public class Person {
  void speakHello() {
    System.out.print("您好，很高兴认识");
    System.out.println(" nice to meet you"); 
  }
    class Xiti {
     public static void main(String[] args) {
      Person person = new Person();
      person.speakHello();
    }
  }
}
}

public static void main(String args[]) {
        int x = 8; 
        byte b = 127; //【代码1】
        b = x; //【代码2】
        x = 12L; //【代码3】
        long y=8.0; //【代码4】
        float z=6.89 ; //【代码5】 
    }

public class Home {
    public static void main (String args[ ]) {
        for(int i = 20302; i <= 20322;i++) {
            System.out.println((char)i);
        }
    }
}
/ *
低
住
佐
佑
佒
体
佔
何
佖
佗
佘
余
佚
佛
作
佝
佞
佟
你
佡
佢
Process finished with exit code 0 */

public class Home {
    public static void main(String args[]) {
        int x = 234,y = 432;
        System.out.println(x + "<" + (2 * x));
        System.out.print("我输出结果后不回车");
        System.out.println("我输出结果后自动回车到下一行");
        System.out.println("x+y= " + (x + y)); 
    }
}
//
234<468
我输出结果后不回车我输出结果后自动回车到下一行
x+y= 666

public class Home {
    public static void main(String args[]) {
        System.out.println("byte取值范围:"+Byte.MIN_VALUE+"至"+Byte.MAX_VALUE);
        System.out.println("short取值范围:"+Short.MIN_VALUE+"至"+Short.MAX_VALUE);
        System.out.println("int取值范围:"+Integer.MIN_VALUE+"至"+Integer.MAX_VALUE);
        System.out.println("long取值范围:"+Long.MIN_VALUE+"至"+Long.MAX_VALUE);
        System.out.println("float取值范围:"+Float.MIN_VALUE+"至"+Float.MAX_VALUE);
        System.out.println("double取值范围:"+ Double.MIN_VALUE+"至"+Double.MAX_VALUE);
    }
}
//
byte取值范围:-128至127
short取值范围:-32768至32767
int取值范围:-2147483648至2147483647
long取值范围:-9223372036854775808至9223372036854775807
float取值范围:1.4E-45至3.4028235E38
double取值范围:4.9E-324至1.7976931348623157E308

public class Home {
    public static void main (String args[ ]) {
        long[] a = {1, 2, 3, 4};
        long[] b = {100, 200, 300, 400, 500};
        b = a;
        System.out.println("数组b的长度:" + b.length); //【代码1】
        System.out.println("b[0]=" + b[0]); //【代码2】
    }
}
//
数组b的长度:4
b[0]=1

public class Home {
    public static void main (String args[ ]) {
        int [] a={10,20,30,40},b[]={{1,2},{4,5,6,7}};
        b[0] = a;
        b[0][1] = b[1][3];
        System.out.println(b[0][3]); //【代码1】
        System.out.println(a[1]); //【代码2】
    }
}
//
40
7

public class Home {
    public static void main (String args[ ]) {
        System.out.println((int)'您');
        System.out.println((int)'我');
        System.out.println((int)'他');
    }
}
//
24744
25105
20182

public class Home {
    public static void main (String args[ ]) {
        char start='α';
        char end='ω';
        for(char ch = start; ch <= end; ch++) {
            System.out.println(" " + ch);
        }
    }
}
//
 α
 β
 γ
 δ
 ε
 ζ
 η
 θ
 ι
 κ
 λ
 μ
 ν
 ξ
 ο
 π
 ρ
 ς
 σ
 τ
 υ
 φ
 χ
 ψ
 ω

public class Home {
    public static void main (String args[]) {
        char x = '你', y = 'e', z = '吃';
        if(x > 'A'){
            y = '苹';
            z = '果';
        } else {
            y = '酸';
        }
        z = '甜';
        System.out.println(x + "," + y + "," + z);
    }
}

public class Home {
    public static void main (String args[]) {
        char c = '\0';
        for(int i = 1;i <= 4; i++) {
            switch(i) {
                case 1: c = 'J';
                    System.out.print(c);
                case 2: c = 'e';
                    System.out.print(c);
                    break;
                case 3: c = 'p';
                    System.out.print(c);
                default:
                    System.out.print("好");
            }
        }
    }
}

public class Home {
    public static void main (String args[]) {
        int x = 1,y = 6;
        while (y-- > 0) {
            x--;
        }
        System.out.print("x="+ x +",y=" + y);
    }
}

