Java Swing，无法显示下拉菜单

Question

我不知道为什么我的Combobox不会出现，我第一次使用摇摆不知道多少。但是我正在尝试使用Jcombobox来实现下拉菜单。

当我运行它时，它似乎似乎没有显示我的下拉菜单（JCombobox），就像我的其他对象，如textfields和lables。

     import javax.swing.*;
     import java.awt.Color;
     import java.awt.event.ActionListener;
     import java.awt.event.ActionEvent;
     import java.awt.*;    

       public class Login implements ActionListener {

    JPanel textPanel, panelForTextFields, completionPanel;
    JLabel titleLabel, usernameLabel, passwordLabel, userLabel, passLabel;
    JTextField usernameField, loginField;
    JButton loginButton;

    //
    JComboBox colorChooser;
    JPanel redBox, blueBox, greenBox, yellowBox;

    public JPanel createContentPane() {

        // We create a bottom JPanel to place everything on.
        JPanel totalGUI = new JPanel();
        totalGUI.setLayout(null);

        //////////////////////////////////////////////////////////////////////

        // To create a JComboBox, we need to pass in an array of Strings.
        // This gives the ComboBox the list of selections you can make.    
        String colors[] = {"Red", "Blue", "Green", "Yellow"};            
        colorChooser = new JComboBox(colors);
        colorChooser.setSelectedIndex(1);
        colorChooser.addActionListener(this);            
        // Now we create a simple JPanel that displays our four coloured boxes.    
        JPanel boxPanel = new JPanel(new GridLayout(2, 2, 20, 20));    
        redBox = createSquareJPanel(Color.red, 50);
        blueBox = createSquareJPanel(Color.blue, 50);
        greenBox = createSquareJPanel(Color.green, 50);
        yellowBox = createSquareJPanel(Color.yellow, 50);    
        // This sets all bar the blue box to be hidden.    
        redBox.setVisible(false);
        greenBox.setVisible(false);
        yellowBox.setVisible(false);    
        boxPanel.add(redBox);
        boxPanel.add(blueBox);
        boxPanel.add(greenBox);
        boxPanel.add(yellowBox);    
        // This sets the widgets on the screen to be layed out in a
        // top to bottom fashion with spacers inbetween.            
        JPanel bottomPanel = new JPanel();
        bottomPanel.setLayout(new BoxLayout(bottomPanel, BoxLayout.PAGE_AXIS));            
        bottomPanel.add(Box.createRigidArea(new Dimension(0,10)));
        bottomPanel.add(colorChooser);
        bottomPanel.add(Box.createRigidArea(new Dimension(0,20)));
        bottomPanel.add(boxPanel);
        bottomPanel.add(Box.createRigidArea(new Dimension(0,10)));            
        totalGUI.add(bottomPanel);          
        ////////////////////////////////////
        titleLabel = new JLabel("User Login Screen");
        titleLabel.setLocation(0, 0);
        titleLabel.setSize(290, 30);
        titleLabel.setHorizontalAlignment(0);
        totalGUI.add(titleLabel);

        // Creation of a Panel to contain the JLabels
        textPanel = new JPanel();
        textPanel.setLayout(null);
        textPanel.setLocation(10, 35);
        textPanel.setSize(70, 80);
        totalGUI.add(textPanel);

        // Username Label
        usernameLabel = new JLabel("Username");
        usernameLabel.setLocation(0, 0);
        usernameLabel.setSize(70, 40);
        usernameLabel.setHorizontalAlignment(4);
        textPanel.add(usernameLabel);

        // Login Label
        passwordLabel = new JLabel("Password");
        passwordLabel.setLocation(0, 40);
        passwordLabel.setSize(70, 40);
        passwordLabel.setHorizontalAlignment(4);
        textPanel.add(passwordLabel);

        // TextFields Panel Container
        panelForTextFields = new JPanel();
        panelForTextFields.setLayout(null);
        panelForTextFields.setLocation(110, 40);
        panelForTextFields.setSize(100, 70);
        totalGUI.add(panelForTextFields);

        // Username Textfield
        usernameField = new JTextField(8);
        usernameField.setLocation(0, 0);
        usernameField.setSize(100, 30);
        panelForTextFields.add(usernameField);

        // Login Textfield
        loginField = new JTextField(8);
        loginField.setLocation(0, 40);
        loginField.setSize(100, 30);
        panelForTextFields.add(loginField);

        // Creation of a Panel to contain the completion JLabels
        completionPanel = new JPanel();
        completionPanel.setLayout(null);
        completionPanel.setLocation(240, 35);
        completionPanel.setSize(70, 80);
        totalGUI.add(completionPanel);

        // Username Label
        userLabel = new JLabel("Wrong");
        userLabel.setForeground(Color.red);
        userLabel.setLocation(0, 0);
        userLabel.setSize(70, 40);
        completionPanel.add(userLabel);

        // Login Label
        passLabel = new JLabel("Wrong");
        passLabel.setForeground(Color.red);
        passLabel.setLocation(0, 40);
        passLabel.setSize(70, 40);
        completionPanel.add(passLabel);

        // Button for Logging in
        loginButton = new JButton("Login");
        loginButton.setLocation(130, 120);
        loginButton.setSize(80, 30);
        loginButton.addActionListener(this);
        totalGUI.add(loginButton);

        totalGUI.setOpaque(true);
        return totalGUI;
    }

    // With this action performed, we simply check to see if the username and
    // password match "Bob" as the username and "Robert" as the password.
    // If they do, we set the labels ajacent to them to "Correct!" and color
    // them green.
    // At the end, we check if both labels are green. If they are, we set the
    // screen to be 'Logging In'.

    public void actionPerformed(ActionEvent e) {
         int temp;

            if(e.getSource() == colorChooser)
            {
                temp = colorChooser.getSelectedIndex();

                switch(temp){
                case 0: 
                    redBox.setVisible(true);
                    blueBox.setVisible(false);
                    greenBox.setVisible(false);
                    yellowBox.setVisible(false);
                    break;
                case 1:
                    redBox.setVisible(false);
                    blueBox.setVisible(true);
                    greenBox.setVisible(false);
                    yellowBox.setVisible(false);
                    break;
                case 2:
                    redBox.setVisible(false);
                    blueBox.setVisible(false);
                    greenBox.setVisible(true);
                    yellowBox.setVisible(false);
                    break;
                case 3:
                    redBox.setVisible(false);
                    blueBox.setVisible(false);
                    greenBox.setVisible(false);
                    yellowBox.setVisible(true);
                    break;
                }
            }
        if (e.getSource() == loginButton) {
            if (usernameField.getText().trim().compareTo("Bob") == 0) {
                userLabel.setForeground(Color.green);
                userLabel.setText("Correct!");
            } else {
                userLabel.setForeground(Color.red);
                userLabel.setText("Wrong!");
            }

            if (loginField.getText().trim().compareTo("Robert") == 0) {
                passLabel.setForeground(Color.green);
                passLabel.setText("Correct!");
            } else {
                passLabel.setForeground(Color.red);
                passLabel.setText("Wrong!");
            }

            if ((userLabel.getForeground() == Color.green)
                    && (passLabel.getForeground() == Color.green)) {
                titleLabel.setText("Logging in....");
                loginButton.setEnabled(false);
            }
        }
    }

    private static void createAndShowGUI() {
        JFrame.setDefaultLookAndFeelDecorated(true);
        JFrame frame = new JFrame("Login");    
        Login demo = new Login();
        frame.setContentPane(demo.createContentPane());    
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(400, 400);
        frame.setVisible(true);
    }

    private JPanel createSquareJPanel(Color color, int size) {
        JPanel tempPanel = new JPanel();
        tempPanel.setBackground(color);
        tempPanel.setMinimumSize(new Dimension(size, size));
        tempPanel.setMaximumSize(new Dimension(size, size));
        tempPanel.setPreferredSize(new Dimension(size, size));
        return tempPanel;
    }

    public static void main(String[] args) {
        // Schedule a job for the event-dispatching thread:
        // creating and showing this application's GUI.
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI();
            }
        });
    }
}'

Answer 1

我建议，你的问题从这里开始......

totalGUI.setLayout(null);

使用适当的布局管理器。这些将允许您开发跨平台应用程序，这些应用程序将具有更大的容量来处理不同平台呈现字体之类的方式的变化......

