如何使用具有空值的 JComboBox 并使用泛型类型？

Question

我将 JComboBox 与泛型 (Java 7) 一起使用，这非常有用，因为我不需要一直进行转换，而且我还使用自定义渲染来正确显示对象。

当我添加一个空值时，我的问题就来了，因为它抛出 java.lang.ClassCastException。我的 ListCellRenderer 没问题，它可以处理 null 值，但是这个异常发生在它可以渲染它之前。

我发现 javax.swing.plaf.basic.BasicComboBoxUI 这样做：

if (value == null) {
      value = " ";
} else if (value instanceof String && "".equals(value)) {
      value = " ";
}
Component component = renderer.getListCellRendererComponent(listBox, value, -1, false, false);

因此它将 null 转换为白色 space String，因此 class 强制转换异常。

我唯一能想到的就是让我的 rederer 接受对象类型并在那里检查值的 class 如果它是 " " 假设就像 null （我认为它不是很正确）。

此行抛出异常 Component component = renderer.getListCellRendererComponent(listBox, value, -1, false, false);，因为值是一个字符串，并且呈现的只接受 MyClass 对象。

可以做什么？这是避免这种转换的方法吗？

这是我使用的代码：

JComboBox plantillaMarcoB = new JComboBox<MyClass>();
plantillaMarcoB.setRenderer(new Visualizador);
DefaultComboBoxModel<MyClass> modeloA = new DefaultComboBoxModel<MyClass>();
modeloB.addElement(null);
modeloB.addElement(new MyClass());
this.plantillaMarcoB.setModel(modeloB);

这是可视化工具 class:

package es.phoneixs;

import java.awt.Component;

import javax.swing.JComboBox;
import javax.swing.JLabel;
import javax.swing.JList;
import javax.swing.ListCellRenderer;

import es.phoneixs.MyClass;


public class Visualizador extends JLabel implements
        ListCellRenderer<MyClass> {

    public Visualizador() {

        setOpaque(true);
        setVerticalAlignment(CENTER);

    }

    @Override
    public Component getListCellRendererComponent(
            JList<? extends MyClass> list,
            MyClass value, int index, boolean isSelected,
            boolean cellHasFocus) {

        this.setFont(list.getFont());

        if (isSelected) {
            setBackground(list.getSelectionBackground());
            setForeground(list.getSelectionForeground());
        } else {
            setBackground(list.getBackground());
            setForeground(list.getForeground());
        }

        if (value == null) {

            this.setText("No usar la plantilla");

        } else {

            this.setText(String.format("%1$s - %2$s - %3$s",  value.getDescripcionCas(), value.getCodigo(), value.getTurno()));

        }

        return this;

    }

}

这是抛出的异常：

Exception in thread "AWT-EventQueue-0" java.lang.ClassCastException: java.lang.String cannot be cast to es.phoneixs.MyClass
    at es.phoneixs.Visualizador.getListCellRendererComponent(Visualizador.java:1)
    at javax.swing.plaf.basic.BasicComboBoxUI.getBaseline(BasicComboBoxUI.java:979)
    at javax.swing.plaf.metal.MetalComboBoxUI.getBaseline(MetalComboBoxUI.java:149)
    at javax.swing.JComponent.getBaseline(JComponent.java:2561)
    at javax.swing.GroupLayout$ComponentSpring.getBaseline(GroupLayout.java:3075)
    at javax.swing.GroupLayout$BaselineGroup.calculateBaselineAndResizeBehavior(GroupLayout.java:2732)
    at javax.swing.GroupLayout$BaselineGroup.calculateSize(GroupLayout.java:2708)
    at javax.swing.GroupLayout$Group.calculatePreferredSize(GroupLayout.java:1602)
    at javax.swing.GroupLayout$Spring.getPreferredSize(GroupLayout.java:1346)
    at javax.swing.GroupLayout$Group.getSpringSize(GroupLayout.java:1638)
    at javax.swing.GroupLayout$Group.calculateSize(GroupLayout.java:1627)
    at javax.swing.GroupLayout$Group.calculatePreferredSize(GroupLayout.java:1602)
    at javax.swing.GroupLayout$Spring.getPreferredSize(GroupLayout.java:1346)
    at javax.swing.GroupLayout$Group.getSpringSize(GroupLayout.java:1638)
    at javax.swing.GroupLayout$Group.calculateSize(GroupLayout.java:1622)
    at javax.swing.GroupLayout$Group.calculatePreferredSize(GroupLayout.java:1602)
    at javax.swing.GroupLayout$Spring.getPreferredSize(GroupLayout.java:1346)
    at javax.swing.GroupLayout$Group.getSpringSize(GroupLayout.java:1638)
    at javax.swing.GroupLayout$Group.calculateSize(GroupLayout.java:1622)
    at javax.swing.GroupLayout$Group.calculatePreferredSize(GroupLayout.java:1602)
    at javax.swing.GroupLayout$Spring.getPreferredSize(GroupLayout.java:1346)
    at javax.swing.GroupLayout$SequentialGroup.setValidSize(GroupLayout.java:2017)
    at javax.swing.GroupLayout$Group.setSize(GroupLayout.java:1587)
    at javax.swing.GroupLayout.calculateAutopadding(GroupLayout.java:1079)
    at javax.swing.GroupLayout.layoutContainer(GroupLayout.java:920)
    at java.awt.Container.layout(Container.java:1503)
    at java.awt.Container.doLayout(Container.java:1492)
    at java.awt.Container.validateTree(Container.java:1688)
    at java.awt.Container.validateTree(Container.java:1697)
    at java.awt.Container.validate(Container.java:1623)
    at java.awt.CardLayout.show(CardLayout.java:547)

Answer 1

我发现是jvm实现的bug。您可以在 https://bugs.openjdk.java.net/browse/JDK-7195179 中看到报告，不幸的是它似乎已针对 Java 8.

修复

我发现的唯一解决方法是更改 Visualizador 以也接受该字符串。

package es.phoneixs;

import java.awt.Component;

import javax.swing.JComboBox;
import javax.swing.JLabel;
import javax.swing.JList;
import javax.swing.ListCellRenderer;

import es.phoneixs.MyClass;


public class Visualizador extends JLabel implements
        ListCellRenderer<Object> {

    public Visualizador() {

        setOpaque(true);
        setVerticalAlignment(CENTER);

    }

    @Override
    public Component getListCellRendererComponent(
            JList<? extends Object> list,
            Object value, int index, boolean isSelected,
            boolean cellHasFocus) {

        this.setFont(list.getFont());

        if (isSelected) {
            setBackground(list.getSelectionBackground());
            setForeground(list.getSelectionForeground());
        } else {
            setBackground(list.getBackground());
            setForeground(list.getForeground());
        }

        if (value == null || (value instanceof String && value.equals(" "))) {

            this.setText("No usar la plantilla");

        } else if (value instanceof MyClass) {

            this.setText(String.format("%1$s - %2$s - %3$s",  value.getDescripcionCas(), value.getCodigo(), value.getTurno()));

        } else {
            this.setText(value.toString());
        }

        return this;

    }

}

如何使用具有空值的 JComboBox 并使用泛型类型？

How to use a JComboBox with null value and using generic types?

java

generics

null

classcastexception

jcombobox