JComboBox自定義渲染器泛型-有解無憂

我已經為 JComboBoxes 撰寫了一個渲染器，以便在禁用狀態時用更清晰的顏色替換乳白色。

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

public class RendererDemo extends JFrame {
  public static final long serialVersionUID = 100L;

  String[] items= {"one", "two", "three"};


  public RendererDemo() {
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setSize(300, 100);

    JComboBox<Object> cmb = new JComboBox<>(items);
//    JComboBox<String> cmb = new JComboBox<>(items);
    cmb.setRenderer(new DisabledDarkTextRenderer(cmb));
    add(cmb, BorderLayout.NORTH);

    JCheckBox check = new JCheckBox("Combo enabled", true);
    add(check, BorderLayout.SOUTH);
    check.addActionListener(e -> cmb.setEnabled(check.isSelected()));

    setVisible(true);
  }


  static public void main(String args[]) {
    EventQueue.invokeLater(RendererDemo::new);
  }


  class DisabledDarkTextRenderer extends DefaultListCellRenderer {
    public static final long serialVersionUID = 50378L;

    JComboBox<Object> cmb;
    ListCellRenderer<Object> origRenderer;
    Object defaultResource= UIManager.get("ComboBox.disabledForeground");

    public DisabledDarkTextRenderer(JComboBox<Object> cmb) {
      this.cmb= cmb;
      origRenderer= cmb.getRenderer();
    }

    public Component getListCellRendererComponent(JList<?> list, Object value,
                        int index, boolean isSelected, boolean cellHasFocus) {
      Component orig= origRenderer.getListCellRendererComponent(list, value,
                                            index, isSelected, cellHasFocus);
      if (cmb.isEnabled())
        UIManager.put("ComboBox.disabledForeground", defaultResource);
      else
        UIManager.put("ComboBox.disabledForeground", new ColorUIResource
                                                            (Color.GRAY));
      return orig;
    }
  }

}

但是，當我將 JComboBox 宣告為 <String> 型別時，代碼無法編譯，原因是"incompatible types: ListCellRenderer<CAP#1> cannot be converted to ListCellRenderer<Object> ..."

所以我試圖讓渲染器更通用。在我修改泛型型別的所有嘗試中，以下版本是只剩下一個錯誤的版本。這又是上面提到的錯誤，只是它現在讀取："incompatible types: JList<CAP#1> cannot be converted to JList<? extends CAP#2> ...".

如何解決這個問題？

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

public class RendererDemo2 extends JFrame {
  public static final long serialVersionUID = 100L;

  String[] items= {"one", "two", "three"};


  public RendererDemo2() {
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setSize(300, 100);

    JComboBox<String> cmb = new JComboBox<>(items);
    cmb.setRenderer(new DisabledDarkTextRenderer2(cmb));
    add(cmb, BorderLayout.NORTH);

    JCheckBox check = new JCheckBox("Combo enabled", true);
    add(check, BorderLayout.SOUTH);
    check.addActionListener(e -> cmb.setEnabled(check.isSelected()));

    setVisible(true);
  }


  static public void main(String args[]) {
    EventQueue.invokeLater(RendererDemo2::new);
  }


  class DisabledDarkTextRenderer2 extends DefaultListCellRenderer {
    public static final long serialVersionUID = 50379L;

    JComboBox<?> cmb;
    ListCellRenderer<?> origRenderer;
    Object defaultResource= UIManager.get("ComboBox.disabledForeground");

    public DisabledDarkTextRenderer2(JComboBox<?> cmb) {
      this.cmb= cmb;
      origRenderer= cmb.getRenderer();
    }

    public Component getListCellRendererComponent(JList<?> list, Object value,
            int index, boolean isSelected, boolean cellHasFocus) {
      Component orig= origRenderer.getListCellRendererComponent(list, value,
                    index, isSelected, cellHasFocus);
      if (cmb.isEnabled())
        UIManager.put("ComboBox.disabledForeground", defaultResource);
      else
        UIManager.put("ComboBox.disabledForeground", new ColorUIResource
                                                            (Color.GRAY));
      return orig;
    }
  }

