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【第18章：图形界面】_下拉列表框：JComboBox

发布时间：2021-01-18 18:02:14编辑：雪饮阅读（）

 实现一个下拉框

用javax.swing.JcomboBox类可以实例化一个下类框实例
import javax.swing.JFrame ;
import java.awt.Container ;
import java.awt.GridLayout ;
import javax.swing.JComboBox ;
import javax.swing.JLabel ;
import javax.swing.BorderFactory ;
import java.util.Vector ;
class MyComboBox{
       private JFrame frame = new JFrame("Welcome TO kasumi") ;
       private Container cont = frame.getContentPane() ; // 得到容器
       private JComboBox jcb1 = null;
       private JComboBox jcb2 = null ;
       public MyComboBox(){
              this.frame.setLayout(new GridLayout(2,1)) ;     
              String nations[] = {"中国","巴西","美国","韩国","意大利","法国"} ;
              Vector<String> v = new Vector<String>() ;
              v.add("kasumi") ;
              v.add("snowDrink") ;
              v.add("ayane") ;
              // 实例化下拉列表框  
              this.jcb1 = new JComboBox(nations) ;
              this.jcb2 = new JComboBox(v) ;
 
              // 为每一个下拉框增加一个边框
              this.jcb1.setBorder(BorderFactory.createTitledBorder("你喜欢去那个国家旅游？")) ;
              this.jcb2.setBorder(BorderFactory.createTitledBorder("选择你喜欢的角色？")) ;
              cont.add(this.jcb1) ;
              cont.add(this.jcb2) ;
              this.frame.setSize(300,150) ;
              this.frame.setVisible(true) ;
       }
}
public class Hello{
       public static void main(String args[]) throws Exception{
              new MyComboBox() ;
       }
};

使用javax.swing.ComboBoxModel抽象类实现下拉框
import javax.swing.JFrame ;
import java.awt.Container ;
import java.awt.GridLayout ;
import javax.swing.JComboBox ;
import javax.swing.AbstractListModel ;
import javax.swing.BorderFactory ;
import javax.swing.ComboBoxEditor ;
import javax.swing.ComboBoxModel ;
import java.util.Vector ;
class MyComboBoxModel extends AbstractListModel implements ComboBoxModel{
       private String nations[] = {"中国","巴西","美国","韩国","意大利","法国"} ;
       private String item = null ;
       public Object getSelectedItem(){ // 得到选择的选项
              return this.item ;
       }
       public void setSelectedItem(Object anItem){     // 设置选项
              this.item = (String)anItem ;
       }
       public int getSize(){
              return this.nations.length ;
       }
       public Object getElementAt(int ind){
              return this.nations[ind] ;
       }
}
class MyComboBox{
       private String defaultMsg = "请选择或输入国家的名称。" ;
       private JFrame frame = new JFrame("Welcome TO kasumi") ;
       private Container cont = frame.getContentPane() ;
       // 实例化下拉列表框
       private JComboBox jcb1 = new JComboBox(new MyComboBoxModel()) ;
       private JComboBox jcb2 = null ;
       public MyComboBox(){
              this.frame.setLayout(new GridLayout(1,1)) ;      // 定义排版
              //让下拉框可编辑（则可以下拉选择也可以输入，其实下拉框中进行输入的操作我个人比较鄙视，感觉很奇葩的操作）
              this.jcb1.setEditable(true) ;
 
              // 为每一个下拉框增加一个边框
              this.jcb1.setBorder(BorderFactory.createTitledBorder("你喜欢去那个国家旅游？")) ;
              //得到编辑对象才可以指定默认值。使得下拉框可编辑则要给输入处一个编辑器并预置一个默认编辑值
              ComboBoxEditor editor = this.jcb1.getEditor() ;      
              this.jcb1.configureEditor(editor,defaultMsg) ;
              //设置下拉列表项最大可见列表项目数，超过此数就要通过下拉滚动条滚动后才可以看到
              this.jcb1.setMaximumRowCount(3) ;
              cont.add(this.jcb1) ;
              this.frame.pack();
              this.frame.setSize(180,150) ;
              this.frame.setVisible(true) ;
       }
}
public class Hello{
       public static void main(String args[]) throws Exception{
              new MyComboBox() ;
       }
};

