如何在JPanel上画画？
我（新手程序员）我试图在JPanel上画一个椭圆形。我正在尝试使用方法绘画。但是，它需要Graphics参数。当我将我的Graphics作为参数包含时，我得到一个NullPointerException，因为它是null，但我不知道如何绘制椭圆。我尝试重新涂抹，但没有发生任何事。任何帮助，将不胜感激。这是我的主要课程：
public class Checkers extends JPanel{
public static final int BOARDSQUARES = 8;
public static final int BOARDSIZE = 75;
private JPanel[][] board;
private final Point point1;
private final LineBorder border1;
public JFrame frame;
checkerPiece piece = new checkerPiece(this);
Graphics g;
public Checkers(){
board = new JPanel[BOARDSQUARES][BOARDSQUARES];
point1 = new Point (BOARDSIZE,BOARDSIZE);
border1=new LineBorder(Color.BLACK, 1);
}
/**
* @param args the command line arguments
*/
public static void main(String[] args) {
Checkers checkers = new Checkers();
checkers.frame=checkers.createJFrame();
checkers.board =checkers.createBoard(checkers.board, checkers.point1, checkers.border1);
for (int y=0;y<BOARDSQUARES;y++){
for (int x = 0;x<BOARDSQUARES;x++){
checkers.frame.getContentPane().add(checkers.board[y][x]);
}
}
checkers.paint(checkers.g);
}
private JPanel[][] createBoard (JPanel[][] board, Point point, LineBorder border1){
for (int row = 0; row<BOARDSQUARES;row++){
point.y=BOARDSIZE;
for (int col = 0; col <BOARDSQUARES;col++){
board[row][col] = new JPanel();
board[row][col].setLocation(point);
board[row][col].setVisible(true);
board[row][col].setSize(BOARDSIZE,BOARDSIZE);
board[row][col].setOpaque(true);
if ((row%2==0&&col%2==0)||(row%2==1&&col%2==1)){
board[row][col].setBackground(new Color (230,200,150));
} else if ((row%2==0&&col%2==1)||(row%2==1&&col%2==0)) {
board[row][col].setBackground(new Color (165, 42,42) );
}
board[row][col].setBorder(border1);
point.y = point.y+BOARDSIZE;
}
point.x=point.x+BOARDSIZE;
}
return board;
}
private JFrame createJFrame (){
JFrame mainFrame = new JFrame("Checkers");
mainFrame.setSize(1000,1000);
mainFrame.setVisible(true);
mainFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
mainFrame.add(new Checkers());
return mainFrame;
}
@Override
public void paint (Graphics g){
System.out.println("hi");
super.paint(g);
Graphics2D g2d = (Graphics2D) g;
piece.paint(g2d,board[0][0].getLocation().x,board[0][0].getLocation().y,Color.BLACK);
}
}
我的其他课程（cherkerPiece篇）的必要片段：
public void paint (Graphics2D g, int x, int y, Color color){
g.setColor(color);
g.fillOval(x, y, DIAMETER, DIAMETER);
}
感谢您的帮助
2 个答案:
答案 0 :(得分：0)
要创建像棋子游戏这样复杂的东西，您应该在创建Java Swing GUI时遵循model / view / controller pattern。
我为棋盘方块创建了一个模型类，为棋子创建了另一个模型类。我为棋盘创建了一个模型类，并为另一个模型类创建了所有的棋子。
我创建了一个用于绘制棋盘的视图类。我创建了另一个视图类来创建主窗口。
我没有创建任何控制器类。此代码仅向您展示如何绘制棋盘格。我将所有类放在一起，以便更容易粘贴代码。您应该将这些类分成单独的Java文件。
package com.ggl.testing;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Point;
import java.awt.Rectangle;
import java.util.ArrayList;
import java.util.List;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
public class CheckerBoard implements Runnable {
private Board board;
private CheckerBoardPanel checkerBoardPanel;
private JFrame frame;
private Pieces pieces;
public static void main(String[] args) {
SwingUtilities.invokeLater(new CheckerBoard());
}
public CheckerBoard() {
this.board = new Board();
this.pieces = new Pieces();
}
@Override
public void run() {
frame = new JFrame("Checker Board");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
checkerBoardPanel = new CheckerBoardPanel(board, pieces);
frame.add(checkerBoardPanel);
frame.pack();
frame.setLocationByPlatform(true);
frame.setVisible(true);
}
public class CheckerBoardPanel extends JPanel {
private static final long serialVersionUID = 3770663347384271771L;
private Board board;
private Pieces pieces;
public CheckerBoardPanel(Board board, Pieces pieces) {
this.board = board;
this.pieces = pieces;
this.setPreferredSize(board.getPreferredSize());
}
@Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
Square[][] checkerBoard = board.getBoard();
for (int row = 0; row < checkerBoard.length; row++) {
for (int column = 0; column < checkerBoard[row].length; column++) {
