在 Java 中创建矩形时，我是否真的调用了我创建的 paintComponent 方法?

2022-01-24 00:00:00 java swing jpanel jframe jcomponent

这是我当前的 RectangleComponent 类，我将它添加到我的主 JFrame 的面板中，但它从未出现.我以为不是画图所以决定在Rectangle的构造函数中调用paintComponent方法，经过4-5个nullPointerExceptions排序后，什么都没有改变.我已经阅读了多篇关于如何绘制矩形的指南，也看过多个代码示例，但我永远无法让面板与多个 JComponent 一起使用.如果可以，请简要查看我的代码，看看是否可以设计解决方案.感谢您的时间.还列出了我在其中调用矩形构造函数的 Frame.

This is my current RectangleComponent class and I add it to a panel in my main JFrame but it never appears. I thought it wasn't drawing so I decided to call the paintComponent method in the Rectangle's constructor, and after sorting through 4-5 nullPointerExceptions, nothing has changed. I've read multiple guides on how to draw rectangles and I have seen multiple code examples, but I can never get the panels to work with more than one JComponent. If you could, please take a brief look at my code and see if you can devise a solution. Thank you for your time. Also listed is the Frame I call the rectangle constructor in.

public class GameFrame extends JFrame
{
    private SpellBarComponent bar;
    private JPanel mainPanel = new JPanel();
    private JPanel buttonPanel = new JPanel();
    private JPanel healthPanel = new JPanel();
    Color green = new Color(29, 180, 29);
    Color red = new Color(255, 0, 0);
    private RectangleComponent life;
    private RectangleComponent death;
    private JFrame frame = new JFrame();

    public GameFrame(char x)
    {
        frame.setSize(1024, 768);
        frame.setTitle("Game");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
        FlowLayout layout = new FlowLayout();
        createPanels(x);
        healthPanel.setLayout(layout);
        buttonPanel.setLayout(layout);
        mainPanel.setLayout(layout);
        frame.getContentPane().add(mainPanel);
        frame.pack();
        repaint();
    }

    public RectangleComponent getLife()
    {
        return life;
    }

    private void createHealth()
    {
        life = new RectangleComponent(green, healthPanel);
        death = new RectangleComponent(red, healthPanel);
    }

    private void createPanels(char x)
    {
        add(healthPanel);
        pack();
        createBar(x);
        createHealth();
        mainPanel.add(buttonPanel);
        mainPanel.add(healthPanel);
        healthPanel.add(death);
        healthPanel.add(life);
        buttonPanel.add(bar.getSpell1());
        buttonPanel.add(bar.getSpell2());
        buttonPanel.add(bar.getSpell3());
        add(mainPanel);
    }

    private void createBar(char x)
    {
        bar = new SpellBarComponent(x, mainPanel);
    }
}


public class RectangleComponent extends JComponent
{
    Color color;
    int width;
    int height = 18;
    RoundRectangle2D roundedRectangle;
    private JPanel panel;
    public RectangleComponent(Color color, JPanel panel)
    {
        this.panel = panel;
        this.color = color;
        paintComponent(panel.getGraphics());
    }

    public void paintComponent(Graphics g)
    {
        Graphics2D graphics2 = (Graphics2D) g;
        width = 125;
        roundedRectangle = new RoundRectangle2D.Float(10, 10, width, height, 10, 10);
        graphics2.setPaint(color);
        graphics2.fill(roundedRectangle);
        graphics2.draw(roundedRectangle); 
    }

    public void subtractLife(int amount)
    {
        width -= amount;
        roundedRectangle.setRoundRect(10, 10, width, height, 10, 10);
        repaint();
    }
}

推荐答案

为了让您的 Swing 应用程序按预期工作，您需要牢记许多事情.由于您以错误的方式编码，因此总是必须遵循某些步骤才能避免可能出现的某些障碍.为此，请坚持 Swing Programming Strictly 的基础知识，并遵循它们.

In order for your Swing Application to work as expected, there are many a things you need to keep in mind. There are always certain steps that one must follow in order to escape certain hurdles, that might can arise, since you coded in the wrong way. For this stick to the basics of Swing Programming Strictly, and follow them.

  • 就像@HovercraftFullOfEels 提到的，你打电话给你的直接画图，千万不要这样做.

  • Like as mentioned by @HovercraftFullOfEels , you calling to your Graphics directly, which one should never do.

