《》ListCellRenderer interface 实例

在上一节中我们利用ListCellRenderer   interface在JList中加入Icon图像，而要在JComboBox中加入图像的方法也是一样的。
我们必须实作ListCellRenderer interface所定义的方法getListCellRendererComponent.以下为这个方法的定义：
要先了解ListCellRenderer interface.我们必须由这个interface所定义的方法，将图像画在JComboBox中的每个项目。
ListCellRenderer interface里只定义了一个方法，那就是getListCellRendererComponent,不过这个参数有点多，我们把它列出来
看看:
public Component getListCellRendererComponent(JList list,
                                               Object value,
                                               int index,
                                               boolean isSelected,
                                               boolean cellHasFocus)
list:即所要画上的图像的JComboBox组件。
value:JComboBox项目值，如JComboBox.getModel().getElementAt(index)所返回的值。
index:为JComboBox项目的索引值，由0开始。
isSelected与cellHasFocus:判断JComboBox中的项目是否有被选取或是有焦点置入。
   上面这4个参数会在你设置JComboBox的绘图样式(setCellRenderer())时自动的由JComboBox组件提供，你只要关心怎么控制
getListCellRendererComponent()方法中的4个参数，而无需担心怎么参数传入。
    要在JList中加入Icon图像的技巧就是将JComboBox中的每一个项目当作是JLabel,因为JLabel在使用文字与图像上非常的方便，要设置JComboBox的图像，
必须使用setRenderer(ListCellRenderer cellRenderer){注:我们在JList中画上图像是利用JList所提供的setCellRenderer(ListCellRenderer 
cellRenderer)方法,读者请小心}这个方法。我们来看下面这个范例，你就能明白了!

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

public class JComboBox4{
   String[] s={"西瓜","苹果","草莓","香蕉","葡萄"}; 
   public JComboBox4(){
   JFrame f=new JFrame("JComboBox");
   Container contentPane=f.getContentPane();
  
   JComboBox combo=new JComboBox(s);
   combo.setBorder(BorderFactory.createTitledBorder("你最喜欢吃哪些水果?"));
   combo.setRenderer(new ACellRenderer());
   combo.setMaximumRowCount(3);
  
   contentPane.add(combo);
       f.pack();
       f.show();
       f.addWindowListener(new WindowAdapter(){
     public void windowClosing(WindowEvent e){
       System.exit(0); 
     }
       });     
   }
   public static void main(String[] args){
     new JComboBox4();
   }
}
   class ACellRenderer extends JLabel implements ListCellRenderer{
   ACellRenderer(){
      setOpaque(true); 
   }
   public Component getListCellRendererComponent(JList   list,
                                                 Object value,
                                                 int index,
                                                 boolean isSelected,
                                                 boolean cellHasFocus){
       if (value!=null){
         setText(value.toString());
         setIcon(new ImageIcon(".\\icons\\fruit"+(index+1)+".jpg"));
       }  
       if (isSelected){
          setBackground(list.getSelectionBackground());
          setForeground(list.getSelectionForeground()); 
       }else{
          setBackground(list.getBackground()); 
          setForeground(list.getForeground());
       }                                    
       return this;       
     }                                               
   }
     各们读者在运行这个程序时会发现，即使JComboBox的选项中有图标，但在选后图标却不会显示在显示列中，原因是在上面程序中
我们以String Array s建立JComboBox:
       JComboBox combo=new JComboBox(s);
       String Array s里面放的只是水果名称，而并没有图标。当我们使用setRenderer()方法来JComboBox时，只会绘制JComboBox的
选项部份，而最后显示在JComboBox上的值还是以String Array s为依据。因此JComboBox显示列就只会显示文字而已，而不会显示出
图形。要解决这个问题，我们必须改变JComboBox所传入的参数内容，也就是将原来的String Array s更改成具有图形的数据项。在
此我们是利用JComboBox(Object[] items)来建立有图像的JComboBox,我们所传进去的Object Array不应该只有文字，而必须连图标一
并传入。我们修改上个范例修改如下:

