/*hello world程序*/
public class HelloWorld {
    public static void main(String []args) {
        System.out.println("Hello World"); //打印Hello World
    }
}

int a = 6;

Integer oa = new Integer(6);

int a[] = new int[10]; //定义了一个长度是10的int类型数组

a[1] = 3; //将下标是1的数组元素赋值为3
System.out.println(a[2]); //打印下标是2的数组元素值

// 用法一
int a;
if (a > 1) {
  //执行代码块
} else {
  //执行代码块
}

// 用法二
int a;
if (a > 1) {
  //执行代码块
} else if (a == 1) {
  //执行代码块
} else {
  //执行代码块
}

int a;
switch (a) {
  case 1:
    //执行代码块
    break;
  case 2:
    //执行代码块
    break;
  default:
    //默认执行代码
}

for (int i = 0; i < 10; ++i) {
  // 循环执行10次此代码块
}

int i = 0;
while (i < 10) {
  // 循环执行10次此代码块
}

for (int i = 0; i < 10; ++i) {
  if (i == 4) {
    continue; //跳过本次循环，不会打印出4这个值
  }
  System.out.println(i);
}

for (int i = 0; i < 10; ++i) {
  if (i == 4) {
    break; //提前终止循环，只会打印0、1、2、3
  }
  System.out.println(i);
}

public void func(int a) {
  if (a == 1) {
    return; //结束一个函数，从此处返回
  }
  System.out.println(a);
}

public class Dog { // 定义了一个Dog类
  private int age; // 属性或者成员变量
  private int weight;

  public Dog(int age, int weight) { // 构造函数
    this.age = age;
    this.weight = weight;
  }

  public int getAge() { // 函数或者方法
    return age;
  }
  
  public int getWeigt() {
    return weight;
  }
  
  public void run() {
    // ...
  }
}

Dog dog1 = new Dog(2, 10);//通过new关键词创建了一个Dog对象dog1
int age = dog1.getAge();//调用dog1的getAge()方法
dog1.run();//调用dog1的run()方法

public class  ClassName{
    public void printNumber（）{
        int a;
    }
    // 其他代码
}

public class  ClassName{
    int a;
    public void printNumber（）{
        // 其他代码
    }
}

public class  ClassName{
    static int a;
    public void printNumber（）{
        // 其他代码
    }
}

// 抽象类
public abstract class Logger {
  private String name;
  private boolean enabled;
  private Level minPermittedLevel;

  public Logger(String name, boolean enabled, Level minPermittedLevel) {
    this.name = name;
    this.enabled = enabled;
    this.minPermittedLevel = minPermittedLevel;
  }
  
  public void log(Level level, String message) {
    boolean loggable = enabled && (minPermittedLevel.intValue() <= level.intValue());
    if (!loggable) return;
    doLog(level, message);
  }
  
  protected abstract void doLog(Level level, String message);
}
// 抽象类的子类：输出日志到文件
public class FileLogger extends Logger {
  private Writer fileWriter;

  public FileLogger(String name, boolean enabled,
    Level minPermittedLevel, String filepath) {
    super(name, enabled, minPermittedLevel);
    this.fileWriter = new FileWriter(filepath); 
  }
  
  @Override
  public void doLog(Level level, String mesage) {
    // 格式化level和message,输出到日志文件
    fileWriter.write(...);
  }
}
// 抽象类的子类: 输出日志到消息中间件(比如kafka)
public class MessageQueueLogger extends Logger {
  private MessageQueueClient msgQueueClient;
  
  public MessageQueueLogger(String name, boolean enabled,
    Level minPermittedLevel, MessageQueueClient msgQueueClient) {
    super(name, enabled, minPermittedLevel);
    this.msgQueueClient = msgQueueClient;
  }
  
  @Override
  protected void doLog(Level level, String mesage) {
    // 格式化level和message,输出到消息中间件
    msgQueueClient.send(...);
  }
}

public class Dog {// public修饰类
  private int age; // private修饰属性，只能在类内部使用
  private int weight;
  
  public Dog(int age, int weight) {
    this.age = age;
    this.weight = weight;
  }

  public int getAge() { //public修饰的方法，任意代码都是可以调用
    return age;
  }
  
  public void run() {
    // ...
  }

}

public class Animal { // 父类
  protected int age;
  protected int weight;
  
  public Animal(int age, int weight) {
    this.age = age;
    this.weight = weight;
  }
  
  public int getAge() { // 函数或者方法
    return age;
  }
  
  public int getWeigt() {
    return weight;
  }
  
  public void run() {
    // ...
  }
}

public class Dog extends Animal { // 子类
  public Dog(int age, int weight) { // 构造函数
    super(age, weight); //调用父类的构造函数
  }

  public void wangwang() {
    //...
  }
}

public class Cat extends Animal { //子类
  public Cat(int age, int weight) { // 构造函数
    super(age, weight); //调用父类的构造函数
  }
  
  public void miaomiao() {
    //...
  }
}

//使用举例
Dog dog = new Dog(2, 8);
dog.run();
dog.wangwang();
Cat cat = new Cat(1, 3);
cat.run();
cat.miaomiao();

public interface Runnable {
  void run();
}

public class Dog implements Runnable {
  private int age; // 属性或者成员变量
  private int weight;

  public Dog(int age, int weight) { // 构造函数
    this.age = age;
    this.weight = weight;
  }

  public int getAge() { // 函数或者方法
    return age;
  }

  public int getWeigt() {
    return weight;
  }

  @Override 
  public void run() { //实现接口中定义的run()方法
    // ...
  }
}

public class DemoA {
  private ArrayList<User> users;
  
  public void addUser(User user) {
    users.add(user);
  }
}

public class UserNotFoundException extends Exception { // 自定义一个异常
  public UserNotFoundException() {
    super();
  }

  public UserNotFoundException(String message) {
    super(message);
  }

  public UserNotFoundException(String message, Throwable e) {
    super(message, e);
  }
}

public class UserService {
  private UserRepository userRepo;
  public UserService(UseRepository userRepo) {
    this.userRepo = userRepo;
  }

  public User getUserById(long userId) throws UserNotFoundException {
    User user = userRepo.findUserById(userId);
    if (user == null) { // throw用来抛出异常
      throw new UserNotFoundException();//代码从此处返回
    }
    return user;
  }
}

public class UserController {
  private UserService userService;
  public UserController(UserService userService) {
    this.userService = userService;
  }
  
  public User getUserById(long userId) {
    User user = null;
    try { //捕获异常
      user = userService.getUserById(userId);
    } catch (UserNotFoundException e) {
      System.out.println("User not found: " + userId);
    } finally { //不管异常会不会发生，finally包裹的语句块总会被执行
      System.out.println("I am always printed.");
    }
    return user;
  }
}

/*class DemoA*/
package com.xzg.cd; // 包名com.xzg.cd

public class DemoA {
  //...
}

/*class DemoB*/
package com.xzg.alg;

import java.util.HashMap; // Java工具包JDK中的类
import java.util.Map;
import com.xzg.cd.DemoA;

public class DemoB {
  //...
}

try{
// 可能产生异常的代码
}catch(Type1 id1){
// 处理Type1异常的代码
}catch(Type2 id2){
// 处理Type2异常的代码
}

catch(exception e){
  System.err.printIn("an exception throw");
  throw e;
}

PrintWriter pw = new PrintWriter(
  new ButteredWriter(
    new OutputStreamWriter(
      new FileOutputStream("abc.txt")
    )
  )
);