public class Home {
    public static void main (String args[]) {
        double sum = 0, a = 1;
        int i = 1;
        while(i <= 10) {
            sum += a;
            i++;
            a *= i;
        }
        System.out.println("sum=" + sum);
    }
}
// sum=4037913.0

public class Home {
    public static void main (String args[]) {
        int i,j;
        for(j = 2; j <= 100; j++) {
            for(i = 2; i <= j /2; i++) {
                if(j % i == 0) {
                    break;
                }
            }
            if(i > j / 2) {
                System.out.println(" " + j);
            }
        }
    }
}

public class Home {
    public static void main (String args[]) {
        double sum = 0, a = 1, i = 1;
        do {
            sum += a;
            i++;
            a = 1.0 / (i * a);
        } while (i <= 20);
        System.out.println("使用do while计算的总和为：" + sum);
        for(sum = 0, i = 1,a = 1; i <= 20; i++) {
            a = 1.0 / (a * i);
            sum += a;
        }
        System.out.println("使用for循环计算的总和为：" + sum);
    }
}

public class Home {
    public static void main (String args[]) {
       int sum = 0, i,j;
       for(i = 1; i <= 1000; i++) {
            for(j = 1, sum = 0; j < i; j++) {
                if(i % j == 0) {
                    sum += j;
                }
            }
            if(sum == j) {
                System.out.println("完数：" + i);
            }
       }
    }
}

public class Home {
    public static void main (String args[]) {
       int m = 8, item = m,i = 1;
       long sum = 0;
       for(i = 1; i <= 10; i++) {
            sum += item;
            item = item * 10 + m;
       }
       System.out.println(sum);
    }
}

public class Home {
    public static void main (String args[]) {
       int n = 1;
       long sum = 0;
       while (true) {
           sum += n;
           n++;
           if(sum >= 8888) {
               break;
           }
       }
       System.out.println("符合题意的最大整数为：" + (n - 1));
    }
}
// 符合题意的最大整数为：133

public class CPU {
    int speed;
    public int getSpeed() {
        return speed;
    }
    public void setSpeed(int speed) {
        this.speed = speed;
    }
}

public class HardDisk {
    int amount;
    public int getAmount() {
        return amount;
    }
    public void setAmount(int amount) {
        this.amount = amount;
    }
}

public class PC {
    HardDisk hd;
    CPU cpu;
    public void setHardDisk(HardDisk hd) {
        this.hd = hd;
    }
    public void setCPU(CPU cpu) {
        this.cpu = cpu;
    }
    public void show() {
        System.out.println("当前cpu的速度为：" + cpu.speed);
        System.out.println("当前磁盘容量为：" + hd.amount);
    }
}

public class main {
    public static void main(String[] args) {
        CPU cpu = new CPU();
        HardDisk hardDisk = new HardDisk();
        PC pc = new PC();
        cpu.setSpeed(2200);
        hardDisk.setAmount(200);
        pc.setCPU(cpu);
        pc.setHardDisk(hardDisk); 
        pc.show();   // 当前cpu的速度为：2200   当前磁盘容量为：200
    }
}

// A.java
public class A {
    int m;
    int getM() {
        return m;
    }
    int seeM() {
        return m;
    }
}
//B.java
public class B extends A {
    int m;
    int getM() {
        return m + 100;
    }
}
// main.java
public class main {
    public static void main(String[] args) {
        B b = new B();
        b.m = 20;
        System.out.println(b.getM());  // 120
        A a = b;
        a.m = -100;  // 上转型对象访问的隐藏的m，即A类中的m
        System.out.println(a.getM());   // 120   上转型对象调用的一定是子类重写的getM()方法，此时子类B的m=20，所以返回120
        System.out.println(b.seeM());   // -100  子类继承的seeM()方法操作的m是被子类隐藏的m，即A中的m
    }
}

public abstract class Animal {
    public abstract void cry();
    public abstract String getAnimalName();
}

public class Dog extends Animal {
    @Override
    public void cry() {
        System.out.println("旺旺旺");
    }
    @Override
    public String getAnimalName() {
        return "Dog";
    }
}

public class Cat extends Animal {
    @Override
    public void cry() {
        System.out.println("喵喵喵");
    }
    @Override
    public String getAnimalName() {
        return "Cat";
    }
}

public class Simulator {
    public void playSound(Animal animal) {
        System.out.println(animal.getAnimalName() + "叫：");
        animal.cry();
    }
}
————————————————

public class main {
    public static void main(String[] args) {
        Simulator simulator = new Simulator();
        simulator.playSound(new Dog());  //Dog叫：旺旺旺
        simulator.playSound(new Cat()); // Cat叫：喵喵喵
    }
}

