代码之家 › 专栏 › 技术社区 › Roman

SimplesGUI倒计时应该如何工作?

user-interface multithreading java

Roman · 技术社区 · 16 年前

我正在写simples gui倒计时。我在网上找到了一些密码,但对我来说已经太花哨了。我尽量保持简单。所以,我只想让一个窗口说“你还有10秒钟”。每秒的秒数应该从10减少到0。我写了一个密码。我想我已经接近工作方案了。但我还是错过了一些东西。你能帮我查出怎么了吗?这是我的代码:

import javax.swing.*;

public class Countdown {

    static JLabel label;

    // Method which defines the appearance of the window.   
    private static void showGUI() {
        JFrame frame = new JFrame("Simple Countdown");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        JLabel label = new JLabel("Some Text");
        frame.add(label);
        frame.pack();
        frame.setVisible(true);
    }

    // Define a new thread in which the countdown is counting down.
    static Thread counter = new Thread() {
        public void run() {
            for (int i=10; i>0; i=i-1) {
                updateGUI(i,label);
                try {Thread.sleep(1000);} catch(InterruptedException e) {};
            }
        }
    };

    // A method which updates GUI (sets a new value of JLabel).
    private static void updateGUI(final int i, final JLabel label) {
        SwingUtilities.invokeLater(new Runnable(i,label) {

            public Runnable(int i, JLabel label) {
                this.i = i;
                this.label = label;
            }

            public void run() {
                label.setText("You have " + i + " seconds.");
            }

        });
    }

    // The main method (entry point).
    public static void main(String[] args) {
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                showGUI();
                //counter.start();
            }
        });
        //counter.start();
    }

}

关于这个代码,我有几个具体的问题:

我应该把 counter.start(); ?(在我的代码中,我把它放在两个地方。哪一个是正确的?)
为什么编译器会抱怨runnable的构造函数?它说我有一个无效的方法声明,我需要指定返回的类型。

补充: 我做了建议的修正。然后我执行代码并得到:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at Worker.run(Worker.java:12)

在第12行的worker.java中,我有: label.setText("You have " + i + " seconds."); .

2 回复 | 直到 15 年前

Kiril 16 年前

呼叫 counter.start() 在可运行文件中:

// The main method (entry point).
public static void main(String[] args) {
    javax.swing.SwingUtilities.invokeLater(new Runnable() {
        public void run() {
            showGUI();
            counter.start();
        }
    });
}

您真的想要一个特定的调用顺序,如果您将它放在线程之外,那么计数器将在GUI存在之前启动,并且它将在您身上失败。

对于第二个问题:

// A method which updates GUI (sets a new value of JLabel).
private static void updateGUI(final int i, final JLabel label) {
    SwingUtilities.invokeLater(new Worker(i, label));
}

这是工人:

import javax.swing.JLabel;

public class Worker implements Runnable{
    private int i;
    private JLabel label;
    public Worker(int i, JLabel label) {
        this.i = i;
        this.label = label;
    }

    public void run() {
        label.setText("You have " + i + " seconds.");
    }
}

现在你的主要任务是:

// The main method (entry point).
public static void main(String[] args) {
    javax.swing.SwingUtilities.invokeLater(new Runnable() {
        public void run() {
            Countdown.showGUI();
            counter.start();
        }
    });
}

更新:
或者如果您仍然想使用匿名模式,那么:

import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.SwingUtilities;

public class Countdown {

    static JLabel label;

    // Method which defines the appearance of the window.   
    public static void showGUI() {
        JFrame frame = new JFrame("Simple Countdown");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        label = new JLabel("Some Text");
        frame.add(label);
        frame.pack();
        frame.setVisible(true);
    }

    // Define a new thread in which the countdown is counting down.
    public static Thread counter = new Thread() {
        public void run() {
            for (int i=10; i>0; i=i-1) {
                updateGUI(i,label);
                try {Thread.sleep(1000);} catch(InterruptedException e) {};
            }
        }
    };

    // A method which updates GUI (sets a new value of JLabel).
    private static void updateGUI(final int i, final JLabel label) {
        SwingUtilities.invokeLater( 
            new Runnable() {
                public void run() {
                    label.setText("You have " + i + " seconds.");
                }
            }
        );
    }
}

Andrew Thompson 15 年前

答案中的匿名示例非常有效。

对于第一个答案,计数器工作,但没有显示GUI,并且每秒钟都会出现以下异常

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
        at com.aramco.ecc.licenseoptimizer.gui.Worker.run(Worker.java:23)
        at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:209)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:597)
        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)