java - 变量在静态时发生变化，在非静态时保持不变

Question

我有一个在窗口上显示时间的工作类，但唯一的问题是当我将字符串变量设置为非静态时，它们根本不会更新，但是当我将字符串变量设置为静态时，它们会自行更新。我不明白为什么会发生这种情况。代码如下:

public class Test extends Panel implements Runnable {
int second;
int minute;
int hour;
static String second_S = "1";
static String minute_S = "1";
static String hour_S = "1";

static JFrame frame = new JFrame("Clock");
static Test panel = new Test(500, 500, 1);

public static void main(String args[]) throws InterruptedException {
    Thread time = new Thread(new Test());

    frame.add(panel);
    Frame.showFrame(frame);

    time.start();

}

public void paintComponent(Graphics g) {
    super.paintComponent(g);
    g.setColor(Color.BLACK);

    g.drawString(hour_S, 250, 250);
    g.drawString(minute_S, 280, 250);
    g.drawString(second_S, 310, 250);

}

public Test() {

    second = 0;
    minute = 0;
    hour = 1;

}

public Test(int width, int length, int minusBy) {
    super(width, length, minusBy);
}

@Override
public void run() {
    while (true) {
        try {
            Thread.sleep(1);
            second++;
            if (second > 60) {
                second = 1;
                minute++;
            }
            if (minute > 60) {
                minute = 1;
                hour++;
            }
            if (hour > 12) {
                hour = 1;
            }

            hour_S = Integer.toString(hour);
            minute_S = Integer.toString(minute);
            second_S = Integer.toString(second);

            System.out.printf("%02d:%02d:%02d\n", hour, minute, second);
            panel.repaint();

        } catch (InterruptedException e) {
        }
    }

}

}

Answer 1

您有两个 Test 实例:

static Test panel = new Test(500, 500, 1); // one which displays the values of the members

public static void main(String args[]) throws InterruptedException {
    Thread time = new Thread(new Test()); // and another which updates the variables

    frame.add(panel);
    Frame.showFrame(frame);

    time.start();

}

当您的成员是静态的时，两个实例共享相同的值。当您的成员不是静态时，更新值的实例与显示值的实例不同，并且每个实例都有自己的成员，因此 panel 实例中的实例变量的值仍然存在不变。

如果您使用相同的实例，您的代码将适用于非 staticc 成员:

static Test panel = new Test(500, 500, 1);

public static void main(String args[]) throws InterruptedException {
    Thread time = new Thread(panel);

    frame.add(panel);
    Frame.showFrame(frame);

    time.start();

}