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CalculatorPanel.java
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package calculator;
import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
/**
* A panel with calculator buttons and a result display.
*/
public class CalculatorPanel extends JPanel {
private JButton display;
private JPanel panel;
private double result;
private String lastCommand;
private boolean start;
public CalculatorPanel() {
setLayout(new BorderLayout());
result = 0;
lastCommand = "=";
start = true;
// add the display
display = new JButton("0");
display.setEnabled(false);
add(display, BorderLayout.NORTH);
ActionListener insert = new InsertAction();
ActionListener command = new CommandAction();
// add the buttons in a 4 x 4 grid
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);
}
/**
* Carries out the pending calculation.
*
* @param x the value to be accumulated with the prior result.
*/
private void calculate(double x) {
switch (lastCommand) {
case "+":
result += x;
break;
case "-":
result -= x;
break;
case "*":
result *= x;
break;
case "/":
result /= x;
break;
case "=":
result = x;
break;
}
display.setText("" + result);
}
/**
* This action inserts the button action string to the end of the display text.
*/
private class InsertAction implements ActionListener {
public void actionPerformed(ActionEvent event) {
String input = event.getActionCommand();
if (start) {
display.setText("");
start = false;
}
display.setText(display.getText() + input);
}
}
/**
* This action executes the command that the button action string denotes.
*/
private class CommandAction implements ActionListener {
public void actionPerformed(ActionEvent event) {
String command = event.getActionCommand();
if (start) {
if (command.equals("-")) {
display.setText(command);
start = false;
} else lastCommand = command;
} else {
calculate(Double.parseDouble(display.getText()));
lastCommand = command;
start = true;
}
}
}
}