Question

我想在applet中显示随机数，只要applet正在运行但不显示。请指导什么是代码

package javaapplication1;
 import java.util.*;
import java.applet.Applet;
import java.awt.Graphics;
import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;

public class set extends Applet implements Runnable {
    private DateFormat formatter = null;
    private Thread t = null;

    public void init() {

        int n = (int)(10.0 * Math.random()) + 1;
       System.out.print(n);
        t = new Thread(this);
    }

    public void start() {
        t.start();
    }

    public void stop() {
        t = null;
    }

    public void paint(Graphics g) {
        Date now = Calendar.getInstance().getTime();
        //
        // Show the current time on the browser status bar
        //
        this.showStatus(formatter.format(now));
    }

    public void run() {
        int delay = 1000;
        try {
            while (t == Thread.currentThread()) {
                //
                // Repaint the applet every on second
                //
                repaint();
                Thread.sleep(delay);
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

并且浮动时出现以下异常

Exception in thread "AWT-EventQueue-1" java.lang.NullPointerException
        at javaapplication1.set.paint(set.java:39)
        at sun.awt.RepaintArea.paintComponent(RepaintArea.java:248)
        at sun.awt.RepaintArea.paint(RepaintArea.java:224)
        at sun.awt.windows.WComponentPeer.handleEvent(WComponentPeer.java:310)
        at java.awt.Component.dispatchEventImpl(Component.java:4706)
        at java.awt.Container.dispatchEventImpl(Container.java:2099)
        at java.awt.Component.dispatchEvent(Component.java:4460)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)
Exception in thread "AWT-EventQueue-1" java.lang.NullPointerException
        at javaapplication1.set.paint(set.java:39)
        at sun.awt.RepaintArea.paintComponent(RepaintArea.java:248)
        at sun.awt.RepaintArea.paint(RepaintArea.java:224)
        at sun.awt.windows.WComponentPeer.handleEvent(WComponentPeer.java:310)
        at java.awt.Component.dispatchEventImpl(Component.java:4706)
        at java.awt.Container.dispatchEventImpl(Container.java:2099)
        at java.awt.Component.dispatchEvent(Component.java:4460)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)
4Exception in thread "AWT-EventQueue-1" java.lang.NullPointerException
        at javaapplication1.set.paint(set.java:39)
        at java.awt.Container.update(Container.java:1801)
        at sun.awt.RepaintArea.updateComponent(RepaintArea.java:239)
        at sun.awt.RepaintArea.paint(RepaintArea.java:216)
        at sun.awt.windows.WComponentPeer.handleEvent(WComponentPeer.java:310)
        at java.awt.Component.dispatchEventImpl(Component.java:4706)
        at java.awt.Container.dispatchEventImpl(Container.java:2099)
        at java.awt.Component.dispatchEvent(Component.java:4460)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)
Exception in thread "AWT-EventQueue-1" java.lang.NullPointerException
        at javaapplication1.set.paint(set.java:39)
        at java.awt.Container.update(Container.java:1801)
        at sun.awt.RepaintArea.updateComponent(RepaintArea.java:239)
        at sun.awt.RepaintArea.paint(RepaintArea.java:216)
        at sun.awt.windows.WComponentPeer.handleEvent(WComponentPeer.java:310)
        at java.awt.Component.dispatchEventImpl(Component.java:4706)
        at java.awt.Container.dispatchEventImpl(Container.java:2099)
        at java.awt.Component.dispatchEvent(Component.java:4460)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)
Exception in thread "AWT-EventQueue-1" java.lang.NullPointerException
        at javaapplication1.set.paint(set.java:39)
        at java.awt.Container.update(Container.java:1801)
        at sun.awt.RepaintArea.updateComponent(RepaintArea.java:239)
        at sun.awt.RepaintArea.paint(RepaintArea.java:216)
        at sun.awt.windows.WComponentPeer.handleEvent(WComponentPeer.java:310)
        at java.awt.Component.dispatchEventImpl(Component.java:4706)
        at java.awt.Container.dispatchEventImpl(Container.java:2099)
        at java.awt.Component.dispatchEvent(Component.java:4460)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)
Exception in thread "AWT-EventQueue-1" java.lang.NullPointerException
        at javaapplication1.set.paint(set.java:39)
        at java.awt.Container.update(Container.java:1801)
        at sun.awt.RepaintArea.updateComponent(RepaintArea.java:239)
        at sun.awt.RepaintArea.paint(RepaintArea.java:216)
        at sun.awt.windows.WComponentPeer.handleEvent(WComponentPeer.java:310)
        at java.awt.Component.dispatchEventImpl(Component.java:4706)
        at java.awt.Container.dispatchEventImpl(Container.java:2099)
        at java.awt.Component.dispatchEvent(Component.java:4460)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)
Exception in thread "AWT-EventQueue-1" java.lang.NullPointerException
        at javaapplication1.set.paint(set.java:39)
        at java.awt.Container.update(Container.java:1801)
        at sun.awt.RepaintArea.updateComponent(RepaintArea.java:239)
        at sun.awt.RepaintArea.paint(RepaintArea.java:216)
        at sun.awt.windows.WComponentPeer.handleEvent(WComponentPeer.java:310)
        at java.awt.Component.dispatchEventImpl(Component.java:4706)
        at java.awt.Container.dispatchEventImpl(Container.java:2099)
        at java.awt.Component.dispatchEvent(Component.java:4460)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)
Exception in thread "AWT-EventQueue-1" java.lang.NullPointerException

Answer 1

这应该不难。当您尝试对NullPointerException执行操作时会出现null。

您的拥有代码中有NullPointerException（而不是库中的代码）。您所要做的就是去那一行，看看可能是null。

在第

行

this.showStatus(formatter.format(now));

formatter（最有可能）null。这就是NullPointerException。

的原因

_{_{我认为stackoverflow让人无限懒惰。}}

Answer 2

您希望如何显示它，您认为您的代码会实现这一目标？

目前，我可以看到您创建一个随机数，然后使用该号码拨打System.out.println。这会将的数字打印到控制台，但它不会显示在您的小程序中。

您需要在paint(Graphics g)方法中添加一些逻辑来显示此数字 - 例如，调用g.drawString()。然而，有很多方法可以输出文本，这取决于你希望它如何在你已经拥有的东西上进行loko，这样对于这个问题给出一个正确或更具体的答案是不合理的。

您应该再看一下applet教程，该教程将介绍如何控制输出，并根据结果发回更多特定的问题。

在applet中显示随机数

2 个答案: