JComboBox的addItem方法报错，请高手帮忙，在线等，立即给分

Question

代码如下：
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;public class Counter
{
public static void main(String[] args)
{
JFrame frame=new CounterFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.show();
}
}class CounterFrame extends JFrame
{
public static final int WIDTH = 350;
public static final int HEIGHT = 150;
//计算显示文本。
private TextField textField1 = new TextField(5);
private TextField textField2 = new TextField(5);
private TextField textresult = new TextField(5);

private JComboBox combo = new JComboBox();
    combo.addItem("+");
    combo.addItem("-");
    combo.addItem("*");
    combo.addItem("/"); public CounterFrame()
{
setSize(WIDTH, HEIGHT);
setTitle("Counter");
Container contentPane = getContentPane(); JPanel textPanel = new JPanel();
textPanel.add(textField1);
textPanel.add(textField2);
textPanel.add(combo);
textPanel.add(textresult);//计算的按钮。
JPanel buttonPanel = new JPanel();
addButton(buttonPanel, "计算",
new ActionListener()
{
public void actionPerformed(ActionEvent evt)
{
computer();
}
}
);

//文本框清空
addButton(buttonPanel, "清除",
new ActionListener()
{
public void actionPerformed(ActionEvent evt)
{
textField1.setText("");
textField2.setText("");
textresult.setText("");
}
}
);//退出的按钮，并退出程序。
addButton(buttonPanel, "退出",
new ActionListener()
{
public void actionPerformed(ActionEvent evt)
{
System.exit(0);
}
}
); contentPane.add(textPanel, BorderLayout.NORTH);
contentPane.add(buttonPanel, BorderLayout.SOUTH);
} public void addButton(Container c, String title, ActionListener 
listener)
{
JButton button = new JButton(title);
c.add(button);
button.addActionListener(listener);
} public void computer()
{
float a,b,result,index;
a=Float.parseFloat(textField1.getText());
b=Float.parseFloat(textField2.getText());
index=combo.getSelectedIndex();
        switch (index)
        {
         case 0:
            result=a+b;
            break;
         case 1:
            result=a-b;
            break;
         case 2:
            result=a*b;
            break;
         case 3:
            result=a/b;
        }

textresult.setText(Float.toString(result)); }
}编译addItem出错。

Answer 1

combo.addItem("+");
combo.addItem("-");
combo.addItem("*");
combo.addItem("/");报这些行出错，请高手指点啊

Answer 2

写成
{
combo.addItem("+");
combo.addItem("-");
combo.addItem("*");
combo.addItem("/");
}
即可

Answer 3

combo.addItem("+");
    combo.addItem("-");
    combo.addItem("*");
    combo.addItem("/");应该要放在初始化里的啊，这个又有不是定义

Answer 4

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;public class Counter
{
public static void main(String[] args)
{
JFrame frame=new CounterFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.show();
}
}class CounterFrame extends JFrame
{
public static final int WIDTH = 350;
public static final int HEIGHT = 150;
//计算显示文本。
private TextField textField1 = new TextField(5);
private TextField textField2 = new TextField(5);
private TextField textresult = new TextField(5);private JComboBox combo = new JComboBox();
   
public CounterFrame()
{
setSize(WIDTH, HEIGHT);
setTitle("Counter");
Container contentPane = getContentPane(); combo.addItem("+");
 combo.addItem("-");
 combo.addItem("*");
 combo.addItem("/");
JPanel textPanel = new JPanel();
textPanel.add(textField1);
textPanel.add(textField2);
textPanel.add(combo);
textPanel.add(textresult);//计算的按钮。
JPanel buttonPanel = new JPanel();
addButton(buttonPanel, "计算",
new ActionListener()
{
public void actionPerformed(ActionEvent evt)
{
computer();
}
}
);//文本框清空
addButton(buttonPanel, "清除",
new ActionListener()
{
public void actionPerformed(ActionEvent evt)
{
textField1.setText("");
textField2.setText("");
textresult.setText("");
}
}
);//退出的按钮，并退出程序。
addButton(buttonPanel, "退出",
new ActionListener()
{
public void actionPerformed(ActionEvent evt)
{
System.exit(0);
}
}
);contentPane.add(textPanel, BorderLayout.NORTH);
contentPane.add(buttonPanel, BorderLayout.SOUTH);
}public void addButton(Container c, String title, ActionListener 
listener)
{
JButton button = new JButton(title);
c.add(button);
button.addActionListener(listener);
}public void computer()
{
float a,b,result=0;
int index = 0;
a=Float.parseFloat(textField1.getText());
b=Float.parseFloat(textField2.getText());
index=combo.getSelectedIndex();
        switch (index)
        {
        case 0:
           result=a+b;
           break;
        case 1:
           result=a-b;
           break;
        case 2:
           result=a*b;
           break;
        case 3:
           result=a/b;
        }textresult.setText(Float.toString(result));}
}

Answer 5

public CounterFrame()
{
setSize(WIDTH, HEIGHT);
setTitle("Counter");
Container contentPane = getContentPane(); combo.addItem("+");
 combo.addItem("-");
 combo.addItem("*");
 combo.addItem("/");public void computer()
{
float a,b,result=0;
int index = 0;

Answer 6

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;class Counter
{
public static void main(String[] args)
{
JFrame frame=new CounterFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.show();
}
}class CounterFrame extends JFrame
{
public static final int WIDTH = 350;
public static final int HEIGHT = 150;
//计算显示文本。
private TextField textField1 = new TextField(5);
private TextField textField2 = new TextField(5);
private TextField textresult = new TextField(5);
private JComboBox combo;

public CounterFrame()
{
setSize(WIDTH, HEIGHT);
setTitle("Counter");
Container contentPane = getContentPane();

ComboBoxModel combomodel=new DefaultComboBoxModel(
   new String[]{"+","-","*","/"});
   JComboBox combo = new JComboBox();
combo.setModel(combomodel);

JPanel textPanel = new JPanel();
textPanel.add(textField1);
textPanel.add(textField2);
textPanel.add(combo);
textPanel.add(textresult);

//计算的按钮。
JPanel buttonPanel = new JPanel();
addButton(buttonPanel, "计算",
new ActionListener()
{
public void actionPerformed(ActionEvent evt)
{
computer();
}
});


//文本框清空
addButton(buttonPanel, "清除",
new ActionListener()
{
public void actionPerformed(ActionEvent evt)
{
textField1.setText("");
textField2.setText("");
textresult.setText("");
}
});
//退出的按钮，并退出程序。
addButton(buttonPanel, "退出",
new ActionListener()
{
public void actionPerformed(ActionEvent evt)
{
System.exit(0);
}
});


contentPane.add(textPanel, BorderLayout.NORTH);
contentPane.add(buttonPanel, BorderLayout.SOUTH);
}

public void addButton(Container c, String title, ActionListener listener)
{
JButton button = new JButton(title);
c.add(button);
button.addActionListener(listener);
}

public void computer()
{
float a,b,result=0,index;
a=Float.parseFloat(textField1.getText());
b=Float.parseFloat(textField2.getText());
index=combo.getSelectedIndex();
        switch ((int)index)
        {
        case 0:
           result=a+b;
           break;
        case 1:
           result=a-b;
           break;
        case 2:
           result=a*b;
           break;
        case 3:
           result=a/b;
        }

textresult.setText(Float.toString(result));

}
}