其次，查看您的 GameFrame() 构造函数，将其设置为可见，甚至在您添加任何组件之前在它的实际尺寸确定之前

Secondly, look at your GameFrame() constructor, you set it to visible, even before you had added any components to it and much before it's real size has been established

编码中的此类漏洞可能会引起很多麻烦，因为当您坐下来编写大型程序时，最好从一开始就走在安全的道路上，然后在后期诅咒自己.正如他们所说的预防胜于治疗.

Such loop holes inside your coding might can give rise to many a headaches, as you sit down to write huge programs, so better to be on the safe road from the beginning, then to curse yourself at the later stage. As they say Prevention is better than Cure.

现在来到你的程序，你错过了主要的事情，因为你没有指定 CustomComponent 即 JComponent 的大小，因此你无法看到它在你的屏幕上.当您将 JCompoent 扩展到您的类时，使其成为一种习惯习惯来覆盖它的 getPreferredSize()，以同样的方式覆盖它的 paintComponent(...) 方法.

Now coming to your program, you missed the main thingy, since you failed to specify the size of your CustomComponent i.e. JComponent, hence you are not been able to see it on your screen. As you extends a JCompoent to your class, make it a customary habbit to override it's getPreferredSize(), in the same manner you override it's paintComponent(...) method.

看看这个我为你制作的小程序，或许能帮到你，让你更明白其中的逻辑.

Have a look at this small program, I had crafted for you, might be this be able to help you out, to understand the logic a bit more.

import java.awt.*;
import java.awt.event.*;
import java.awt.geom.RoundRectangle2D;
import javax.swing.*;

public class CustomPainting {

    private RectangleComponent life;
    private RectangleComponent death;

    private void createAndDisplayGUI() {
        JFrame frame = new JFrame("Custom Painting");
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);

        JPanel centerPanel = new JPanel();
        centerPanel.setLayout(new GridLayout(0, 2, 5, 5));
        // Specifying the WIDTH, HEIGHT and Colour for this JComponent.
        life = new RectangleComponent(Color.GREEN.darker(), 20, 20);
        death = new RectangleComponent(Color.RED.darker(), 20, 20);
        centerPanel.add(life);
        centerPanel.add(death);

        JPanel buttonPanel = new JPanel();
        buttonPanel.setLayout(new FlowLayout(FlowLayout.LEFT, 5, 5));
        JButton incLifeButton = new JButton("INCREASE LIFE");
        incLifeButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent ae) {
                life.addLife(1);
            }
        });

        JButton decLifeButton = new JButton("DECREASE LIFE");
        decLifeButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent ae) {
                life.subtractLife(1);
            }
        });

        JButton incDeathButton = new JButton("INCREASE DEATH");
        incDeathButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent ae) {
                death.addLife(1);
            }
        });

        JButton decDeathButton = new JButton("DECREASE DEATH");
        decDeathButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent ae) {
                death.subtractLife(1);
            }
        }); 

        buttonPanel.add(incLifeButton);
        buttonPanel.add(decLifeButton);
        buttonPanel.add(incDeathButton);
        buttonPanel.add(decDeathButton);

        frame.getContentPane().add(centerPanel, BorderLayout.CENTER);
        frame.getContentPane().add(buttonPanel, BorderLayout.PAGE_END);
        frame.pack();
        frame.setLocationByPlatform(true);
        frame.setVisible(true);
    }

    public static void main(Stringu005Bu005D args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                new CustomPainting().createAndDisplayGUI();
            }
        });
    }
}

class RectangleComponent extends JComponent {

    private Color colour;
    private static final int MARGIN = 10;
    private int width;
    private int height;
    private int originalWidth;
    private RoundRectangle2D roundedRectangle;

    public RectangleComponent(Color c, int w, int h) {
        colour = c;
        width = w;
        height = h;
        originalWidth = width;
    }

    /*
     * Overriding this method, so that
     * the size of the JComponent
     * can be determined, on the screen
     * or by the LayoutManager concern.
     */
    @Override 
    public Dimension getPreferredSize() {
        return (new Dimension(width, height));
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2d = (Graphics2D) g;
        roundedRectangle = new RoundRectangle2D.Float(MARGIN, MARGIN,
                                        width, height, MARGIN, MARGIN);
        g2d.setPaint(colour);
        g2d.draw(roundedRectangle);
        g2d.fill(roundedRectangle);
    }

    public void subtractLife(int amount) {
        width -= amount;
        System.out.println("ORIGINAL Width : " + originalWidth);
        System.out.println("Width : " + width);
        if (width > 0) {
            roundedRectangle.setRoundRect(MARGIN, MARGIN, width, height,
                                            MARGIN, MARGIN);
            /*
             * This repaint() will call the paintComponent(...)
             * by itself, so nothing else to be done.
             */
            repaint();
        } else {
            width += amount;
        }
    }

    public void addLife(int amount) {
        width += amount;
        System.out.println("ORIGINAL Width : " + originalWidth);
        System.out.println("Width : " + width);
        if (width < originalWidth) {
            roundedRectangle.setRoundRect(MARGIN, MARGIN, width, height,
                                            MARGIN, MARGIN);
            repaint();
        } else {
            width -= amount;
        }
    }
}

