返回數組NullPointerExeption

我一直在創建一個包含Segments數組的Snakes程序。在這種情況下，我認爲這是至關重要的，因爲順序很重要。第一個問題是，即使在創建「新的Segment實例」之後，它也不會顯示任何正方形（這是蛇的第一部分）。但是，當我在paint方法內部放置一個System.out.println()語句（稍後顯示）時，它會拋出NullPointerException。這裏是代碼：返回數組NullPointerExeption

注意：方法increaseSegmentCount()被從外部類調用一次。

@Override 
    public void paint(Graphics g) { 
     for (Segment segment : segments) { 
      if (segment != null) 
       System.out.println("Called") 
       segment.paint(g); 
     } 
    }

Segment類（相當簡單類）

import java.awt.Color; 
import java.awt.Dimension; 
import java.awt.Graphics; 
import java.awt.Point; 

public class Segment { 
    Point location; 

    public Segment(Point location) { 
     this.location = location; 
    } 

    public static Dimension getSize() { 
     return new Dimension(20, 20); 
    } 

    public void paint(Graphics g) { 
     g.setColor(Color.WHITE); 
     g.fillRect(location.x, location.y, 20, 20); 
    } 
}

：此外，這是不完整的代碼...

蛇

把 System.out.println聲明後

import java.awt.Graphics; 
import java.awt.Point; 

public class Snake implements Entity { 
    Point location; 
    Segment[] segments; 
    int segmentCount = 0; 

    public Snake(Point location) { 
     this.location = location; 
     segments = new Segment[25]; 
    } 

    public void increaseSegmentCount() { 

     segments[segmentCount] = new Segment(new Point(location.x + (Segment.getSize().width * segments.length), 
       location.y + (Segment.getSize().height * segments.length))); 

     segmentCount++; 

    } 

    public Segment[] getSegments() { 
     return segments; 
    } 

    public int getSegmentCount() { 
     return segmentCount; 
    } 

    @Override 
    public void paint(Graphics g) { 
     for (Segment segment : segments) { 
      if (segment != null) 
       segment.paint(g); 
     } 
    } 
}

paint()方法

paintComponent() 01方法類（調試）

@Override 
    public void paintComponent(Graphics g) { 
     super.paintComponent(g); 
     for (Entity entity : model.getEntities()) { 
      entity.paint(g); 
     } 
    }

最後的錯誤（插入打印語句之後...有沒有我之前插入它）：

Called 
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException 
    at Snake.paint(Snake.java:61) 
    at View.paintComponent(View.java:50) 
    at javax.swing.JComponent.paint(Unknown Source) 
    at javax.swing.JComponent.paintToOffscreen(Unknown Source) 
    at javax.swing.RepaintManager$PaintManager.paintDoubleBuffered(Unknown Source) 
    at javax.swing.RepaintManager$PaintManager.paint(Unknown Source) 
    at javax.swing.RepaintManager.paint(Unknown Source) 
    at javax.swing.JComponent._paintImmediately(Unknown Source) 
    at javax.swing.JComponent.paintImmediately(Unknown Source) 
    at javax.swing.RepaintManager$4.run(Unknown Source) 
    at javax.swing.RepaintManager$4.run(Unknown Source) 
    at java.security.AccessController.doPrivileged(Native Method) 
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source) 
    at javax.swing.RepaintManager.paintDirtyRegions(Unknown Source) 
    at javax.swing.RepaintManager.paintDirtyRegions(Unknown Source) 
    at javax.swing.RepaintManager.prePaintDirtyRegions(Unknown Source) 
    at javax.swing.RepaintManager.access$1200(Unknown Source) 
    at javax.swing.RepaintManager$ProcessingRunnable.run(Unknown Source) 
    at java.awt.event.InvocationEvent.dispatch(Unknown Source) 
    at java.awt.EventQueue.dispatchEventImpl(Unknown Source) 
    at java.awt.EventQueue.access$500(Unknown Source) 
    at java.awt.EventQueue$3.run(Unknown Source) 
    at java.awt.EventQueue$3.run(Unknown Source) 
    at java.security.AccessController.doPrivileged(Native Method) 
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source) 
    at java.awt.EventQueue.dispatchEvent(Unknown Source) 
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source) 
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source) 
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source) 
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source) 
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source) 
    at java.awt.EventDispatchThread.run(Unknown Source)

任何想法？此外，請不要將此標記爲重複，並指向另一個顯示如何解決NPE的帖子...我已經知道該怎麼做，發生了什麼（大部分），但這是一件奇怪的事情我遇到過，這很混亂。謝謝。

來源

2016-03-06 Eames

你需要{}你如果當你添加打印語句...

if (segment != null) { 
      System.out.println("Called") 
      segment.paint(g); 
}

你可能會考慮使用自動代碼格式化，這將使得這個明顯與縮進。

來源

2016-03-06 10:39:59

返回數組NullPointerExeption

回答

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