package com.example.six;
public interface Animal {
    public void cry();
    public String getAnimalName();
}

package com.example.six;
public class Simulator {
    public void playSound(Animal animal) {
        System.out.println(animal.getAnimalName() + "叫：");
        animal.cry();
    }
}

package com.example.six;
public class Dog implements Animal {
    @Override
    public void cry() {
        System.out.println("旺旺旺");
    }
    @Override
    public String getAnimalName() {
        return "Dog";
    }
}

package com.example.six;
public class Cat implements Animal{
    @Override
    public void cry() {
        System.out.println("喵喵喵");
    }
    @Override
    public String getAnimalName() {
        return "Cat";
    }
}

package com.example.six;
public class main {
    public static void main(String[] args) {
        Simulator simulator = new Simulator();
        simulator.playSound(new Dog());
        simulator.playSound(new Cat());
    }
}

package com.example.six;
import java.util.*;
public class Main {
    public static void main(String[] args) {
        Scanner reader = new Scanner(System.in);
        double sum = 0;
        int m = 0;
        while(reader.hasNextDouble()) {
            Double num = reader.nextDouble();
            assert num <= 100 && num >= 0:"输入的成绩非法！";
            m = m + 1;
            sum = sum + num;
        }
        System.out.println(m + "个数的和为" + sum);
        System.out.println(m + "个数的平均值为" + sum / m);
    }
}

package com.example.six;
import java.util.*;
public class Main {
    public static void main(String[] args) {
        Calendar calendar = Calendar.getInstance();
        calendar.setTime(new Date());
        String s = String.format("%tF(%<tA)",calendar);
        System.out.println(s);
        int n = 25;
        System.out.println("向后滚动（在月内）" + n + "天");
        /*
         不应该通过类实例访问静态成员
         */
        calendar.roll(calendar.DAY_OF_MONTH, n);
        s = String.format("%tF(%<ta)",calendar);
        System.out.println(s);
        System.out.println("再向后滚动（在年内）" + n + "天");
        calendar.roll(calendar.DAY_OF_YEAR, n);
        s = String.format("%tF(%<ta)",calendar);
        System.out.println(s);
    }
}
/*
    2022-05-05(星期四)
    向后滚动（在月内）25天
    2022-05-30(星期一)
    再向后滚动（在年内）25天
    2022-06-24(星期五)
*/

package com.example.six;
import java.util.*;
public class Main {
    public static void main(String[] args) {
        Runtime runtime = Runtime.getRuntime();
        long free = runtime.freeMemory();
        System.out.println("Java虚拟机可用空闲内存" + free + "bytes");
        long total = runtime.totalMemory();
        System.out.println("Java虚拟机占用总内存" + total + "bytes");
        long n1 = System.currentTimeMillis();
        for(int i = 1; i <= 100; i++) {
            int j = 2;
            for(; j < i / 2; j++) {
                if(i % j == 0) {
                    break;
                }
                if(j > i / 2) {
                    System.out.println(" " + i);
                }
            }
        }
        long n2 = System.currentTimeMillis();
        System.out.println("\n循环用时：" + (n2 - n1) + "毫秒\n");
        free = runtime.freeMemory();
        System.out.println("Java虚拟机可用空闲内存" + free + "bytes");
        total = runtime.totalMemory();
        System.out.println("Java虚拟机占用总内存" + total + "bytes");
    }
}
/*
    Java虚拟机可用空闲内存188345056bytes
    Java虚拟机占用总内存191365120bytes
    循环用时：0毫秒
    Java虚拟机可用空闲内存188345056bytes
    Java虚拟机占用总内存191365120bytes
*/

public class Main {
  public static void main(String[] args) {
    String str1 = "Come On Forever!";
    System.out.println(str1.toUpperCase());
    System.out.println(str1.toLowerCase());
    System.out.println(str1.concat("Fredoom！"));
  }
}

public class Main {
    public static void main(String[] args) {
        String str1 = "Come On Forever!";
        System.out.println(str1 + "的第一个字符是：" + str1.charAt(0));
        System.out.println(str1 + "的最后一个字符是：" + str1.charAt(str1.length() - 1));
    }
}
"""
Come On Forever!的第一个字符是：C
Come On Forever!的最后一个字符是：!7

