如何使用具有null值和使用泛型类型的JComboBox

Question

我使用带有泛型的JComboBox（Java 7），这非常有用，因为我不需要一直进行强制转换，而且我使用自定义渲染来正确显示对象。

当我在抛出java.lang.ClassCastException时添加一个null值时，问题就来了。我的ListCellRenderer可以，它可以处理null值，但这个异常在它渲染之前发生。

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

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转换为空白空间String，因此类抛出异常。

我唯一能想到的就是让我的rederer接受Object类型，并在那里检查值的类，如果它是" "，假设它像null（我认为它不是很正确）。

这一行抛出异常Component component = renderer.getListCellRendererComponent(listBox, value, -1, false, false);，因为值是String，并且呈现的仅接受MyClass对象。

能做些什么？这是避免这种转换的一种方法吗？

---

这是一个类似于我使用的代码：

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Visualizador类：

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;
    }
}

这是异常抛出：

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

我发现这是jvm实现中的一个bug。您可以在中查看报告https://bugs.openjdk.java.net/browse/JDK-7195179不幸的是，它似乎是为Java 8修复的。

我找到的唯一解决办法是将Visualizador更改为也接受字符串。

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;
    }
}