在Java中創建矩形時，我實際上調用了paintComponent方法嗎？

Question

這是我當前的RectangleComponent類，我將它添加到我的主JFrame中的面板，但它從不出現。我認爲它沒有繪製，所以我決定在Rectangle的構造函數中調用paintComponent方法，在對4-5 nullPointerExceptions進行排序後，沒有任何變化。我已經閱讀了關於如何繪製矩形的多個指南，並且我已經看到了多個代碼示例，但是我無法讓面板與多個JComponent一起工作。如果可以的話，請簡單看看我的代碼，看看你能否設計一個解決方案。 謝謝你的時間。還列出了我所說的矩形構造框架。

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(); 
    } 
} 

Answer 1

爲了讓您的Swing應用程序按預期工作，您需要記住許多事項。爲避免可能出現的某些障礙，總會有一些步驟必須遵循，因爲您以錯誤的方式編碼。爲此，嚴格遵守Swing編程的基礎知識，並遵循它們。

• 通過@HovercraftFullOfEels提到喜歡，你叫你的 圖形直接，哪一個不應該做的。

• 其次，看你的GameFrame()構造，可以將其設置爲 可見，你又增加了，甚至之前，它和任何組件多 之前它的真實大小已經建立

這樣的循環孔內你的編碼可能會引起許多令人頭疼的問題，當你坐下來編寫大型節目時，最好是從一開始就安全的路上，然後在後期詛咒自己。正如他們所說的預防勝於治療。

由於未能指定CustomComponent的大小（即JComponent），因此您無法在屏幕上看到它，現在即將到達您的程序中，您錯過了主要的東西。當您將JCompoent擴展到您的班級時，請將其設爲習慣性習慣來覆蓋它的getPreferredSize()，方法與覆蓋方法相同。

看看這個我爲你製作的小程序，可能會幫助你理解邏輯。

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(String\u005B\u005D 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; 
     } 
    } 
} 

可能會問任何問題，可能會出現，可能與您經過這個程序:-)，我很樂意幫助上:-)

**兩種顏色最新編輯：**

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(String\u005B\u005D 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; 
     } 
    } 
} 

Answer 2

無需通過JPanel到RectangleComponent構造只是爲了獲得Graphics，並且無需手動調用paintComponent。請參閱Painting in AWT and Swing。看看這個example，演示了一個繪製矩形的自定義組件。

Answer 3

你的代碼有點創意，有點瘋狂，而且邏輯非常難以遵循。最不尋常的方面是它有兩個JFrames，一個叫做「frame」，另一個叫做GameFrame對象本身，它們都會添加組件，但其中只有一個顯示。你也有很多方法返回void（如果過度使用會增加代碼的氣味），只會增加代碼的混淆。

例如，

public GameFrame(char x) { 

    // here you set up the "frame" JFrame 
    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); 

    // here you add content to the frame JFrame, and pack it 
    frame.getContentPane().add(mainPanel); 
    frame.pack(); 
    repaint(); // and then call repaint on the "this" JFrame? 
} 

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); // now you add content to the "this" JFrame 
    pack(); // and pack it 
    createBar(x); 
    createHealth(); 
    mainPanel.add(buttonPanel); 
    mainPanel.add(healthPanel); // and then re-add a JPanel into a second JPanel? 
    healthPanel.add(death); 
    healthPanel.add(life); 
    buttonPanel.add(bar.getSpell1()); 
    buttonPanel.add(bar.getSpell2()); 
    buttonPanel.add(bar.getSpell3()); 
    add(mainPanel); // and then re -add the mainPanel into the "this" JFrame??? 
} 

這是非常混亂，不太可能去上班。

然後，您就試圖直接調用paintComponent並在JComponent上調用getGraphics，而這兩者都不應該這樣做。你會想通過圖形教程來看看如何正確地做到這一點。

我建議您考慮重新編寫此代碼，並首先僅使用一個JFrame，並更好地組織代碼。