import java.util.Calendar;
import java.util.Date;
public class Main {
    public static void main(String[] args) {
        Calendar calendar1 = Calendar.getInstance();
        calendar1.set(Calendar.YEAR, Integer.parseInt(args[0]));
        calendar1.set(Calendar.MONTH, Integer.parseInt(args[1]));
        calendar1.set(Calendar.DAY_OF_MONTH, Integer.parseInt(args[2]));
        Calendar calendar2 = Calendar.getInstance();
        calendar2.set(Calendar.YEAR, Integer.parseInt(args[3]));
        calendar2.set(Calendar.MONTH, Integer.parseInt(args[4]));
        calendar2.set(Calendar.DAY_OF_MONTH, Integer.parseInt(args[5]));
        long time1 = calendar1.getTimeInMillis();
        long time2 = calendar2.getTimeInMillis();
        long subDay = (time1 - time2) / (1000 * 60 * 60 * 24);
        System.out.println(new Date(time1) + " and " + new Date(time2) + "be apart " + subDay + " days.");
    }
}
"""
D:\idea_project\class_work\homework\src\com\example\six>javac Main.java
> cd ../
> cd ../
D:\idea_project\class_work\homework\src>java com.example.six.Main 2022 5 19 2003 4 20
Sun Jun 19 09:12:41 CST 2022 and Tue May 20 09:12:41 CST 2003be apart 6969 days.
"""

import java.math.BigInteger;
public class Main {
    public static void main(String[] args) {
        double num1 = 5.0;
        double num2 = 5.2;
        double num3 = 5.9;
        double num4 = 5.6;
        double st = Math.sqrt(num1);
        System.out.println(num1 + "的平方根：" + st);
        System.out.println("大于等于 "+ num2 + "的最小整数:" + (int)Math.ceil(num2));
        System.out.println("小于等于 "+ -num2 + "的最小整数:" + (int)Math.floor(-num2));
        System.out.println(num3+ "四舍五入的整数： " + Math.round(num3));
        System.out.println(-num4+ "四舍五入的整数： " + Math.round(-num4));
        BigInteger result = new BigInteger("0");
        BigInteger one = new BigInteger("123456789");
        BigInteger two = new BigInteger("987654321");
        result = one.add(two);
        System.out.println(one + " + " + two + "的和为：" + result);
        result = one.multiply(two);
        System.out.println(one + " + " + two + "的积为：" + result);
    }
}
"""
5.0的平方根：2.23606797749979
大于等于 5.2的最小整数:6
小于等于 -5.2的最小整数:-6
5.9四舍五入的整数： 6
-5.6四舍五入的整数： -6
123456789 + 987654321的和为：1111111110
123456789 + 987654321的积为：121932631112635269
"""

public class Main {
    public static void main(String[] args) {
        String target = "Happy 420 Birthday to you! 519";
        String regex = "\\D+";  // 匹配非数字
        String[] digitWord = target.split(regex);   
        for(String s : digitWord) {
            System.out.println(s) ;
        }
        System.out.println(digitWord.length); // length=3,  ["","420","519"]
    }
}

import java.util.InputMismatchException;
import java.util.Scanner;
public class GetPrice {
    int len = 0;
    double sum = 0;
    public double getSum(String cost) {
        String regex = "[^0123456789.]+";
        Scanner scanner = new Scanner(cost);
        scanner.useDelimiter(regex);
        while (scanner.hasNext()) {
            try {
                double price = scanner.nextDouble();
                len++;
                sum += price;
            } catch (InputMismatchException exp) {
                String t = scanner.next();
            }
        }
        return sum;
    }
    public double getAvg() {
        return sum / len;
    }
}

public class Main {
    public static void main(String[] args) {
        String message = "数学87分，物理76分，英语96分";
        GetPrice getPrice = new GetPrice();
        System.out.println("总成绩为：：" + getPrice.getSum(message));
        System.out.println("平均分数为：：" + getPrice.getAvg());
    }
}

public class Main {
    public static void main(String[] args) {
        ComputeWindow win = new ComputeWindow();
        win.setBounds(100,100,600,600);
        win.setTitle("计算");
  }
}

import javax.swing.*;
import java.awt.*;
public class ComputeWindow extends JFrame {
    JTextArea inputText,showText;
    TextListener textChangeListener;
    ComputeWindow() {
        init();
        setLayout(new FlowLayout());
        setVisible(true);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }
    private void init() {
        inputText = new JTextArea(15,20);
        showText = new JTextArea(15,20);
        showText.setLineWrap(true);
        showText.setWrapStyleWord(true);
        add(new JScrollPane(inputText));
        add(new JScrollPane(showText));
        showText.setEditable(false);
        textChangeListener = new TextListener();
        textChangeListener.setInputText(inputText);
        textChangeListener.setShowText(showText);
        (inputText.getDocument()).addDocumentListener(textChangeListener);
    }
}

