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Thread: Assigning Colors in a 2D array in a random order

  1. #1
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    Default Assigning Colors in a 2D array in a random order

    Alright so basically what I'm trying to do is create a 16x16 square board of tiles (256 total) and when I run the program I need it to set up the tiles randomly into the following colors
    39 yellow tiles
    46 red tiles
    34 blue tiles
    52 green tiles
    41 magenta tiles
    44 cyan tiles

    When I run the code, I'm getting a null pointer at
    tiles[i].setBackground(Color.YELLOW);

    Heres the GUI class
    import java.awt.BorderLayout;
    import java.awt.Color;
    import java.awt.Container;
    import java.awt.Dimension;
    import java.awt.FlowLayout;
    import java.awt.GridLayout;
    import java.awt.event.ActionEvent;
    import java.awt.event.ActionListener;
    import java.util.Random;
    import java.util.Scanner;
     
    import javax.swing.JButton;
    import javax.swing.JFileChooser;
    import javax.swing.JFrame;
    import javax.swing.JMenu;
    import javax.swing.JMenuBar;
    import javax.swing.JMenuItem;
    import javax.swing.JPanel;
     
    public class GameBoard extends JFrame{
    	Tile[][] myTiles;
    	private int numRows = 16, numCols = 16;
    	//private MyEventHandler eh = new MyEventHandler();
     
    	public static void main(String[] args) {
    		new GameBoard();
    	}
     
    	public GameBoard() {
    		myTiles = new Tile[numRows][numCols];
    		setSize(800,800);
     
    		Container c = getContentPane();
    		c.setLayout(new BorderLayout());
     
    		setDefaultCloseOperation(EXIT_ON_CLOSE);
    		setTitle("Random Colors");
     
    		JPanel tilePanel = buildTilePanel();
    		c.add(tilePanel, BorderLayout.CENTER);
     
    		setVisible(true);
    	}
     
    	private JPanel buildTilePanel() {
    		JPanel tilePanel = new JPanel();
    		tilePanel.setBackground(Color.BLACK);
    		tilePanel.setPreferredSize(new Dimension(800, 800));
    		tilePanel.setLayout(new GridLayout(numRows, numCols));
     
    		for (int i = 0; i < myTiles.length; i++)
    			for (int j = 0; j < myTiles[0].length; j++) {
    				myTiles[i][j] = new Tile();
    				myTiles[i][j].setColors();
    				myTiles[i][j].shuffle();
    				//myTiles[i][j].addActionListener(eh);
    				tilePanel.add(myTiles[i][j]);
    			}
    		return tilePanel;
    	}
    }

    and heres the tiles class
    import java.awt.Color;
    import java.awt.Dimension;
    import javax.swing.JButton;
     
    public class Tile extends JButton{
     
    	private Tile[] tiles = new Tile[256];
     
     
    	public Tile() {
    		setPreferredSize(new Dimension(10, 10));
    	}
     
    	public void setColors() {
    		for(int i = 0; i < 39; i++){
    			tiles[i].setBackground(Color.YELLOW);
    			paintImmediately(0, 0, getWidth(), getHeight());
     
    		}
     
    		for(int i = 39; i < 85; i++){
    			tiles[i].setBackground(Color.RED);
    			paintImmediately(0, 0, getWidth(), getHeight());
    		}
     
    		for(int i = 85; i < 119; i++){
    			tiles[i].setBackground(Color.BLUE);
    			paintImmediately(0, 0, getWidth(), getHeight());
    		}
     
    		for(int i = 119; i < 171; i++){
    			tiles[i.]setBackground(Color.GREEN);
    			paintImmediately(0, 0, getWidth(), getHeight());
    		}
     
    		for(int i = 171; i < 212; i++){
    			tiles[i].setBackground(Color.MAGENTA);
    			paintImmediately(0, 0, getWidth(), getHeight());
    		}
     
    		for(int i = 212; i < 256; i++){
    			tiles[i].setBackground(Color.CYAN);
    			paintImmediately(0, 0, getWidth(), getHeight());
    		}
    	}
     
    	public void shuffle() {
    		int r = tiles.length;
    		for(int i = 0; i < tiles.length; i++){
    			Tile t = tiles[i];
    			tiles[i] = tiles[r-1];
    			tiles[r-1] = t;
    		}
    	}
    }
    Last edited by captain; February 20th, 2012 at 09:18 PM.


  2. #2
    Super Moderator Norm's Avatar
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    Default Re: Assigning Colors in a 2D array in a random order

    I'm getting a null pointer.
    Please copy and paste here the full text of the error message.
    Look at the line number referenced in your program and find the variable that has a null value. Then backtrack in the code to see why that variable does not have a non-null value.

  3. #3
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    Default Re: Assigning Colors in a 2D array in a random order

    Exception in thread "main" java.lang.NullPointerException
    at Tile.setColor(Tile.java:16)
    at GameBoard.buildTilePanel(GameBoard.java:55)
    at GameBoard.<init>(GameBoard.java:39)
    at GameBoard.main(GameBoard.java:26)

    which would be
    tiles[i].setBackground(Color.YELLOW);

  4. #4
    Super Moderator Norm's Avatar
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    Default Re: Assigning Colors in a 2D array in a random order

    Is the value of tiles[i] null?
    If so, where do you assign it a value?
    You have defined an array, but you have not put any objects into the array. All its slots are empty.
    You must assign a value to all the slots in the array before you can call a method using the value of a slot.

  5. #5
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    Default Re: Assigning Colors in a 2D array in a random order

    Well my thought process for that piece of code was that it would set the i-th tile in the array to yellow. I'm not sure where I would assign a value to it initially though.

  6. #6
    Super Moderator Norm's Avatar
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    Default Re: Assigning Colors in a 2D array in a random order

    The problem is there is no tile in the array at the i-th slot. You need to assign tiles to the array before you can set their colors.
    where I would assign a value to it
    Any time before you want to access it.

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