请提出任何问题，在您完成此程序时可能会出现 :-)，我很乐意为您提供帮助 :-)

Do ask any question, that might can arise as you go through this program :-), I be HAPPY to help on that :-)

**两种颜色的最新**

**LATEST EDIT WITH TWO COLOURS : **

import java.awt.*;
import java.awt.event.*;
import java.awt.geom.RoundRectangle2D;
import javax.swing.*;

public class CustomPainting {

    private RectangleComponent lifeDeath;

    private void createAndDisplayGUI() {
        JFrame frame = new JFrame("Custom Painting");
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);

        JPanel centerPanel = new JPanel();
        centerPanel.setLayout(new GridLayout(0, 2, 5, 5));
        // Specifying the WIDTH, HEIGHT and Colour for this JComponent.
        lifeDeath = new RectangleComponent(Color.GREEN, Color.RED, 20, 20);
        centerPanel.add(lifeDeath);

        JPanel buttonPanel = new JPanel();
        buttonPanel.setLayout(new GridLayout(1, 2, 5, 5));
        JButton incLifeButton = new JButton("INCREASE LIFE");
        incLifeButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent ae) {
                lifeDeath.addLife(1);
            }
        });

        JButton decLifeButton = new JButton("DECREASE LIFE");
        decLifeButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent ae) {
                lifeDeath.subtractLife(1);
            }
        });

        buttonPanel.add(incLifeButton);
        buttonPanel.add(decLifeButton);

        frame.getContentPane().add(centerPanel, BorderLayout.CENTER);
        frame.getContentPane().add(buttonPanel, BorderLayout.PAGE_END);
        frame.pack();
        frame.setLocationByPlatform(true);
        frame.setVisible(true);
    }

    public static void main(Stringu005Bu005D args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                new CustomPainting().createAndDisplayGUI();
            }
        });
    }
}

class RectangleComponent extends JComponent {

    private Color lifeColour;
    private Color deathColour;
    private static final int MARGIN = 10;
    private int widthLife;
    private int widthDeath;
    private int height;
    private int originalWidth;
    private RoundRectangle2D roundedRectangle;

    public RectangleComponent(Color lc, Color dc, int w, int h) {
        lifeColour = lc;
        deathColour = dc;
        widthLife = w;
        height = h;
        originalWidth = widthLife;
        widthDeath = 0;     
    }

    /*
     * Overriding this method, so that
     * the size of the JComponent
     * can be determined, on the screen
     * or by the LayoutManager concern.
     */
    @Override 
    public Dimension getPreferredSize() {
        return (new Dimension(originalWidth, height));
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2d = (Graphics2D) g;

        roundedRectangle = new RoundRectangle2D.Float((MARGIN + widthDeath), MARGIN,
                                        widthLife, height, MARGIN, MARGIN);
        g2d.setPaint(lifeColour);
        g2d.draw(roundedRectangle);
        g2d.fill(roundedRectangle);

        roundedRectangle.setRoundRect(MARGIN, MARGIN,
                                        widthDeath, height, MARGIN, MARGIN);
        g2d.setPaint(deathColour);
        g2d.draw(roundedRectangle);
        g2d.fill(roundedRectangle);
    }

    public void subtractLife(int amount) {
        widthLife -= amount;
        widthDeath += amount;
        System.out.println("ORIGINAL Width : " + originalWidth);
        System.out.println("Width Life : " + widthLife);
        System.out.println("Width Death : " + widthDeath);
        if (widthLife > 0 && widthDeath < originalWidth) {
            /*
             * This repaint() will call the paintComponent(...)
             * by itself, so nothing else to be done.
             */
            repaint();
        } else {
            widthLife += amount;
            widthDeath -= amount;
        }
    }

    public void addLife(int amount) {
        widthLife += amount;
        widthDeath -= amount;
        System.out.println("ORIGINAL Width : " + originalWidth);
        System.out.println("Width Life : " + widthLife);
        System.out.println("Width Death : " + widthDeath);
        if (widthLife < originalWidth && widthDeath > 0) {
            repaint();
        } else {
            widthLife -= amount;
            widthDeath += amount;
        }   
    }
}

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