import javax.swing.*;
import javax.swing.event.DocumentEvent;
import javax.swing.event.DocumentListener;
import java.util.Scanner;
public class TextListener implements DocumentListener {
    JTextArea inputText,showText;
    double sum;
    double avg;
    int cnt;
    @Override
    public void insertUpdate(DocumentEvent e) {
        changedUpdate(e);
    }
    @Override
    public void removeUpdate(DocumentEvent e) {
        changedUpdate(e);
    }
    @Override
    public void changedUpdate(DocumentEvent e) {
        String str = inputText.getText();
        sum = 0;   // 注意在这里需要全部初始化，不然每打一个空格，所有数的和或者平均值都会变化
        avg = 0;
        cnt = 0;
        Scanner scanner = new Scanner(str);
        while(scanner.hasNextDouble()) {
            cnt++;
            sum += scanner.nextDouble();
        }
        avg = sum / cnt;
        showText.setText(null);
        showText.append("\n和为：" + sum);
        showText.append("\n平均值为：" + avg);
    }
    public void setInputText(JTextArea text) {
        inputText = text;
    }
    public void setShowText(JTextArea text) {
        showText = text;
    }
}

public class Main {
    public static void main(String[] args) {
        ComputeWindow win = new ComputeWindow();
        win.setBounds(100,100,600,600);
        win.setTitle("计算");
    }
}

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
public class ComputeWindow extends JFrame implements ActionListener {
  JTextField inputField1, inputField2, outputFiled;
  JLabel label;
  JPanel panel;
  Box vbox;
  JButton buttonAdd, buttonSub, buttonMul, buttonDiv;
  ComputeWindow() {
    init();
    setContentPane(panel);
    setLayout(new FlowLayout());
    setVisible(true);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  }
  void init() {
    inputField1 = new JTextField(10);
    inputField2 = new JTextField(10);
    outputFiled = new JTextField(10);
    label  = new JLabel("=");
    label.setBackground(Color.green);
    vbox = Box.createVerticalBox();
    panel = new JPanel();
    buttonAdd = new JButton("加");
    buttonSub = new JButton("减");
    buttonMul = new JButton("乘");
    buttonDiv = new JButton("除");
    buttonAdd.addActionListener(this);
    buttonSub.addActionListener(this);
    buttonMul.addActionListener(this);
    buttonDiv.addActionListener(this);
    vbox.add(buttonAdd);
    vbox.add(buttonSub);
    vbox.add(buttonMul);
    vbox.add(buttonDiv);
    panel.add(inputField1);
    panel.add(vbox);
    panel.add(inputField2);
    panel.add(label);
    panel.add(outputFiled);
  }
  @Override
  public void actionPerformed(ActionEvent e) {
    if(e.getSource() == buttonAdd) {
      try {
        double a = Double.parseDouble(inputField1.getText());
        double b = Double.parseDouble(inputField2.getText());
        double result = a + b;
        outputFiled.setText("" + result);
      } catch (NumberFormatException exception) {
        JOptionPane.showMessageDialog(this,"你输入了非法数字","消息对话框",JOptionPane.WARNING_MESSAGE);
      }
    } else if(e.getSource() == buttonSub) {
      try {
        double a = Double.parseDouble(inputField1.getText());
        double b = Double.parseDouble(inputField2.getText());
        double result = a - b;
        outputFiled.setText("" + result);
      } catch (NumberFormatException exception) {
        JOptionPane.showMessageDialog(this,"你输入了非法数字","消息对话框",JOptionPane.WARNING_MESSAGE);
      }
    } else if(e.getSource() == buttonMul) {
      try {
        double a = Double.parseDouble(inputField1.getText());
        double b = Double.parseDouble(inputField2.getText());
        double result = a * b;
        outputFiled.setText("" + result);
      } catch (NumberFormatException exception) {
        JOptionPane.showMessageDialog(this,"你输入了非法数字","消息对话框",JOptionPane.WARNING_MESSAGE);
      }
    }  else if(e.getSource() == buttonDiv) {
      try {
        double a = Double.parseDouble(inputField1.getText());
        double b = Double.parseDouble(inputField2.getText());
        double result = a / b;
        outputFiled.setText("" + result);
      } catch (NumberFormatException exception) {
        JOptionPane.showMessageDialog(this,"你输入了非法数字","消息对话框",JOptionPane.WARNING_MESSAGE);
      }
    }
  }
}

public class Main {
    public static void main(String[] args) {
        WindowTrapezoid win = new WindowTrapezoid();
        win.setBounds(100,100,420,260);
        win.setTitle("使用MVC结构计算梯形面积");
    }
}