}

按照 MadProgrammer 的建議，我的渲染器現在看起來像這樣：

  class DisabledDarkTextRenderer2<T> extends DefaultListCellRenderer {
    public static final long serialVersionUID = 50379L;

    JComboBox<T> cmb;
    ListCellRenderer<?> origRenderer;
    Object defaultResource= UIManager.get("ComboBox.disabledForeground");

    public DisabledDarkTextRenderer2(JComboBox<T> cmb) {
      this.cmb= cmb;
      origRenderer= cmb.getRenderer();
    }

    public Component getListCellRendererComponent(JList<?> list, Object value,
            int index, boolean isSelected, boolean cellHasFocus) {
      Component orig= origRenderer.getListCellRendererComponent(list, value,
                    index, isSelected, cellHasFocus);
      if (cmb.isEnabled())
        UIManager.put("ComboBox.disabledForeground", defaultResource);
      else
        UIManager.put("ComboBox.disabledForeground", new ColorUIResource
                                                            (Color.GRAY));
      return orig;
    }
  }

但編譯時錯誤仍然存??在。
我正在將此渲染器用于在運行時更改其狀態的組合。因此，我無需在每個程式中都在需要時設定顏色，因為渲染器會自動為我執行此操作。

uj5u.com熱心網友回復：

例如，您需要“通用化”您DisabledDarkTextRenderer2的 .

class DisabledDarkTextRenderer2<T> implements ListCellRenderer<T> {
    public static final long serialVersionUID = 50379L;

    private JComboBox<T> combobox;
    private ListCellRenderer<? super T> origRenderer;

    public DisabledDarkTextRenderer2(JComboBox<T> combobox) {
        this.combobox = combobox;
        this.origRenderer = combobox.getRenderer();
    }
    
    public static <T> void install(JComboBox<T> combobox) {
        combobox.setRenderer(new DisabledDarkTextRenderer2<T>(combobox));
    }

    @Override
    public Component getListCellRendererComponent(JList<? extends T> list, T value, int index, boolean isSelected, boolean cellHasFocus) {
        Component orig = origRenderer.getListCellRendererComponent(list, value, index, isSelected, cellHasFocus);
        if (combobox.isEnabled()) {
            orig.setForeground(null);
        } else {
            orig.setForeground(Color.GRAY);
        }
        return orig;
    }
}

現在，話雖如此，我不鼓勵您UIManager在渲染器中使用，因為這會影響更改后渲染的所有組件，這可能是也可能不是您想要的目標，如果是這樣，那么只需在開始時設定值應用。

以下示例將為所有組合框設定默認禁用的前景色RED，而無需您執行任何其他操作。

import java.awt.Color;
import java.awt.EventQueue;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.border.EmptyBorder;
import javax.swing.plaf.ColorUIResource;

public final class Main {
    public static void main(String[] args) {
        new Main();
    }

    public Main() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                UIManager.put("ComboBox.disabledForeground", new ColorUIResource(Color.RED));
                JFrame frame = new JFrame();
                frame.add(new TestPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class TestPane extends JPanel {
        public TestPane() {
            setLayout(new GridBagLayout());
            setBorder(new EmptyBorder(32, 32, 32, 32));

            JComboBox<String> cmb = new JComboBox<>(new String[]{"Hello", "Goodbye"});
            cmb.setSelectedIndex(0);

            GridBagConstraints gbc = new GridBagConstraints();
            gbc.gridwidth = GridBagConstraints.REMAINDER;

            add(cmb, gbc);

            JButton toggle = new JButton("Toggle");
            toggle.addActionListener(new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent e) {
                    cmb.setEnabled(!cmb.isEnabled());
                }
            });
            add(toggle, gbc);
        }
    }

}

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