下拉框选项选择事件监听
要实现下拉框选项选项事件的监听，需要实现两个接口，接口java.awt.event.ItemListener与接口java.awt.event.ActionListener
import javax.swing.JFrame ;
import java.awt.Container ;
import java.awt.Font ;
import java.awt.event.ActionListener ;
import java.awt.event.ItemListener ;
import java.awt.event.ItemEvent ;
import java.awt.event.ActionEvent ;
import java.awt.GridLayout ;
import javax.swing.JComboBox ;
import javax.swing.JLabel ;
import javax.swing.ComboBoxEditor ;
class MyComboBox implements ItemListener,ActionListener{
       private JFrame frame = new JFrame("Welcome TO kasumi") ;
       private String defaultMsg="输入或选择一个字号";
       private Container cont = frame.getContentPane() ;
       private JComboBox jcb1 = null ;
       private JLabel label = new JLabel("www.kasumi.cn") ;
       private String fontSize[] = {"10","12","14","16","18","20","22","24","26","48","72"} ;
       public MyComboBox(){
              this.frame.setLayout(new GridLayout(2,1)) ;
              this.jcb1 = new JComboBox(fontSize);
              //监听下拉选项或输入值
              this.jcb1.addItemListener(this) ;
              this.jcb1.addActionListener(this) ;
 
              this.jcb1.setEditable(true);
 
 
              ComboBoxEditor editor = this.jcb1.getEditor() ;
              this.jcb1.configureEditor(editor,defaultMsg) ;
              this.jcb1.setMaximumRowCount(3) ;
              //设置默认字号
              this.changeFontSize(12) ;
              cont.add(this.jcb1) ;
 
              cont.add(label) ;
              this.frame.setSize(300,150) ;
              this.frame.setVisible(true) ;
       }
       public void itemStateChanged(ItemEvent e){
              //当发生了状态变化，判断该状态是选中状态还是去除选中状态。每个option都会触发该事件，则这里只针对状态改变为选中状态的进行处理
              if(e.getStateChange()==ItemEvent.SELECTED){
                     String itemSize = (String)e.getItem() ;
                     try{
                            this.changeFontSize(Integer.parseInt(itemSize)) ;
                     }catch(Exception e1){
                           
                     }
              }
       }
       /**
       *一般情况下，这是在编写GUI程序时，组件发生“有意义”的事件时会调用这个方法，比如按钮被按下，文本框内输入回车时都会触发这个事件，然后调用你编写的事件处理程序。
       */
       public void actionPerformed(ActionEvent e){
              String item = (String)this.jcb1.getSelectedItem() ;
              int size = 12 ; // 设置一个默认大小
              try{
                     size = Integer.parseInt(item) ;
              }catch(Exception e1){}
              this.changeFontSize(size) ;
              // 判断给定的size是否存在，如果不存在则需要将新的内容设置上去(用户输入的值不存在于下拉框中则自动将该输入值增加到下拉框中)
              if(!this.isExists(item))
              {
                     this.jcb1.addItem(jcb1.getSelectedItem()) ;
              }
       }
       public boolean isExists(String item){
              boolean flag = false ;
              for(int i=0;i<this.jcb1.getItemCount();i++){
                     //getItemAt方法用于获取下拉框中某个option（根据索引）
                     if(item.equals(this.jcb1.getItemAt(i))){
                            flag = true ;
                     }
              }
              return flag ;
       }
       public void changeFontSize(int size){
              Font fnt = new Font("Serief",Font.BOLD,size) ;
              this.label.setFont(fnt) ;
       }
 
}
public class Hello{
       public static void main(String args[]) throws Exception{
              new MyComboBox() ;
       }
};