示例

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.Box;
import javax.swing.BoxLayout;
import javax.swing.JButton;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;

public class Login implements ActionListener {

    JPanel textPanel, panelForTextFields, completionPanel;
    JLabel titleLabel, usernameLabel, passwordLabel, userLabel, passLabel;
    JTextField usernameField, loginField;
    JButton loginButton;

//
    JComboBox colorChooser;
    JPanel redBox, blueBox, greenBox, yellowBox;

    public JPanel createContentPane() {

        // We create a bottom JPanel to place everything on.
        JPanel totalGUI = new JPanel();
        totalGUI.setLayout(new BorderLayout());

        //////////////////////////////////////////////////////////////////////
        // To create a JComboBox, we need to pass in an array of Strings.
        // This gives the ComboBox the list of selections you can make.
        String colors[] = {"Red", "Blue", "Green", "Yellow"};

        colorChooser = new JComboBox(colors);
        colorChooser.setSelectedIndex(1);
        colorChooser.addActionListener(this);

        // Now we create a simple JPanel that displays our four coloured boxes.
        JPanel boxPanel = new JPanel(new GridLayout(2, 2, 20, 20));

        redBox = createSquareJPanel(Color.red, 50);
        blueBox = createSquareJPanel(Color.blue, 50);
        greenBox = createSquareJPanel(Color.green, 50);
        yellowBox = createSquareJPanel(Color.yellow, 50);

        // This sets all bar the blue box to be hidden.
        redBox.setVisible(false);
        greenBox.setVisible(false);
        yellowBox.setVisible(false);

        boxPanel.add(redBox);
        boxPanel.add(blueBox);
        boxPanel.add(greenBox);
        boxPanel.add(yellowBox);

        // This sets the widgets on the screen to be layed out in a
        // top to bottom fashion with spacers inbetween.
        JPanel bottomPanel = new JPanel();
        bottomPanel.setLayout(new BoxLayout(bottomPanel, BoxLayout.PAGE_AXIS));

        bottomPanel.add(Box.createRigidArea(new Dimension(0, 10)));
        bottomPanel.add(colorChooser);
        bottomPanel.add(Box.createRigidArea(new Dimension(0, 20)));
        bottomPanel.add(boxPanel);
        bottomPanel.add(Box.createRigidArea(new Dimension(0, 10)));

        totalGUI.add(bottomPanel, BorderLayout.SOUTH);

        JPanel loginPane = new JPanel(new GridBagLayout());
        GridBagConstraints gbc = new GridBagConstraints();
        gbc.gridx = 0;
        gbc.gridy = 0;
        gbc.gridwidth = GridBagConstraints.REMAINDER;

        ////////////////////////////////////
        titleLabel = new JLabel("User Login Screen");
        titleLabel.setHorizontalAlignment(0);
        loginPane.add(titleLabel, gbc);

        gbc.gridwidth = 1;
        gbc.gridy++;

        // Username Label
        usernameLabel = new JLabel("Username");
        usernameLabel.setHorizontalAlignment(4);
        loginPane.add(usernameLabel, gbc);

        gbc.gridx++;

        // Username Textfield
        usernameField = new JTextField(8);
        loginPane.add(usernameField, gbc);

        gbc.gridx++;

        // Username Label
        userLabel = new JLabel("Wrong");
        userLabel.setForeground(Color.red);
        loginPane.add(userLabel, gbc);

        gbc.gridx = 0;
        gbc.gridy++;

        // Login Label
        passwordLabel = new JLabel("Password");
        passwordLabel.setHorizontalAlignment(4);
        loginPane.add(passwordLabel, gbc);

        gbc.gridx++;

        // Login Textfield
        loginField = new JTextField(8);
        loginPane.add(loginField, gbc);

        gbc.gridx++;

        // Login Label
        passLabel = new JLabel("Wrong");
        passLabel.setForeground(Color.red);
        loginPane.add(passLabel, gbc);

        gbc.gridx = 0;
        gbc.gridy++;
        gbc.gridwidth = GridBagConstraints.REMAINDER;

        // Button for Logging in
        loginButton = new JButton("Login");
        loginButton.addActionListener(this);
        loginPane.add(loginButton, gbc);

        totalGUI.add(loginPane);

        totalGUI.setOpaque(true);
        return totalGUI;
    }

// With this action performed, we simply check to see if the username and
// password match "Bob" as the username and "Robert" as the password.
// If they do, we set the labels ajacent to them to "Correct!" and color
// them green.
// At the end, we check if both labels are green. If they are, we set the
// screen to be 'Logging In'.
    public void actionPerformed(ActionEvent e) {
        int temp;

        if (e.getSource() == colorChooser) {
            temp = colorChooser.getSelectedIndex();

            switch (temp) {
                case 0:
                    redBox.setVisible(true);
                    blueBox.setVisible(false);
                    greenBox.setVisible(false);
                    yellowBox.setVisible(false);
                    break;
                case 1:
                    redBox.setVisible(false);
                    blueBox.setVisible(true);
                    greenBox.setVisible(false);
                    yellowBox.setVisible(false);
                    break;
                case 2:
                    redBox.setVisible(false);
                    blueBox.setVisible(false);
                    greenBox.setVisible(true);
                    yellowBox.setVisible(false);
                    break;
                case 3:
                    redBox.setVisible(false);
                    blueBox.setVisible(false);
                    greenBox.setVisible(false);
                    yellowBox.setVisible(true);
                    break;
            }
        }
        if (e.getSource() == loginButton) {
            if (usernameField.getText().trim().compareTo("Bob") == 0) {
                userLabel.setForeground(Color.green);
                userLabel.setText("Correct!");
            } else {
                userLabel.setForeground(Color.red);
                userLabel.setText("Wrong!");
            }

            if (loginField.getText().trim().compareTo("Robert") == 0) {
                passLabel.setForeground(Color.green);
                passLabel.setText("Correct!");
            } else {
                passLabel.setForeground(Color.red);
                passLabel.setText("Wrong!");
            }

            if ((userLabel.getForeground() == Color.green)
                    && (passLabel.getForeground() == Color.green)) {
                titleLabel.setText("Logging in....");
                loginButton.setEnabled(false);
            }
        }
    }

    private static void createAndShowGUI() {

        JFrame.setDefaultLookAndFeelDecorated(true);
        JFrame frame = new JFrame("Login");

        Login demo = new Login();
        frame.setContentPane(demo.createContentPane());

        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(400, 400);
        frame.setVisible(true);

    }

    private JPanel createSquareJPanel(Color color, int size) {
        JPanel tempPanel = new JPanel();
        tempPanel.setBackground(color);
        tempPanel.setMinimumSize(new Dimension(size, size));
        tempPanel.setMaximumSize(new Dimension(size, size));
        tempPanel.setPreferredSize(new Dimension(size, size));
        return tempPanel;
    }

    public static void main(String[] args) {
        // Schedule a job for the event-dispatching thread:
        // creating and showing this application's GUI.
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI();
            }
        });
    }
}

Answer 2

阅读How to Use Combo Boxes上的Swing教程中的部分以获取工作示例。

这是从一个简单的例子开始，让它运作起来。然后添加几个组件并使其正常工作。不要编写整个程序并开始测试，特别是当您不熟悉该语言时。

不要使用null布局。 Swing旨在与布局管理器一起使用。