在JRadioButtons的ButtonGroup中获得焦点，以转到当前选定的项目，而不是第一个

Question

我在ButtonGroup有一些JRadioButtons 。 这些都在容器内，并且组中还有其他项目。 每当我切换到按钮组时，焦点始终移至该组中的第一项。 但是，我宁愿去选择的项目。

要重现此问题，请使用下面的代码（添加导入，现在忽略我懒得将其全部放入SwingUtilities.invokeLater调用中这一事实）。 向下移至单选按钮，然后向下移至较晚的一项，例如“蓝色”。 再按4次返回单选按钮，您会发现焦点位于顶部的“红色”单选按钮上。 我希望焦点放在“蓝色”单选按钮上。

public static void main(String[] args)
{
    JFrame f = new JFrame("JRadioButton Test");
    f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    Container parent = f.getContentPane();
    parent.setLayout(new GridBagLayout());

    GridBagConstraints gbc = new GridBagConstraints();
    gbc.anchor = GridBagConstraints.NORTHWEST;
    gbc.gridx = 1;
    gbc.gridy = 1;
    parent.add(new JLabel("Start"), gbc);
    gbc.gridx++;
    parent.add(new JTextField(10), gbc);

    gbc.gridx = 1;
    gbc.gridy++;
    parent.add(new JLabel("End"), gbc);
    gbc.gridx++;
    parent.add(new JTextField(10), gbc);

    gbc.gridx = 1;
    gbc.gridy++;
    parent.add(new JLabel("Colors"), gbc);

    gbc.gridx++;
    final Box buttons = Box.createVerticalBox();
    parent.add(buttons, gbc);
    final ButtonGroup bg = new ButtonGroup();
    for (String s : "Red,Orange,Yellow,Green,Blue,Indigo,Violet".split(","))
    {
        JRadioButton radioBtn = new JRadioButton(s);
        buttons.add(radioBtn);
        radioBtn.addItemListener(new ItemListener() {
            @Override
            public void itemStateChanged(ItemEvent e)
            {
                System.out.printf("Itemstate changed to %s\n",
                    e.getStateChange() == ItemEvent.SELECTED ? "SELECTED" : "DESELECTED");
            }
        });
        radioBtn.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e)
            {
                System.out.println("JRadioButton action listener");
            }
        });
        bg.add(radioBtn);
    }

    gbc.gridx = 1;
    gbc.gridy += 2;
    gbc.gridwidth = 2;
    final JLabel currentValue = new JLabel("none");
    parent.add(currentValue, gbc);
    gbc.gridy--;
    JButton btn = new JButton("Show Button Group Value");
    btn.addActionListener(new ActionListener() {
        @Override
        public void actionPerformed(ActionEvent e)
        {
            ButtonModel selection = bg.getSelection();
            for (int i = 0; i < buttons.getComponentCount(); i++)
            {
                JRadioButton radioBtn = (JRadioButton)buttons.getComponent(i);
                ButtonModel loopModel = radioBtn.getModel();
                if (loopModel == selection)
                {
                    currentValue.setText(radioBtn.getText());
                    return;
                }
                currentValue.setText("none");
            }
        }
    });
    parent.add(btn, gbc);

    f.pack();
    f.setVisible(true);
}

Answer 1

您可能可以使用FocusTraversalPolicy ：

buttons.setFocusTraversalPolicyProvider(true);
buttons.setFocusTraversalPolicy(new LayoutFocusTraversalPolicy() {
  @Override public Component getDefaultComponent(Container focusCycleRoot) {
    ButtonModel selection = bg.getSelection();
    for (Component c: focusCycleRoot.getComponents()) {
      JRadioButton radioBtn = (JRadioButton) c;
      ButtonModel loopModel = radioBtn.getModel();
      if (loopModel == selection) {
        return radioBtn;
      }
    }
    return super.getDefaultComponent(focusCycleRoot);
  }
});

• 自定义焦点遍历： 如何使用焦点子系统（Java™教程>使用JFC / Swing创建GUI>使用其他Swing功能）

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

public class ButtonGroupFocusTraversalTest {
  public JComponent makeUI() {
    JPanel parent = new JPanel(new GridBagLayout());
    GridBagConstraints gbc = new GridBagConstraints();
    gbc.anchor = GridBagConstraints.NORTHWEST;
    gbc.gridx = 1;
    gbc.gridy = 1;
    parent.add(new JLabel("Start"), gbc);
    gbc.gridx++;
    parent.add(new JTextField(10), gbc);

    gbc.gridx = 1;
    gbc.gridy++;
    parent.add(new JLabel("End"), gbc);
    gbc.gridx++;
    JTextField textField = new JTextField(10);
    parent.add(textField, gbc);

    gbc.gridx = 1;
    gbc.gridy++;
    parent.add(new JLabel("Colors"), gbc);

    gbc.gridx++;
    final Box buttons = Box.createVerticalBox();
    parent.add(buttons, gbc);
    final ButtonGroup bg = new ButtonGroup();
    for (String s : "Red,Orange,Yellow,Green,Blue,Indigo,Violet".split(",")) {
      JRadioButton radioBtn = new JRadioButton(s);
      buttons.add(radioBtn);
      bg.add(radioBtn);
    }

    gbc.gridx = 1;
    gbc.gridy += 2;
    gbc.gridwidth = 2;
    final JLabel currentValue = new JLabel("none");
    parent.add(currentValue, gbc);
    gbc.gridy--;
    JButton btn = new JButton("Show Button Group Value");
    parent.add(btn, gbc);

    buttons.setFocusTraversalPolicyProvider(true);
    buttons.setFocusTraversalPolicy(new LayoutFocusTraversalPolicy() {
      @Override public Component getDefaultComponent(Container focusCycleRoot) {
        ButtonModel selection = bg.getSelection();
        for (Component c: focusCycleRoot.getComponents()) {
          JRadioButton radioBtn = (JRadioButton) c;
          ButtonModel loopModel = radioBtn.getModel();
          if (loopModel == selection) {
            return radioBtn;
          }
        }
        return super.getDefaultComponent(focusCycleRoot);
      }
    });
    return parent;
  }
  public static void main(String... args) {
    EventQueue.invokeLater(() -> {
      JFrame f = new JFrame();
      f.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
      f.getContentPane().add(new ButtonGroupFocusTraversalTest().makeUI());
      f.setSize(320, 320);
      f.setLocationRelativeTo(null);
      f.setVisible(true);
    });
  }
}

Answer 2

    buttonGroup.setFocus(selectedRadioButton); 

尝试这个。 这将焦点放在selectedRadioButton而不是first