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在jLabel中显示jComboBox选定项

jframe combobox java

Maxxx · 技术社区 · 6 年前

JFrame 图形用户界面拖放元素,我目前正在处理组合框。下面是GUI。

private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {                                         
    for (int i = 0; i<10; i++) {
        String str_rep = Integer.toString(i);
        jComboBox1.addItem(str_rep);
    }
    jComboBox1.setSelectedItem(null);
 }

一旦数字被添加到组合框中,我 将所选项目设置为空 因为我 我不想 组合框 显示第一个元素 一旦按下按钮。

jLabel2 显示基于组合框选择的内容,但一旦按下“按下”按钮,标签将自动显示第一个数字0,即使我将组合框选定项设置为 null .

private void jComboBox1ActionPerformed(java.awt.event.ActionEvent evt) {
    jLabel2.setText(jComboBox1.getSelectedItem().toString());
}

防止标签显示第一个数字 按钮已按下 ? 它应该在我从组合框中选择一个数字之前,它将更新为该数字。

0 回复 | 直到 6 年前

Prasad Karunagoda 6 年前

在 ComboBoxAndLabel 同学们,我试图重现你们的问题。

import javax.swing.*;
import java.awt.BorderLayout;
import java.awt.event.*;

public class ComboBoxAndLabel {

  public static void main(String[] args) {

    JLabel jLabel2 = new JLabel("jLabel2");

    JComboBox<String> jComboBox1 = new JComboBox<>();
    jComboBox1.addActionListener(new ActionListener() {
      @Override
      public void actionPerformed(ActionEvent e) {
        jLabel2.setText(jComboBox1.getSelectedItem().toString());
      }
    });

    JButton button = new JButton("Press");
    button.addActionListener(new ActionListener() {
      @Override
      public void actionPerformed(ActionEvent e) {
        for (int i = 0; i<10; i++) {
          String str_rep = Integer.toString(i);
          jComboBox1.addItem(str_rep);
        }
        jComboBox1.setSelectedItem(null);
      }
    });

    JFrame frame = new JFrame();
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.getContentPane().add(button, BorderLayout.NORTH);
    frame.getContentPane().add(jComboBox1, BorderLayout.CENTER);
    frame.getContentPane().add(jLabel2, BorderLayout.SOUTH);
    frame.setBounds(400, 300, 400, 300);
    frame.setVisible(true);
  }
}

当我运行上面的程序时,我在控制台中看到下面的错误(我想您也会得到这个错误)。

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
  at ComboBoxAndLabel$1.actionPerformed(ComboBoxAndLabel.java:21)
  at javax.swing.JComboBox.fireActionEvent(JComboBox.java:1258)
  at javax.swing.JComboBox.setSelectedItem(JComboBox.java:586)
  at ComboBoxAndLabel$2.actionPerformed(ComboBoxAndLabel.java:35)
  at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
  at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2348)
  at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
  at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
  at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)

jComboBox1.addItem() 方法在组合框为空时设置所选项(请参阅 DefaultComboBoxModel.addElement() jComboBox1.setSelectedItem(null)

但是如果你用 jComboBox1.setModel() jComboBox1.setSelectedItem(空) jButton1ActionPerformed 这样地:

private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {
    DefaultComboBoxModel<String> comboBoxModel = new DefaultComboBoxModel<>();
    for (int i = 0; i<10; i++) {
      String str_rep = Integer.toString(i);
      comboBoxModel.addElement(str_rep);
    }
    jComboBox1.setModel(comboBoxModel);
}

import javax.swing.*;
import java.awt.BorderLayout;
import java.awt.event.*;

public class ComboBoxAndLabelSolution {

  public static void main(String[] args) {

    JLabel jLabel2 = new JLabel("jLabel2");

    JComboBox<String> jComboBox1 = new JComboBox<>();
    jComboBox1.addActionListener(new ActionListener() {
      @Override
      public void actionPerformed(ActionEvent e) {
        jLabel2.setText(jComboBox1.getSelectedItem().toString());
      }
    });

    JButton button = new JButton("Press");
    button.addActionListener(new ActionListener() {
      @Override
      public void actionPerformed(ActionEvent e) {
        DefaultComboBoxModel<String> comboBoxModel = new DefaultComboBoxModel<>();
        for (int i = 0; i<10; i++) {
          String str_rep = Integer.toString(i);
          comboBoxModel.addElement(str_rep);
        }
        jComboBox1.setModel(comboBoxModel);
      }
    });

    JFrame frame = new JFrame();
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.getContentPane().add(button, BorderLayout.NORTH);
    frame.getContentPane().add(jComboBox1, BorderLayout.CENTER);
    frame.getContentPane().add(jLabel2, BorderLayout.SOUTH);
    frame.setBounds(400, 300, 400, 300);
    frame.setVisible(true);
  }
}