Question

我一直在创建一个包含Segments数组的Snakes程序。在这种情况下，我认为使用数组至关重要，因为顺序很重要。第一个问题是，即使我创建了一个＆＃34;新的Segment实例＆＃34;，它也不会显示任何方块（这是蛇的第一段）。但是，当我在paint方法中放置一个System.out.println()语句（稍后显示）时，它会抛出一个NullPointerException。这是代码：

注意：方法increaseSegmentCount()是从外部类调用一次的。此外，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);
        }
    }
}

将paint()语句放入：

后的

System.out.println方法

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

Segment课程（相当简单的课程）

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()类中的

View方法（用于调试目的）

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

最后错误（在插入print语句之后......在我插入之前没有）：

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，请不要将此标记为重复内容，并指出另一篇文章，说明如何解决NPE ...我已经知道该做什么以及发生了什么（最重要的）部分），但这是一个奇怪的，我遇到的事情，它非常令人困惑。感谢。

Answer 1

当你添加print语句时，你需要{} ...

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

您可能会考虑使用自动代码格式化程序，这会使缩进显而易见。

数组返回NullPointerExeption

1 个答案: