JLabel setText（）方法不起作用

Question

This code should create a frame, which contains a Label, that prints out a clock. The method zeitLaeuft() makes the clock work and with the Button 'start' the clock starts to run. When I call the method zeitLaeuft() the code fails. I've tried a few things and now I know it's because of the Label jLUhr . In the method zeitLaeuft() the two orders which call the method jLUhr.setText() fail. I've tried to set the Label text and commented out the method but it doesn't work.

What is the problem?

package uhr;

import javax.swing.*;
import javax.swing.JLabel;

import java.awt.Font;
import java.awt.event.*;
import java.text.DecimalFormat;
import java.util.concurrent.TimeUnit;

public class Uhr1 extends javax.swing.JFrame {

    public Uhr1() {
        super();
        initGUI();
    }

    private static JLabel jLUhr;
    private static JButton jBtnStart;
    private static int stunden = 0, minuten = 0;

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                Uhr1 uhr = new Uhr1();
                uhr.setVisible(true);
                uhr.setLocationRelativeTo(null);
            }
        });

    }

    public void initGUI() {
        try {
            setDefaultCloseOperation(WindowConstants.DISPOSE_ON_CLOSE);
            this.setTitle("Uhr");
            getContentPane().setLayout(null);

            {
                JLabel jLUhr = new JLabel(); //Uhr = clock in german
                add(jLUhr);
                jLUhr.setBounds(49, 89, 300, 100);
                jLUhr.setHorizontalAlignment(SwingConstants.CENTER);
                jLUhr.setVerticalAlignment(SwingConstants.CENTER);

            }

            {
                JButton jBtnStart = new JButton();
                add(jBtnStart);
                jBtnStart.setBounds(49, 219, 80, 30);
                jBtnStart.setText("Start");
                jBtnStart.addActionListener(new ActionListener() {
                    public void actionPerformed(ActionEvent evt) {
                        start(evt);
                    }
                });
            }

            pack();
            setSize(400,300);

        } catch (Exception e) {
            e.printStackTrace();
        }
    }


    public static void zeitLaeuft() {

        while(true) {

                if(minuten < 60) {
                    int i;

                    try {
                        for(i = 0; i < 60; i++){
                        jLUhr.setText(Integer.toString(stunden) + " : " + Integer.toString(minuten));   //this is where the code fails
                        TimeUnit.SECONDS.sleep(1);
                        jLUhr.setText(Integer.toString(stunden) + "  " + Integer.toString(minuten));   //this is where the code fails
                        }
                        minuten++;
                        i = 0;
                    } catch (Exception e) {
                            e.printStackTrace();
                        }
                    }
                else if(minuten == 60) {
                    minuten = 0;
                    if(stunden < 24) {  
                        stunden ++;
                        }
                    else {
                        stunden = 0;
                    }
                }
            }
    }

    public void start(ActionEvent evt) {
        zeitLaeuft(); //this is where the code fails
    }

}

Answer 1

This:

JLabel jLUhr = new JLabel(); //Uhr = clock in german

...declares & creates a local variable whose scope is lost at the end of the method. It should be:

jLUhr = new JLabel(); //Uhr = clock in german

This is accesing the global variable.