java - 数组返回 NullPointerException

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 7 年前。

我一直在创建一个 Snakes 程序，其中包含 Segments 数组。在这种情况下，我认为使用数组至关重要，因为顺序很重要。第一个问题是，即使在我创建“新 Segment 实例”之后，它也不会显示任何正方形(这是蛇的第一段)。但是，当我输入 System.out.println() 时在 Paint 方法中声明(稍后显示)，它会抛出 NullPointerException 。这是代码:

注意:方法 increaseSegmentCount()被外部类(class)调用过一次。另外，这不是完整的代码...

蛇

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()放置后的方法System.out.println声明于:

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

Segment类(非常简单的类)

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);
    }
}

paintComponent() View中的方法类(用于调试目的)

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

最后是错误(插入打印语句后...在插入之前没有错误):

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)

有什么想法吗？另外，请不要将其标记为重复，并让我指向另一篇展示如何解决 NPE 的帖子...我已经知道该做什么以及发生了什么(大部分)，但这是一件奇怪的事情我遇到过，非常困惑。谢谢。

Answer 1

当您添加打印语句时，您的 if 需要 { }...

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

您可以考虑使用自动代码格式化程序，这会使缩进变得明显。