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
public class WindowTrapezoid extends JFrame implements ActionListener {
    Trapezoid trapezoid;
    JTextField textFieldA,textFieldB,textFieldC;
    JTextArea showArea;
    JButton controlButton;
    WindowTrapezoid() {
        init();
        setVisible(true);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }
    void init() {
        trapezoid = new Trapezoid();
        textFieldA = new JTextField(5);
        textFieldB = new JTextField(5);
        textFieldC = new JTextField(5);
        showArea = new JTextArea();
        controlButton = new JButton("计算面积");
        JPanel pNorth = new JPanel();
        pNorth.add(new JLabel("上底:"));
        pNorth.add(textFieldA);
        pNorth.add(new JLabel("下底:"));
        pNorth.add(textFieldB);
        pNorth.add(new JLabel("高:"));
        pNorth.add(textFieldC);
        pNorth.add(controlButton);
        controlButton.addActionListener(this);
        add(pNorth, BorderLayout.NORTH);
        add(new JScrollPane(showArea), BorderLayout.CENTER);
    }
    @Override
    public void actionPerformed(ActionEvent e) {
        try {
            double upperBottom = Double.parseDouble(textFieldA.getText().trim());
            double belowBottom = Double.parseDouble(textFieldB.getText().trim());
            double height = Double.parseDouble(textFieldC.getText().trim());
            trapezoid.setUpperBottom(upperBottom);
            trapezoid.setBelowBottom(belowBottom);
            trapezoid.setHeight(height);
            String area = trapezoid.getArea();
            showArea.append("梯形的上底为：" + upperBottom + ",下底为：" + belowBottom + ",高为：" + height + "，所以面积为：");
            showArea.append(area + "\n");
        } catch (Exception exception){
            showArea.append("\n" + exception + "\n");
        }
    }
}

public class Trapezoid  {
    double uBottom,bBottom,h,area;
    public void setUpperBottom(double upperBottom) {
        uBottom = upperBottom;
    }
    public void setBelowBottom(double belowBottom) {
        bBottom = belowBottom;
    }
    public void setHeight(double height) {
        h = height;
    }
    public String getArea() {
        area = (bBottom + uBottom) * h / 2.0;
        return String.valueOf(area);
    }
}

public class Main {
    public static void main(String[] args) {
        File file = new File("D:\\idea_project\\class_work\\homework\\src\\com\\example\\six\\Main.java");
        try {
            RandomAccessFile in = new RandomAccessFile(file,"rw");
            System.out.println(file.length());  // 579
            FileOutputStream out = new FileOutputStream(file);
            System.out.println(file.length());  // 0
        } catch (IOException e) {
            System.out.println("File read error" + e);
        }
    }
}

import java.io.*;
public class Main {
    public static void main(String[] args) {
        int n = -1;
        File file = new File("D:\\idea_project\\class_work\\homework\\src\\com\\example\\six\\hello.txt");
        byte[] a = "abcd".getBytes();
        try {
            FileOutputStream out = new FileOutputStream(file);
            out.write(a);
            out.close();
            FileInputStream in = new FileInputStream(file);
            byte[] tom = new byte[3];
            int m = in.read(tom,0,3);
            System.out.println(m);      // 3
            String s = new String(tom,0,3);
            System.out.println(s);      // abc
            m = in.read(tom,0,3);
            System.out.println(m);      // 1
            s = new String(tom,0,3);
            System.out.println(s);       // dbc  
        } catch (IOException e) {
            System.out.println("File read error" + e);
        }
    }
}

public class Main {
    public static void main(String[] args) {
        File file = new File("D:\\idea_project\\class_work\\homework\\src\\com\\example\\six\\hello.txt");
        try {
            FileOutputStream out = new FileOutputStream(file);
            PrintStream printStream = new PrintStream(out);
            printStream.print(12345.6789);
            printStream.println("how are you");
            printStream.println(true);
            printStream.close();
        } catch (IOException e) {
        }
    }
}

public class Main {
    public static void main(String[] args) {
        File file = new File("D:\\idea_project\\class_work\\homework\\src\\com\\example\\six\\hello.txt");
        try {
            RandomAccessFile in = new RandomAccessFile(file,"rw");
            in.seek(0);
            long m = in.length();
            while (m >= 0) {
                m -= 1;
                in.seek(m);
                int c = in.readByte();
                if(c <= 255 && c > 0) {
                    System.out.print((char)c);
                } else {
                    m -= 1;
                    in.seek(m);
                    byte[] cc = new byte[2];
                    in.readFully(cc);
                    System.out.print(new String(cc));
                }
            }
        } catch (IOException e) {
        }
    }
}
/*
12345.6789how are you
true
☟☟☟
eurt
uoy era woh9876.54321
*/

