java计算器代码有的地方不懂，请大神来解释一下41行到105行！！

Question

代码是对的，请大神们帮我看看第41行到105行之间，你们帮忙注释解释下，我不太懂。
就是解释下 public void actionPerformed(ActionEvent e) 这个中的代码和void setResult() 中的代码，非常感谢，我原来的注释可以忽略。

package jisuanqi;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class 计算 implements ActionListener {
JFrame frame;
JTextArea field;
JButton[] bits;
JButton bit;
JButton[] cites;

public 计算() {
frame = new JFrame("计算器");
field = new JTextArea( 2, 24);
bits = new JButton[16];
bit = new JButton("清空");
cites=new JButton[4];
String [] str = {"1","2","3","+","4","5","6","-","7","8","9","*","0",".","=","/"};
for (int i = 0; i < 16; i++) {
bits[i] = new JButton(str[i]);
}
String [] chan={"ME","AE","BE","CE",};
for(int j=0;j<4;j++){
cites [j]=new JButton(chan[j]);
}
setBorder();
setFontAndColor();
addEventHandler();
}
public void addEventHandler() {
for (int i = 0; i < 16; i++) {
bits[i].addActionListener(this);
bit.addActionListener(this);
}
}

String x = null;
String m = null;
String n = null;
boolean flag = false; //??

void setResult() {
if (field.getText().equals("")) {
flag = true;
m = null;
n = null;
}
else if (m != null) {
n = field.getText();
if (x.equals("+")) {
field.setText(Double.parseDouble(m) + Double.parseDouble(n)
+ "");
}
else if (x.equals("-")) {
field.setText(Double.parseDouble(m) - Double.parseDouble(n)
+ "");
} else if (x.equals("*")) {
field.setText(Double.parseDouble(m) * Double.parseDouble(n)
+ "");
} else if (x.equals("/")) {
field.setText(Double.parseDouble(m) / Double.parseDouble(n)
+ "");
}
flag = true;
m = null;
n = null;
}
}

public void actionPerformed(ActionEvent e) {
String str = e.getActionCommand();// 得到被点的按钮名称
if ("0123456789.".indexOf(str) != -1) {//如果输入的是数字
if (flag) {
field.setText("");//文本框清空
flag = false;
}
field.setText(field.getText() + str); //后面加上数字
}
else if ("+-*/".indexOf(str) != -1) {
if (field.getText().equals("")) {
x = str;
}
else if (m == null) {
m = field.getText();
x = str;
field.setText("");
}
else if (m != null) {
setResult();
m = field.getText();
x = str;
}

}
else if (str.equals("=")) {
setResult(); //获得运算结果
}
else if (str.equals("清空")) {
field.setText(""); //清空文本框
m = null;
n = null;
}
}

public void setBorder() {
frame.setLayout(new BorderLayout());
JPanel northPannel = new JPanel();
JPanel centerPannel = new JPanel();
JPanel southPannel = new JPanel();
JPanel westPannel = new JPanel();
northPannel.setLayout(new FlowLayout(1,8,8));
centerPannel.setLayout(new GridLayout(4,4,2,2));
southPannel.setLayout(new FlowLayout(1,8,8));
westPannel.setLayout(new GridLayout(4,1,0,14));
northPannel.add(field);
for (int i = 0; i < 16; i++) {
centerPannel.add(bits[i]);
}
southPannel.add(bit);
for(int j=0;j<4;j++){
westPannel.add(cites[j]);
}

frame.add(northPannel, BorderLayout.NORTH);
frame.add(centerPannel, BorderLayout.CENTER);
frame.add(southPannel, BorderLayout.SOUTH);
frame.add(westPannel, BorderLayout.WEST);

}

public void setFontAndColor() {
field.setFont(new Font("隶书", Font.BOLD, 30));
field.setForeground(Color.BLUE);
field.setBackground(new Color(192,192,192));

}

public void window() {
frame.setSize(500, 400);
frame.setLocation(500, 300);
frame.setVisible(true);
frame.setDefaultCloseOperation(3);
}

public static void main(String[] args) {
new 计算().window();
}
}

Answer 1

 void setResult() { 
if (field.getText().equals("")) {  //判断文本框是否为空
flag = true; //flag用来判断是输入第一个数还是第二个数
m = null; //第一个数
n = null;  // 第二个数
} 
else if (m != null) { 
n = field.getText(); 
if (x.equals("+")) {  //如果是加法
field.setText(Double.parseDouble(m) + Double.parseDouble(n) 
+ "");  //结果框显示m+n
} 
else if (x.equals("-")) {  //减法
field.setText(Double.parseDouble(m) - Double.parseDouble(n) 
+ ""); 
} else if (x.equals("*")) { //乘法
field.setText(Double.parseDouble(m) * Double.parseDouble(n) 
+ ""); 
} else if (x.equals("/")) {  //除法
field.setText(Double.parseDouble(m) / Double.parseDouble(n) 
+ ""); 
} 
flag = true; //初始化，原理同上
m = null; 
n = null; 
} 
}
public void actionPerformed(ActionEvent e) { 
String str = e.getActionCommand();// 得到被点的按钮名称 
if ("0123456789.".indexOf(str) != -1) {//如果输入的是数字
if (flag) { 
field.setText("");//文本框清空 
flag = false; 
} 
field.setText(field.getText() + str); //后面加上数字
} 
else if ("+-*/".indexOf(str) != -1) { 字符串不包含+-*/
if (field.getText().equals("")) { 
x = str; 
}
else if (m == null) { 
m = field.getText();  //得到第一个数
x = str; 
field.setText(""); //清空文本框以便输入第二个数
}
else if (m != null) { 
setResult(); //第二个数也有了，那么计算结果
m = field.getText(); 
x = str; 
}
} 
else if (str.equals("=")) { 
setResult(); //获得运算结果
} 
else if (str.equals("清空")) { 
field.setText(""); //清空文本框
m = null; 
n = null; 
} 
}