ItemListener不工作

Question

为什么我的组合框不能执行我在ItemListener中声明的内容？当我单击组合框中的一项时，程序挂起，我需要关闭整个BlueJ。请看一下我的代码中有什么地方错了

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

您所做的基本错误是使用==操作符比较两个String对象。

您应该改用以下条件：

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

作为JOptionPane的简短演示..我在这里放了你的代码的修改版本…我希望它能帮助你理解如何正确处理这种情况……

    //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

问题是您阻塞了调用Scanner#nextInt的EDT，等待来自ItemListener中的System.in的输入

int bilang = inp.nextInt(); 

不要使用Scanner来读取Swing应用程序中的用户输入。这只会阻止EDT，直到控制台窗口中提供数据为止。有许多方法可以读取输入，例如使用JOptionPane#showInputDialog。有关此选项的更多信息，请参阅How to Make Dialogs。

除此之外，在比较String内容时使用.equals而不是==运算符。后者比较Object引用。