import java.io.*;
public class Main {
    public static void main(String[] args) {
        File file1 = new File("D:\\idea_project\\class_work\\homework\\src\\com\\example\\six\\Main.java");
        File file2 = new File("D:\\idea_project\\class_work\\homework\\src\\com\\example\\six\\hello.txt");
        try {
            FileReader inOne = new FileReader(file1);
            BufferedReader inTwo = new BufferedReader(inOne);
            FileWriter toFile = new FileWriter(file2);
            BufferedWriter out = new BufferedWriter(toFile);
            String s = null;
            int i = 0;
            s = inTwo.readLine();
            while (s != null) {
                i++;
                out.write(i +  " " + s);
                out.newLine();
                s = inTwo.readLine();
            }
            inOne.close();
            inTwo.close();
            out.flush();
            out.close();
            toFile.close();
        } catch (IOException e) {
        }
    }
}

public class Main {
    public static void main(String[] args) {
        File file = new File("D:\\idea_project\\class_work\\homework\\src\\com\\example\\six\\student.txt");
        Scanner sc = null;
        int count = 0;
        double sum = 0;
        try {
            double score = 0;
            sc = new Scanner(file);
            sc.useDelimiter("[^123456789.]+");
            while (sc.hasNextDouble()) {
                score = sc.nextDouble();
                count++;
                sum += score;
                System.out.println(score);
            }
            System.out.println("平均价格:" +  sum / count);
        } catch (Exception exp) {
            System.out.println(exp);
        }
    }
}

public class Home {
    public static void main(String[] args) {
        String url = "jdbc:mysql://localhost:3306/test_homework?useSSL=true&characterEncoding=utf-8";
        String user = "xx";
        String password = "xxxxx";
        try {
            Connection connection = DriverManager.getConnection(url, user, password);
            Statement sql = connection.createStatement();
            ResultSet res = sql.executeQuery("select * from mess order by birthday");
            while (res.next()) {
                String number = res.getString(1);
                String name = res.getString(2);
                Date date = res.getDate(3);
                float height = res.getFloat(4);
                System.out.printf("%s\t" , number);
                System.out.printf("%s\t" , name);
                System.out.printf("%s\t" , date);
                System.out.println(height);
            }
            connection.close();
        } catch (SQLException throwables) {
            System.out.println(throwables);
        }
    }
}

// Home.java
import javax.swing.*;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
public class Home {
    public static void main(String[] args) throws IOException {
        String[] tableHead;
        String[][] content;
        JTable table;
        JFrame win = new JFrame();
        Query findRecord = new Query();
        System.out.println("请输入数据库名和表名并以空格分割（一行内）:");
        BufferedReader buffer = new BufferedReader(new InputStreamReader(System.in));
        String[] input = buffer.readLine().split(" ");
        String dataBaseName = input[0];
        String tableName = input[1];
        findRecord.setDatabaseName(dataBaseName);
        findRecord.setSQL("select * from " + tableName);
        content = findRecord.getRecord();
        tableHead = findRecord.getColumnName();
        table = new JTable(content,tableHead);
        win.add(new JScrollPane(table));
        win.setBounds(12,100,400,200);
        win.setVisible(true);
        win.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }
}
// Query.java
import java.sql.*;
public class Query {
    String databaseName="";
    String SQL;
    String[] columnName;
    String[][] record;
    public Query() {}
    public void setDatabaseName(String s) {
        databaseName = s.trim();
    }
    public void setSQL(String SQL) {
        this.SQL = SQL.trim();
    }
    public String[] getColumnName() {
        if(columnName == null) {
            System.out.println("先查询记录");
            return null;
        }
        return columnName;
    }
    public String[][] getRecord() {
        startQuery();
        return record;
    }
    private void startQuery() {
        Connection con;
        ResultSet res;
        Statement sql;
        String url = "jdbc:mysql://localhost:3306/"+databaseName+"?useSSL=true&characterEncoding=utf-8";
        try {
            con = DriverManager.getConnection(url,"root","203420");
            sql = con.createStatement(ResultSet.TYPE_SCROLL_SENSITIVE,ResultSet.CONCUR_READ_ONLY);
            res = sql.executeQuery(SQL);
            ResultSetMetaData metaData = res.getMetaData();
            int columnCount = metaData.getColumnCount();
            columnName = new String[columnCount];
            for(int i = 1; i <= columnCount; i++) {
                columnName[i - 1] = metaData.getColumnName(i);
            }
            res.last();
            int recordAmount = res.getRow();
            record = new String[recordAmount][columnCount];
            int i = 0;
            res.beforeFirst();
            while (res.next()) {
                for(int j = 1; j <= columnCount; j++) {
                    record[i][j - 1] = res.getString(j);
                }
                i++;
            }
            con.close();
        } catch (SQLException throwables) {
            System.out.println("请输入正确的表名" + throwables);
        }
    }
}

// Home.java
public class Home {
    public static void main(String[] args) {
        Target target = new Target();
        Thread thread = new Thread(target);
        thread.start();
        for(int i = 0; i <= 10; i++) {
            System.out.println("yes");
            try  {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}
// Target.java
public class Target implements Runnable {
    @Override
    public void run() {
        for (int i = 0; i <= 10; i++) {
            System.out.println("ok");
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

// Home.java
public class Home {
    public static void main(String[] args) {
        Target target = new Target();
        Thread thread = new Thread(target);
        thread.run();
        for(int i = 0; i <= 10; i++) {
            System.out.println("yes");
            try  {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}
// Target.java
public class Target implements Runnable {
    @Override
    public void run() {
        for (int i = 0; i <= 10; i++) {
            System.out.println("ok");
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

// Home.java
public class Home {
    public static void main(String[] args) {
        Target target = new Target();
        Thread thread1 = new Thread(target);
        Thread thread2 = new Thread(target);
        thread1.start();
        try  {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            thread2.start();
        }
    }
}
// Target.java
public class Target implements Runnable {
    int i = 0;
    @Override
    public void run() {
        i++;
        System.out.println("i=" + i);
    }
}
//   i=1

public class Home {
    public static void main(String[] args) {
        Target target1 = new Target();
        Target target2 = new Target();
        Thread thread1 = new Thread(target1);
        Thread thread2 = new Thread(target2);
        thread1.start();
        try  {
            Thread.sleep(1000);
        } catch (Exception e) {
            thread2.start();
        }
    }
}
// i=1

public class Home {
    public static void main(String[] args) {
        javax.swing.Timer time = new javax.swing.Timer(500,new A());
        time.setInitialDelay(0);
        time.start();
    }
}
// A.java
import javax.swing.*;
import java.util.Date;
public class A  extends JLabel implements java.awt.event.ActionListener {
    @Override
    public void actionPerformed(java.awt.event.ActionEvent e) {
        System.out.println(new Date());
    }
}

public class A implements java.awt.event.ActionListener {
    @Override
    public void actionPerformed(java.awt.event.ActionEvent e) {
        System.out.println(new Date());
    }
}
// 直接退出了，计时器启动失败

public class A implements Runnable {
    Thread t1,t2;
    StringBuffer buffer = new StringBuffer();
    A() {
        t1 = new Thread(this);
        t2 = new Thread(this);
    }
    public synchronized void addChar(char c) {
        if(Thread.currentThread() == t1) {
            while (buffer.length() == 0) {
                try {
                    wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            buffer.append(c);
        }
        if(Thread.currentThread() == t2) {
            buffer.append(c);
            notifyAll();
        }
    }
    @Override
    public void run() {
        if(Thread.currentThread() == t1) {
            addChar('A');
        }
        if(Thread.currentThread() == t2) {
            addChar('B');
        }
    }
    public static void main(String[] args) {
        A hello = new A();
        hello.t1.start();
        hello.t2.start();
        while(hello.t1.isAlive() || hello.t2.isAlive()) {
        }
        System.out.println(hello.buffer);
    }
}
// BA
// 线程2启动之后在通知线程1

public class Bank implements Runnable {
    Thread t1,t2;
    Bank() {
        t1 = new Thread(this);
        t2 = new Thread(this);
    }
    @Override
    public void run() {
        printMess();
    }
    public void printMess() {
        System.out.println(Thread.currentThread().getName() + "正在使用这个方法");
        synchronized (this) {
            // 当一个线程使用同步块时，其他线程必须等待
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                System.out.println(Thread.currentThread().getName() + "正在使用这个同步块");
            }
        }
    }
    public static void main(String[] args) {
        Bank hello = new Bank();
        hello.t1.start();
        hello.t2.start();
    }
}
// Thread-1正在使用这个方法
// Thread-0正在使用这个方法

map.put("协议", url.getProtocol());
map.put("主机名称", url.getHost());
map.put("请求端口", url.getPort());
map.put("请求默认端口", url.getDefaultPort());
map.put("请求路径", url.getPath());
map.put("请求参数", url.getQuery());
map.put("请求文件", url.getFile());
map.put("认证信息", url.getAuthority());
map.put("用户信息", url.getUserInfo());
map.put("引用信息", url.getRef());