一个小计算器中的小问题

Question

private class CommandAction implements ActionListener{ public void actionPerformed(ActionEvent e) {
String command = e.getActionCommand();
if(start){
if(command.equals("-")){
display.setText(command);
start = false;

}else
lastCommand = command;

}else{
calculate(Double.parseDouble(display.getText()));
lastCommand = command;
start = true;
}
}

}

Answer 1

全部代码在这里import java.awt.BorderLayout;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
public class Calculator {
public static void main(String[] args) {
CalculatorFrame frame = new CalculatorFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);
frame.setResizable(true);

}}class CalculatorFrame extends JFrame{
public CalculatorFrame(){
// this.setSize(arg0);
setTitle("超哥计算器");
CalculatorPanel panel = new CalculatorPanel();
add(panel);
pack();
}
}class CalculatorPanel extends JPanel{
public CalculatorPanel(){
setLayout(new BorderLayout());

result = 0;
lastCommand = "=";
start = true;

display = new JButton("0");
display.setEnabled(false);
add(display,BorderLayout.NORTH);

ActionListener insert = new InsertAction();
ActionListener command = new CommandAction();

panel = new JPanel();
panel.setLayout(new GridLayout(4,4));

addButton("7",insert);
addButton("8",insert);
addButton("9",insert);
addButton("/", command);

addButton("4",insert);
addButton("5",insert);
addButton("6",insert);
addButton("*", command);

addButton("1",insert);
addButton("2",insert);
addButton("3",insert);
addButton("+", command);

addButton("0", insert);
addButton(".", insert);
addButton("=", command);
addButton("-", command);

add(panel,BorderLayout.CENTER);
}

private void addButton(String  label,ActionListener listener){
JButton button = new JButton(label);
button.addActionListener(listener);
panel.add(button);
}
private class InsertAction implements ActionListener{ public void actionPerformed(ActionEvent e) {
String input = e.getActionCommand();
if(start){
display.setText("");
start = false;
}
display.setText(display.getText() + input);
}

}

private class CommandAction implements ActionListener{ public void actionPerformed(ActionEvent e) {
String command = e.getActionCommand();
if(start){
if(command.equals("-")){
display.setText(command);
start = false;

}else
lastCommand = command;

}else{
calculate(Double.parseDouble(display.getText()));
lastCommand = command;
start = true;
}
}

}


public void calculate(double x){
if(lastCommand.equals("+"))  result += x;
else if(lastCommand.equals("-"))  result -= x;
else if(lastCommand.equals("*"))  result *= x;
else if(lastCommand.equals("/"))  result /= x;
display.setText(" " + result);
}

private JButton display;
private JPanel panel;
private boolean start;
private double result;
private String lastCommand;
}

Answer 2

java core 上面的例子  但是我不能完全理解那两个监听器

Answer 3

CommandAction ==>对应+-*/ =
InsertAction==>对应数字

Answer 4

按你写的程序解读出来， 可以告诉你 你这个程序有问题.
我下面给红色表的是两个监听器，与问题 有一些无用代码，我给你个程序思路。
strart=true  or false 有意思么  点击按钮了 ，就会触发事件了
还有以下这块代码  有意思么 ，点按钮，就触发事件，把符号赋值给字符串不就完了么。用判断么.
 我有说的不对可以指出来.
共同学习，进步么
    if(start){//如果没有start是true的话进入 条件循环   这里判断的是否点击是符号键
            
                if(command.equals("-")){//判断你是否点的是‘-’号
                 System.out.println("进入了减号");
                    display.setText(command);
                    start = false;
                    
            }else//如果不是'-'号
                lastCommand = command;//把该符号赋值 给lastCommand
            
        }else{//如果不是点击是符号键 的话进入
                calculate(Double.parseDouble(display.getText()));
                lastCommand = command;
                start = true;
            }
源代码：
package test;
import java.awt.BorderLayout;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
public class test {
    public static void main(String[] args) {
      CalculatorFrame frame = new CalculatorFrame();
         frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
         frame.setVisible(true);
         frame.setResizable(true);    }}class CalculatorFrame extends JFrame{
    public CalculatorFrame(){
    //    this.setSize(arg0);
        setTitle("超哥计算器");
        CalculatorPanel panel = new CalculatorPanel();
        add(panel);//把组件添加容器上
        pack();
    }
}class CalculatorPanel extends JPanel{
    
    private JButton display;
    private JPanel panel;
    private boolean start;
    private double result;
    private String lastCommand;
    public CalculatorPanel(){
        setLayout(new BorderLayout());
        
        result = 0;
        lastCommand = "=";
        start = true;//strart初始化
        
        display = new JButton("0");
        display.setEnabled(false);
        add(display,BorderLayout.NORTH);
        
