be of type: ChessPiece (as far as the compiler is concerned, runtime knows it's "real" type), so you only have access to methods declared in the ChessPiece's anyway.
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    Default Re: Class heirarchy

    Thanks , it's abit more clear now

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