Question

我正在尝试编写简单的GUI倒计时。我在网上发现了一些代码，但它对我来说已经过于花哨了。我想尽量保持简单。所以，我只想要一个窗口说“你剩下10秒钟”。秒数应该每秒从10减少到0.我写了一个代码。而且我认为我接近工作解决方案。但我仍然遗漏了一些东西。你能请求帮助我找出问题所在吗？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();
    }

}

我对这段代码有几个具体的问题：

我应该在哪里放置counter.start();？（在我的代码中，我把它放在2个位置。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。

增加： 我提出了建议的更正。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.");。

Answer 1

在runnable中调用counter.start()：

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

你真的想要一个特定的调用顺序，如果你将它放在线程之外，那么计数器将在GUI存在之前启动，它将失败。

关于第二个问题：

// 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.");
    }
}

现在你的主力：

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

<强>更新
或者，如果您仍想使用匿名模式，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.");
                }
            }
        );
    }
}

Answer 2

答案中的匿名示例完美无缺。

对于第一个答案，计数器工作，但没有显示GUI，并且每秒出现以下异常

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)

简单的GUI倒计时应该如何工作？

2 个答案: