當actionListener被激活時，製作一個繪製applet的方法

Question

問題：我試圖添加多個使用Java內置圖形方法的方法。繪製屏幕的方法使用if語句。

我試過了：玩弄方法名稱並更改（圖形g）中的字符。此外，我試圖將我遇到問題的方法放入paint方法塊中。

中使用的變量：

• 字長=存儲的字的量具有一定長度的數組。例如。如果我有1個1個字母的單詞，wordLengths [0]將爲1。

以下代碼打印我的聲母，空白出來的文字字段和油漆圖表的x和y軸：

public void paint(Graphics g) { 
     g.drawString("BWD", 450, 490); // Puts my signature in the bottom corner 
     inputField.setText(""); 
     inputText = (""); 
     g.drawLine(50, 450, 460, 450);//Paints x-axis 
     g.drawLine(50, 450, 50, 100);//Paints the y-axis 
    } 

下面的代碼具有的方法名稱塗料。我試圖改變這一點，但它仍然返回問題。當點擊提交按鈕，我想下面的代碼繪製到屏幕上：

public void paint (Graphics g) 
{ 
     for(int j=0, x=50, y = 100; j< wordLengths.length;j++) 
     { 
      if (wordLengths[j] == 1) 
      { 
       y = y+15; 
       g.drawString(wordLengths[j]+ " word of length "+ (j+1), x, y);        
      } 

      g.drawString("There are: ", 50, 90); 

      if (wordLengths[j] > 1) 
      { 
       y = y+15; 
       g.drawString(wordLengths[j]+ " words of length "+ (j+1), x, y); 
      } 
     } 
    } 

理想的代碼會是這樣：

public void paint(Graphics g) { 
      g.drawString("BWD", 450, 490); // Puts my signature in the bottom corner 
      inputField.setText(""); 
      inputText = (""); 
      g.drawLine(50, 450, 460, 450);//Paints x-axis 
      g.drawLine(50, 450, 50, 100);//Paints the y-axis 

      for(int j=0, x=50, y = 100; j< wordLengths.length;j++) 
      { 
       if (wordLengths[j] == 1) 
       { 
        y = y+15; 
        g.drawString(wordLengths[j]+ " word of length "+ (j+1), x, y);        
       } 

       g.drawString("There are: ", 50, 90); 

       if (wordLengths[j] > 1) 
       { 
        y = y+15; 
        g.drawString(wordLengths[j]+ " words of length "+ (j+1), x, y); 
       } 
      } 

     } 

如果需要任何其他信息，請不要猶豫添加評論，我會盡快收錄它！

在此先感謝！

Exception in thread "AWT-EventQueue-1" java.lang.NullPointerException 
at java_assignment.JavaAppletMain.paint(JavaAppletMain.java:114) 
at sun.awt.RepaintArea.paintComponent(RepaintArea.java:264) 
at sun.lwawt.LWRepaintArea.paintComponent(LWRepaintArea.java:54) 
at sun.awt.RepaintArea.paint(RepaintArea.java:240) 
at sun.lwawt.LWComponentPeer.handleJavaPaintEvent(LWComponentPeer.java:1267) 
at sun.lwawt.LWComponentPeer.handleEvent(LWComponentPeer.java:1150) 
at java.awt.Component.dispatchEventImpl(Component.java:4937) 
at java.awt.Container.dispatchEventImpl(Container.java:2287) 
at java.awt.Component.dispatchEvent(Component.java:4687) 
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java: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.doIntersectionPrivilege(ProtectionDomain.java:76) 
at java.security.ProtectionDomain$1.doIntersectionPrivilege(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.doIntersectionPrivilege(ProtectionDomain.java:76) 
at java.awt.EventQueue.dispatchEvent(EventQueue.java:705) 
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242) 
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161) 
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150) 
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146) 
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138) 
at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)) 

    Thread [AWT-EventQueue-1] (Suspended (breakpoint at line 112 in JavaAppletMain))  
    JavaAppletMain.paint(Graphics) line: 112  
    LWRepaintArea(RepaintArea).paintComponent(Component, Graphics) line: 264  
    LWRepaintArea.paintComponent(Component, Graphics) line: 54 
    LWRepaintArea(RepaintArea).paint(Object, boolean) line: 240 
    LWPanelPeer(LWComponentPeer<T,D>).handleJavaPaintEvent() line: 1267 
    LWPanelPeer(LWComponentPeer<T,D>).handleEvent(AWTEvent) line: 1150 
    JavaAppletMain(Component).dispatchEventImpl(AWTEvent) line: 4937  
    JavaAppletMain(Container).dispatchEventImpl(AWTEvent) line: 2287  
    JavaAppletMain(Component).dispatchEvent(AWTEvent) line: 4687  
    EventQueue.dispatchEventImpl(AWTEvent, Object) line: 735  
    EventQueue.access$200(EventQueue, AWTEvent, Object) line: 103 
    EventQueue$3.run() line: 694  
    EventQueue$3.run() line: 692  
    AccessController.doPrivileged(PrivilegedAction<T>, AccessControlContext) line: not available [native method]  
    ProtectionDomain$1.doIntersectionPrivilege(PrivilegedAction<T>, AccessControlContext, AccessControlContext) line: 76  
    ProtectionDomain$1.doIntersectionPrivilege(PrivilegedAction<T>, AccessControlContext) line: 87 
    EventQueue$4.run() line: 708  
    EventQueue$4.run() line: 706  
    AccessController.doPrivileged(PrivilegedAction<T>, AccessControlContext) line: not available [native method]  
    ProtectionDomain$1.doIntersectionPrivilege(PrivilegedAction<T>, AccessControlContext, AccessControlContext) line: 76  
    EventQueue.dispatchEvent(AWTEvent) line: 705  
    EventDispatchThread.pumpOneEventForFilters(int) line: 242 
    EventDispatchThread.pumpEventsForFilter(int, Conditional, EventFilter) line: 161  
    EventDispatchThread.pumpEventsForHierarchy(int, Conditional, Component) line: 150 
    EventDispatchThread.pumpEvents(int, Conditional) line: 146 
    EventDispatchThread.pumpEvents(Conditional) line: 138 
    EventDispatchThread.run() line: 91 

Answer 1

你只是初始化在actionPerformed你的陣列wordLenghts。但在程序啓動時會調用paint，並且actionPerformed永遠不會被初始化爲數組。這就是爲什麼你會得到NPE。

您應該在構造函數或init方法中初始化它。

private int[] wordLength; 

@Overrde 
public void init() { 
    wordLength = new int[0]; 
}