Square square = checkerBoard[row][column];
Rectangle drawingRectangle = square.getDrawingRectangle();
g.setColor(square.getColor());
g.fillRect(drawingRectangle.x, drawingRectangle.y,
drawingRectangle.width, drawingRectangle.height);
}
}
List<Piece> checkerPieces = pieces.getPieces();
for (Piece checkerPiece : checkerPieces) {
Point coordinate = checkerPiece.getCoordinate();
Rectangle drawingRectangle = checkerBoard[coordinate.x][coordinate.y]
.getDrawingRectangle();
int x = drawingRectangle.x + drawingRectangle.width / 2;
int y = drawingRectangle.y + drawingRectangle.height / 2;
int radius = board.getSquareWidth() * 2 / 6;
g.setColor(checkerPiece.getColor());
g.fillOval(x - radius, y - radius, radius + radius, radius
+ radius);
}
}
}
public class Board {
private static final int BOARD_WIDTH = 8;
private static final int SQUARE_WIDTH = 64;
private Square[][] board;
public Board() {
this.board = initalizeBoard(BOARD_WIDTH, SQUARE_WIDTH);
}
private Square[][] initalizeBoard(int boardWidth, int squareWidth) {
Square[][] board = new Square[boardWidth][boardWidth];
int x = 0;
int y = 0;
for (int row = 0; row < boardWidth; row++) {
for (int column = 0; column < boardWidth; column++) {
Square square = new Square();
if (isLightSquare(row, column)) {
square.setColor(Color.WHITE);
} else {
square.setColor(Color.LIGHT_GRAY);
}
square.setCoordinate(new Point(row, column));
square.setDrawingRectangle(new Rectangle(x, y, squareWidth,
squareWidth));
board[row][column] = square;
x += squareWidth;
}
x = 0;
y += squareWidth;
}
return board;
}
public boolean isLightSquare(int row, int column) {
if (row % 2 == 0) {
if (column % 2 == 0) {
return true;
} else {
return false;
}
} else {
if (column % 2 == 0) {
return false;
} else {
return true;
}
}
}
public Dimension getPreferredSize() {
int width = SQUARE_WIDTH * 8 + 1;
return new Dimension(width, width);
}
public Square[][] getBoard() {
return board;
}
public int getSquareWidth() {
return SQUARE_WIDTH;
}
}
public class Square {
private Color color;
private Point coordinate;
private Rectangle drawingRectangle;
public Point getCoordinate() {
return coordinate;
}
public void setCoordinate(Point coordinate) {
this.coordinate = coordinate;
}
public Rectangle getDrawingRectangle() {
return drawingRectangle;
}
public void setDrawingRectangle(Rectangle drawingRectangle) {
this.drawingRectangle = drawingRectangle;
}
public Color getColor() {
return color;
}
public void setColor(Color color) {
this.color = color;
}
}
public class Pieces {
private List<Piece> pieces;
public Pieces() {
this.pieces = addPieces();
}
private List<Piece> addPieces() {
List<Piece> pieces = new ArrayList<Piece>();
Piece piece = new Piece();
piece.setColor(Color.RED);
piece.setCoordinate(new Point(2, 1));
pieces.add(piece);
piece = new Piece();
piece.setColor(Color.BLACK);
piece.setCoordinate(new Point(5, 0));
pieces.add(piece);
// Add the rest of the red and black pieces here
return pieces;
}
public List<Piece> getPieces() {
return pieces;
}
public void addPiece(Piece piece) {
this.pieces.add(piece);
}
}
public class Piece {
private Color color;
private Point coordinate;
public Color getColor() {
return color;
}
public void setColor(Color color) {
this.color = color;
}
public Point getCoordinate() {
return coordinate;
}
public void setCoordinate(Point coordinate) {
this.coordinate = coordinate;
}
}
}
答案 1 :(得分：-1)
你需要添加一个扩展Canvas的类，并在该类中完成所有绘画。像这样
public class FirstWindow extends JFrame
{
/**
*
*/
private static final long serialVersionUID = 1L;
public FirstWindow()
{
super("Kayte does not go to water parks");
setSize(500, 500);
setDefaultCloseOperation(EXIT_ON_CLOSE);
setResizable(false);
}
}
public class Start extends Canvas implements Runnable
public class Main
{
public static void main(String arg[])
{
FirstWindow fw = new FirstWindow();
Start game = new Start(500, 500);
fw.add(game);
fw.setVisible(true);
new Thread(game).start();
}
}
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