ItemListener不起作用

Question

爲什麼我的組合框不執行我在ItemListener中聲明的事物？當我點擊組合框內的一個項目時，程序掛起，我需要關閉整個BlueJ。請大家看看什麼是錯在我的代碼

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

public class HulaHoops { 

private Scanner inp = new Scanner(System.in); 

public static void main(String[]args) { 
    EventQueue.invokeLater(new Runnable() 
    { 
     @Override 
     public void run() 
     { 
      new HulaHoops(); 
     } 
    }); 
} 

public HulaHoops() { 
    JFrame frame = new JFrame(); 
    JPanel panel = new JPanel(); 
    String choices[] = {"Shoes","Comb","Ball"}; 
    JComboBox combo = new JComboBox(choices); 
    combo.setBackground(Color.gray); 
    combo.setForeground(Color.red); 
    panel.add(combo); 
    frame.add(panel); 
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
    frame.setSize(300,100); 
    frame.setVisible(true); 


    combo.addItemListener(new ItemListener() { 
     @Override 
     public void itemStateChanged(ItemEvent e) 
     { 
      String item = (String)e.getItem(); 
      if (e.getStateChange() == ItemEvent.SELECTED) 
      { 
       System.out.println("You chose " +item); 
        if (item == "Shoes") 
        { 
         System.out.println("Enter quantity: "); 
         int bilang = inp.nextInt(); 
         int total = bilang * 99; 
         String order = bilang + " " + "Shoes " + "  " + total; 
         System.out.print("" + order); 
        } 

        else if (item == "Comb") 
        { 
         System.out.println("Enter quantity: "); 
         int bilang = inp.nextInt(); 
         int total1 = bilang * 99; 
         String order1 = bilang + " " + "Comb " + "  " + total1; 
         System.out.print("" + order1); 
        } 

        else if (item == "Ball") 
        { 
         System.out.println("Enter quantity: "); 
         int bilang = inp.nextInt(); 
         int total2 = bilang * 99; 
         String order2 = bilang + " " + "Ball " + "  " + total2; 
         System.out.print("" + order2); 
        } 
      } 

     } 
    }); 
} 

}

Answer 1

，你正在做的是使用==操作比較兩個String對象的基本錯誤。
應使用下列條件改爲：

if (item .equals("Shoes")) 
if (item .equals("Comb")) 
if (item .equals("Ball")) 

由於JOptionPane的一個簡短的演示..我在這裏把更改的版本代碼的...我希望它會幫助你瞭解如何處理這種情況正常...

//combo box, the program hangs I need to close the entire BlueJ. Please take a look what is wrong in my code 
import java.awt.*; 
import java.awt.event.*; 
import javax.swing.*; 
import java.util.*; 

public class HulaHoops { 

private Scanner inp = new Scanner(System.in); 

public static void main(String[]args) { 
    EventQueue.invokeLater(new Runnable() 
    { 
     @Override 
     public void run() 
     { 
      new HulaHoops(); 
     } 
    }); 
} 

public HulaHoops() { 
    JFrame frame = new JFrame(); 
    JPanel panel = new JPanel(); 
    String choices[] = {"Shoes","Comb","Ball"}; 
    JComboBox combo = new JComboBox(choices); 
    combo.setBackground(Color.gray); 
    combo.setForeground(Color.red); 
    panel.add(combo); 
    frame.add(panel); 
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
    frame.setSize(300,100); 
    frame.setVisible(true); 


    combo.addItemListener(new ItemListener() { 
     @Override 
     public void itemStateChanged(ItemEvent e) 
     { 
      String item = (String)e.getItem(); 
      if (e.getStateChange() == ItemEvent.SELECTED) 
      { 
       System.out.println("You chose " +item); 
        if (item.equals("Comb")) 
        { 
      Object val = JOptionPane.showInputDialog("Please input quantity of comb"); 
      System.out.println(val); 

         Integer bilang= Integer.valueOf((String)val); 
         int total = bilang * 99; 
         String order = bilang + " " + "Shoes " + "  " + total; 
         System.out.print("" + order); 
        } 


      } 
    } 
    }); 
    } 
} 

Answer 2

的問題是，你擋住了EDT叫Scanner#nextInt在ItemListener等待輸入從System.in：

int bilang = inp.nextInt(); 

唐使用Scanner來讀取Swing應用程序中的用戶輸入。這隻會阻止EDT，直到數據在控制檯窗口中提供。有許多方法可以讀取輸入，例如使用JOptionPane#showInputDialog。有關此選項的更多信息，請參見How to Make Dialogs。

---

從這個

除此之外，比較String內容時，而不是==運營商使用.equals。後者比較Object參考。