        ActionListener insert = new InsertAction();
        ActionListener command = new CommandAction();
        
        panel = new JPanel();
        panel.setLayout(new GridLayout(4,4));
        
        addButton("7",insert);
        addButton("8",insert);
        addButton("9",insert);
        addButton("/", command);
        
        addButton("4",insert);
        addButton("5",insert);
        addButton("6",insert);
        addButton("*", command);
        
        addButton("1",insert);
        addButton("2",insert);
        addButton("3",insert);
        addButton("+", command);
        
        addButton("0", insert);
        addButton(".", insert);
        addButton("=", command);
        addButton("-", command);
        
        add(panel,BorderLayout.CENTER);
    }
    
    private void addButton(String  label,ActionListener listener){
        JButton button = new JButton(label);
        button.addActionListener(listener);
        panel.add(button);
    }    
    private class InsertAction implements ActionListener{//数字键 和点键 点击事件        public void actionPerformed(ActionEvent e) {
            String input = e.getActionCommand();
            if(start){//如果没有start是true的话进入 条件循环   这里判断的是否是按了数字键和点键 
                display.setText("");//你这个按钮的值清空
                start = false;
            }
            display.setText(display.getText() + input);
        }
        
    }
    
    private class CommandAction implements ActionListener{//符号的事件        public void actionPerformed(ActionEvent e) {
            String command = e.getActionCommand();
            if(start){//如果没有start是true的话进入 条件循环   这里判断的是否点击是符号键
            
                if(command.equals("-")){//判断你是否点的是‘-’号
                 System.out.println("进入了减号");
                    display.setText(command);
                    start = false;
                    
            }else//如果不是'-'号
                lastCommand = command;//把该符号赋值 给lastCommand
            
        }else{//如果不是点击是符号键 的话进入
                calculate(Double.parseDouble(display.getText()));
                lastCommand = command;
                start = true;
            }
        }
        
    }
    
    
    public void calculate(double x){//判断运算符号
        if(lastCommand.equals("+"))  result += x;
        else if(lastCommand.equals("-"))  result -= x;
        else if(lastCommand.equals("*"))  result *= x;
        else if(lastCommand.equals("/"))  result /= x;
        display.setText(" " + result);
    }}

Answer 5

<Core Java I>中这段计算器代码我看过，是在讲Swing的组建布局里的实例代码，两种类型的ActionListener分别侦听数字按钮和运算符按钮。

Answer 6

“Exception in thread "AWT-EventQueue-0" java.lang.NumberFormatException: For input string: "-"
at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1224)
at java.lang.Double.parseDouble(Double.java:510)
at CalculatorPanel$CommandAction.actionPerformed(Calculator.java:108)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2318)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:236)
at java.awt.Component.processMouseEvent(Component.java:6216)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3265)
at java.awt.Component.processEvent(Component.java:5981)
at java.awt.Container.processEvent(Container.java:2041)
at java.awt.Component.dispatchEventImpl(Component.java:4583)
at java.awt.Container.dispatchEventImpl(Container.java:2099)
at java.awt.Component.dispatchEvent(Component.java:4413)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4556)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4220)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4150)
at java.awt.Container.dispatchEventImpl(Container.java:2085)
at java.awt.Window.dispatchEventImpl(Window.java:2475)
at java.awt.Component.dispatchEvent(Component.java:4413)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)
Exception in thread "AWT-EventQueue-0" java.lang.NumberFormatException: For input string: "-"
at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1224)
at java.lang.Double.parseDouble(Double.java:510)
at CalculatorPanel$CommandAction.actionPerformed(Calculator.java:108)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2318)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:236)
at java.awt.Component.processMouseEvent(Component.java:6216)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3265)
at java.awt.Component.processEvent(Component.java:5981)
at java.awt.Container.processEvent(Container.java:2041)
at java.awt.Component.dispatchEventImpl(Component.java:4583)
at java.awt.Container.dispatchEventImpl(Container.java:2099)
at java.awt.Component.dispatchEvent(Component.java:4413)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4556)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4220)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4150)
at java.awt.Container.dispatchEventImpl(Container.java:2085)
at java.awt.Window.dispatchEventImpl(Window.java:2475)
at java.awt.Component.dispatchEvent(Component.java:4413)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)
”
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