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

public class JComboBox5
{
     String[] s = {"西瓜","苹果","草莓","香蕉","葡萄"};
     ImageIcon[] icons = new ImageIcon[5];;
    
     public JComboBox5()
     {
         JFrame f = new JFrame("JComboBox");
         Container contentPane = f.getContentPane();
         ItemObj[] obj = new ItemObj[5];
        
         for(int i=0; i < 5; i++)
         {
             icons[i] = new ImageIcon(".\\icons\\fruit"+(i+1)+".jpg");
             obj[i] = new ItemObj(s[i],icons[i]);
         }
        
         JComboBox combo = new JComboBox(obj);//利用ItemObj Array obj当作是JComboBox的参数传入，构造出JComboBox.
         combo.setBorder(BorderFactory.createTitledBorder("您喜欢吃哪些水果？"));
         combo.setRenderer(new ACellRenderer());
         combo.setMaximumRowCount(3);
        
         contentPane.add(combo);
         f.pack();
         f.show();
         f.addWindowListener(new WindowAdapter() {
             public void windowClosing(WindowEvent e) {
                     System.exit(0);
             }
         });
     }
    
     public static void main(String args[])
     {
         new JComboBox5();
     }
}

class ItemObj
{
     String name;
     ImageIcon icon;
    
     public ItemObj(String name, ImageIcon icon){
         this.name = name;
         this.icon = icon;
     }
}
    
class ACellRenderer extends JLabel implements ListCellRenderer
{
     ACellRenderer()
     {
         setOpaque(true);
     }
    
     public Component getListCellRendererComponent(JList list,
                                                   Object value,
                                                   int index,
                                                   boolean isSelected,
                                                   boolean cellHasFocus)
     {
         if (value != null)
         {
             setText(((ItemObj)value).name);
             setIcon(((ItemObj)value).icon);
         }

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

         return this;
     }    
}

    你可以发现，第一栏显示有图标显示出来了。当然你也可以利用ComboBoxModel方式来构造出有图标的JComboBox.我们来看下面
的例子:

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

public class JComboBox6{
   String[] s={"西瓜","苹果","草莓","香蕉","葡萄"};
   ImageIcon[] icons=new ImageIcon[5];
   public JComboBox6(){
     JFrame f=new JFrame("JComboBox"); 
     Container contentPane=f.getContentPane();
         for(int i=0; i < 5; i++)
         {
             icons[i] = new ImageIcon(".\\icons\\fruit"+(i+1)+".jpg");
         }
         ComboBoxModel mode=new AModel();
         JComboBox combo=new JComboBox(mode);
         combo.setBorder(BorderFactory.createTitledBorder("您喜欢吃哪些水果？"));
         combo.setRenderer(new ACellRenderer());
         combo.setMaximumRowCount(3);
        
         contentPane.add(combo);
         f.pack();
         f.show();
         f.addWindowListener(new WindowAdapter() {
             public void windowClosing(WindowEvent e) {
                     System.exit(0);
             }
         });
   } 
   public static void main(String[] args){
   new JComboBox6();
   }
/*我们用JComboBox(ComboBoxModel aModel)来构造图标的JComboBox,因此我们在程序中编写一个继承DefaultComboBoxModel的
ComboBoxModel.
*/
   class AModel extends DefaultComboBoxModel{
   AModel(){
      for (int i=0;i<s.length;i++){
         ItemObj obj=new ItemObj(s[i],icons[i]); 
         addElement(obj);
      } 
   }
   }
}
class ItemObj
{
     String name;
     ImageIcon icon;
    
     public ItemObj(String name, ImageIcon icon){
         this.name = name;
         this.icon = icon;
     }
}
    
class ACellRenderer extends JLabel implements ListCellRenderer
{
     ACellRenderer()
     {
         setOpaque(true);
     }
    
     public Component getListCellRendererComponent(JList list,
                                                   Object value,
                                                   int index,
                                                   boolean isSelected,
                                                   boolean cellHasFocus)
     {
         if (value != null)
         {
             setText(((ItemObj)value).name);
             setIcon(((ItemObj)value).icon);
         }

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

         return this;
     }    
}

    我们用JComboBox(ComboBoxModel aModel)来构造图标的JComboBox,因此我们在程序中编写一个继承DefaultComboBoxModel的
ComboBoxModel.