返回数组NullPointerExeption

我一直在创建一个包含Segments数组的Snakes程序。在这种情况下，我认为这是至关重要的，因为顺序很重要。第一个问题是，即使在创建“新的Segment实例”之后，它也不会显示任何正方形（这是蛇的第一部分）。但是，当我在paint方法内部放置一个System.out.println()语句（稍后显示）时，它会抛出NullPointerException。这里是代码：返回数组NullPointerExeption

注意：方法increaseSegmentCount()被从外部类调用一次。

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

Segment类（相当简单类）

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，这是不完整的代码...

蛇

把 System.out.println声明后

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方法类（调试）

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

最后的错误（插入打印语句之后...有没有我之前插入它）：

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，请不要将此标记为重复，并指向另一个显示如何解决NPE的帖子...我已经知道该怎么做，发生了什么（大部分），但这是一件奇怪的事情我遇到过，这很混乱。谢谢。

来源

2016-03-06 Eames

你需要{}你如果当你添加打印语句...

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

你可能会考虑使用自动代码格式化，这将使得这个明显与缩进。

来源

2016-03-06 10:39:59

返回数组NullPointerExeption

回答

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