Question

我在使用Jlabel和setText（）方法时遇到了一些问题。通常，setText（）应该清除以前的文本，并在按钮单击时替换为新文本，而是将文本相互堆叠。

    public class JFrameDemo extends JFrame{
    private JButton clearButton;
    private JLabel display;
    private JButton exitButton;
    private JButton go;
    private JTextField nameField;
    private JLabel name;

    public JFrameDemo(){
        doEnglish();
    }

    private void doEnglish(){
        setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        getContentPane().setLayout(null);        

        name = new JLabel("Enter Name");
        getContentPane().add(name);
        name.setBounds(5,30,300,30);        
        nameField = new JTextField();
        getContentPane().add(nameField);    
        nameField.setBounds(80,33,150,25);        

        go = new JButton("Go");
        getContentPane().add(go);
        go.setBounds(5, 65, 50, 30);
        go.addActionListener(new ActionListener() {                        
            @Override
            public void actionPerformed(ActionEvent e) {
                goButtonActionPerformed();
            }
        });
    }

    private void goButtonActionPerformed(){
        String na = nameField.getText();
                display = new JLabel();                               
                display.setText(na);
                getContentPane().add(display);
                display.setBounds(85,110,400,20);                
    }

    public static void main(String args[]){
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                new JFrameDemo().setVisible(true);
            }
        });
    }
   }

Answer 1

您正在使用null布局
您正在创建JLabel的新实例，然后将其添加到内容窗格中......这并不奇怪

避免使用null布局，像素完美布局是现代ui设计中的一种幻觉。影响组件个体大小的因素太多，您无法控制。 Swing旨在与布局管理器一起工作，放弃这些将导致问题和问题的终结，您将花费越来越多的时间来纠正

请查看What is a Null Pointer Exception, and how do I fix it?了解更多详情......

相反，您应检查display标签的状态，并仅在需要时创建...

private void goButtonActionPerformed(){
    String na = nameField.getText();
    if (display == null) {
        display = new JLabel(); 
        getContentPane().add(display);
        // Get rid of this as soon as you can...
        display.setBounds(85,110,400,20);                
    }
    display.setText(na);
}

Answer 2

每次按下add按钮，都会向contentPane添加新的JLabel。所以将代码更改为：

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;
import javax.swing.WindowConstants;

        public class JFrameDemo extends JFrame{
        private JButton clearButton;
        private JLabel display = new JLabel();
        private JButton exitButton;
        private JButton go;
        private JTextField nameField;
        private JLabel name;

        public JFrameDemo(){
            doEnglish();
        }

        private void doEnglish(){
            setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
            getContentPane().setLayout(null);        

            name = new JLabel("Enter Name");
            getContentPane().add(name);
            name.setBounds(5,30,300,30);        
            nameField = new JTextField();
            getContentPane().add(nameField);    
            nameField.setBounds(80,33,150,25);        

            go = new JButton("Go");
            getContentPane().add(go);
            go.setBounds(5, 65, 50, 30);

            getContentPane().add(display);
            display.setBounds(85,110,400,20);                

            go.addActionListener(new ActionListener() {                        
                @Override
                public void actionPerformed(ActionEvent e) {
                    goButtonActionPerformed();
                }
            });
        }

        private void goButtonActionPerformed(){
            String na = nameField.getText();
                    //display = new JLabel();                               
                    display.setText(na);
        }

        public static void main(String args[]){
            SwingUtilities.invokeLater(new Runnable() {
                @Override
                public void run() {
                    new JFrameDemo().setVisible(true);
                }
            });
        }
       }

JLabel setText（）的问题

2 个答案: