Java基础（三）- 多线程、网络通信、单元测试、反射、注解、动态代理

多线程基础

线程：一个程序内部的一条执行流程，只有一条执行流程就是单线程

java.lang.Thread代表线程

主线程退出，子线程存在，进程不会退出

可以使用jconsole查看

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创建线程

有多个方法可以创建线程

继承Thread类
- 优点：编码简单
- 缺点：无法继承其他类，不利于功能的扩展
实现Runnable接口
- 优点：任务类只是实现了接口，可以继续继承其他类、实现其他接口，扩展性强
- 缺点：需要多创建一个Runnable对象
实现Callable接口和FutureTask类
- 优点：可以返回线程执行结束之后的结果
- 缺点：编码复杂
执行为什么是start()？

使用run不是多线程，相当于直接调用方法还是单线程
```
start->start0(本地方法 JVM调用 C/C++实现的)
```

方法一

public class Demo1 {public static void main(String[] args) throws Exception {//main是主线程执行的//新建了一个t线程Thread t = new primeThread();//启动线程 start自动调用run方法 必须要调用start方法//如果是t.run() 相当于直接调用方法 还是单线程t.start();for (int i = 0; i < 5; i++) {System.out.println("主线程");Thread.sleep(500);}}
}class primeThread extends Thread{public primeThread(){}@Overridepublic void run() {//描述线程的执行的任务for (int i = 0; i < 5; i++) {System.out.println("子线程");try {Thread.sleep(500);} catch (Exception e) {e.printStackTrace();}}}
}

方法二

public class Demo2 {public static void main(String[] args) throws Exception {//runnable只是一个任务对象Runnable target = new prime1Thread();//需要线程对象接受任务对象 开辟新的线程new Thread(target).start();for (int i = 0; i < 5; i++) {System.out.println("主线程");Thread.sleep(500);}}
}class prime1Thread implements Runnable{@Overridepublic void run() {for (int i = 0; i < 5; i++) {System.out.println("子线程");try {Thread.sleep(500);} catch (Exception e) {e.printStackTrace();}}}
}//可以使用匿名内部类
public class Demo2 {public static void main(String[] args) throws Exception {//需要线程对象进行调用任务对象开辟新的线程new Thread(()-> {for (int i = 0; i < 5; i++) {System.out.println("子线程");try {Thread.sleep(500);} catch (InterruptedException e) {throw new RuntimeException(e);}	}}).start();for (int i = 0; i < 5; i++) {System.out.println("主线程");Thread.sleep(500);}}
}

方法三

import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.FutureTask;public class Demo3 {public static void main(String[] args) throws ExecutionException, InterruptedException {//创建一个Callable对象Callable<String> myCallable = new MyCallable(100);// 把Callable的对象封装成一个FutureTask对象(任务对象)// 未来任务对象的作用?// 1、是一个任务对象,实现下Runnable对象// 2、可以在线程执行完毕之后，用未来任务对象调用get方法获取线程执行完毕的结果//也可以使用匿名内部类FutureTask<String> stringFutureTask = new FutureTask<>(myCallable);new Thread(stringFutureTask).start();//获取结果会阻塞线程System.out.println(stringFutureTask.get());}
}//泛型
class MyCallable implements Callable<String>{private  int n;public MyCallable(int n) {this.n = n;}@Overridepublic String call() throws Exception {int sum = 0;for (int i = 1; i <= n; i++) {sum+=i;}return sum+"";}
}

线程方法

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setPriority()更改线程的优先级
getPriority()获取线程的优先级
interrupt中断线程，并不是真正的结束线程所以一般用于中断正在休眠的线程
yield线程的礼让，不一定礼让成功（和join相反，线程的插队）

public class Demo4 {public static void main(String[] args) throws InterruptedException {Thread t1 = new Thread1("1号线程");
//        t1.setName("1号线程");//启动之前取名字t1.start();t1.join();
//        System.out.println(t1.getName());Thread t2 = new Thread1("2号线程");
//        t2.setName("2号线程");//启动之前取名字t2.start();t2.join();//t2线程执行完成之后才能继续往下执行
//        System.out.println(t2.getName());Thread t3 = new Thread1("3号线程");t3.start();t3.join();Thread m = Thread.currentThread();m.setName("最牛逼的名字");
//        System.out.println(m.getName());for (int i = 0; i < 5; i++) {System.out.println(m.getName()+"输出"+(i+1));}}
}class Thread1 extends Thread{public Thread1(String name) {super(name);}@Overridepublic void run() {Thread t= Thread.currentThread();for (int i = 0; i < 3; i++) {System.out.println("子线程"+t.getName()+"输出:"+(i+1));}}
}

线程终止

当线程执行完成时，自动退出
使用变量来控制run方法退出的方式停止线程

守护线程

当所有的用户线程都退出时，守护线程自动退出

垃圾回收机制

public class Test {public static void main(String[] args) {//子线程设置为守护线程myDaemonThread myDaemonThread = new myDaemonThread();myDaemonThread.setDaemon(true);myDaemonThread.start();for (int i = 0; i < 10; i++) {System.out.println(Thread.currentThread().getName() + " 执行");try {Thread.sleep(1000);} catch (InterruptedException e) {throw new RuntimeException(e);}}}
}/*
守护线程：
当用户线程退出后 子线程也自动退出*/
class myDaemonThread extends Thread {@Overridepublic void run() {while (true) {System.out.println(Thread.currentThread().getName() + " 正在执行");try {Thread.sleep(500);} catch (InterruptedException e) {throw new RuntimeException(e);}}}
}

线程安全

概念

多个线程同时操作同一个共享资源的时候可能出现业务安全问题

模拟线程安全问题

package Thread_;public class Demo5 {public static void main(String[] args) {Thread xiaoHong = new DrawThread("小红");Thread xiaoMing = new DrawThread("小明");xiaoMing.start();xiaoHong.start();}
}
class Account{private static double moneys = 100000;private Account(){}public static double getMoneys() {return moneys;}public static void setMoneys(double moneys) {Account.moneys = moneys;}public static boolean drawMoneys(double moneys){String name = Thread.currentThread().getName();if (moneys>Account.getMoneys()){System.out.println(name+"来取钱，钱不够");return false;}Account.moneys-=moneys;System.out.println(name+"来取钱，取钱成功，剩余"+Account.moneys);return true;}
}class DrawThread extends Thread{public DrawThread(String name) {super(name);}@Overridepublic void run() {Account.drawMoneys(100000.0);}
}

线程同步

认识线程同步

多个线程实现先后依次访问共享资源

**加锁：**每次只允许一个线程加锁，加锁之后才能访问，访问完毕之后自动解锁，然后其他线程才能再加锁继续

方法一：同步代码块

把访问共享资源的核心代码给上锁，保证线程安全

synchronized(同步锁){访问共享资源的核心代码
}

对于当前同时执行的线程来说，同步锁必须是同一把（同一对象）

锁对象的选择：

实例对象：使用this
静态对象：使用类型.class

public class Demo5 {public static void main(String[] args) throws InterruptedException {Account acc1 = new Account(100000);Thread xiaoHong = new DrawThread("小红",acc1);Thread xiaoMing = new DrawThread("小明",acc1);xiaoMing.start();xiaoHong.start();Account acc2 = new Account(100000);Thread daGang = new DrawThread("大纲",acc2);Thread daLi = new DrawThread("大力",acc2);daGang.start();daLi.start();}
}class Account {private double moneys;public Account() {}public Account(double moneys) {this.moneys = moneys;}public double getMoneys() {return moneys;}public void setMoneys(double moneys) {this.moneys = moneys;}public void drawMoneys(double moneys) throws InterruptedException {String name = Thread.currentThread().getName();/** 两个人同时竞争lock这个对象（这把锁），只有一个人能够得到* 上锁之后另外一个人要等待开锁** 但是这个lock对于所有的对象是一个锁* 一个对象上锁的时候 和该对象无关的对象也无法进入核心代码* 非static建议使用 this* static建议使用 ClassName.class* */synchronized (this) {
//            Thread.sleep(5000); 测试if (moneys > this.getMoneys()) {System.out.println(name + "来取钱，钱不够");} else {this.moneys -= moneys;System.out.println(name + "来取钱，取钱" + moneys + "成功，剩余" + this.moneys);}}}
}class DrawThread extends Thread {private Account acc;public DrawThread(String name,Account acc) {super(name);this.acc = acc;}@Overridepublic void run() {try {acc.drawMoneys(100000);} catch (InterruptedException e) {throw new RuntimeException(e);}}
}

方法二：同步方法

访问共享资源的核心方法给上锁

修饰符 synchronized 返回值类型 方法名称(形参列表){操作共享资源的代码
}

public class Demo5 {public static void main(String[] args) throws InterruptedException {Account acc1 = new Account(100000);Thread xiaoHong = new DrawThread("小红", acc1);Thread xiaoMing = new DrawThread("小明", acc1);xiaoMing.start();xiaoHong.start();Account acc2 = new Account(100000);Thread daGang = new DrawThread("大纲", acc2);Thread daLi = new DrawThread("大力", acc2);daGang.start();daLi.start();}
}class Account {private double moneys;public Account() {}public Account(double moneys) {this.moneys = moneys;}public double getMoneys() {return moneys;}public void setMoneys(double moneys) {this.moneys = moneys;}/*有一个隐含的锁 实例方法是 this  静态方法是 类型.class*/public synchronized void drawMoneys(double moneys) throws InterruptedException {String name = Thread.currentThread().getName();if (moneys > this.getMoneys()) {System.out.println(name + "来取钱，钱不够");} else {this.moneys -= moneys;System.out.println(name + "来取钱，取钱" + moneys + "成功，剩余" + this.moneys);}}
}class DrawThread extends Thread {private Account acc;public DrawThread(String name, Account acc) {super(name);this.acc = acc;}@Overridepublic void run() {try {acc.drawMoneys(100000);} catch (InterruptedException e) {throw new RuntimeException(e);}}
}

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方法三：Lock锁

Lock锁是IDK5开始提供的一个新的锁定操作，通过它可以创建出锁对象进行加锁和解锁，更灵活、更方便、更强大

Lock是接口，不能直接实例化，可以采用它的实现类**ReentrantLock**来构建Lock锁对象。

package Thread_;import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;public class Demo5 {public static void main(String[] args) throws InterruptedException {Account acc1 = new Account(100000);Thread xiaoHong = new DrawThread("小红", acc1);Thread xiaoMing = new DrawThread("小明", acc1);xiaoMing.start();xiaoHong.start();Account acc2 = new Account(100000);Thread daGang = new DrawThread("大纲", acc2);Thread daLi = new DrawThread("大力", acc2);daGang.start();daLi.start();}
}class Account {/*创建了一个锁对象 每一个账户都有一个自己的锁对象不允许二次赋值*/private final Lock lk = new ReentrantLock();private double moneys;public Account() {}public Account(double moneys) {this.moneys = moneys;}public double getMoneys() {return moneys;}public void setMoneys(double moneys) {this.moneys = moneys;}public void drawMoneys(double moneys) throws InterruptedException {String name = Thread.currentThread().getName();try {lk.lock();if (moneys > this.getMoneys()) {System.out.println(name + "来取钱，钱不够");} else {this.moneys -= moneys;System.out.println(name + "来取钱，取钱" + moneys + "成功，剩余" + this.moneys);}} catch (Exception e) {throw new RuntimeException(e);} finally {lk.unlock();//无论try中代码是否有错误 都会解锁}}
}class DrawThread extends Thread {private Account acc;public DrawThread(String name, Account acc) {super(name);this.acc = acc;}@Overridepublic void run() {try {acc.drawMoneys(100000);} catch (InterruptedException e) {throw new RuntimeException(e);}}
}

释放锁的时机

当前线程的同步方法、同步代码块执行结束
当前线程在同步方法、同步代码块中遇到break、return
当前线程在同步方法、同步代码块中出现了未处理的Error或者Exception，导致异常结束
当前线程在同步方法、同步代码块中执行了线程对象的wait()方法，当前线程暂停释放锁，等待唤醒

不释放锁

Thread.sleep()、Thread.yeild不会释放锁
suspend()挂起方法，也不会释放锁
- suspend、resume控制线程，不推荐使用

线程死锁

多个线程都占用了对方的锁资源，但是不肯相让，导致了死锁

public class Demo {public static void main(String[] args) {new Thread(new MyDeadThread(false)).start();new Thread(new MyDeadThread(true)).start();}
}
class MyDeadThread implements Runnable{private boolean flag;private static Object o1 = new Object();private static Object o2 = new Object();public MyDeadThread() {}public MyDeadThread(boolean flag) {this.flag = flag;}@Overridepublic void run() {while (true){/*flag=true 占用o1锁 抢夺o2锁flag=false 占用o2锁 抢夺o1锁如果两个线程 一个占用o1 一个占用o2 那么就造成死锁*/if (flag){synchronized (o1){System.out.println("o1");synchronized (o2){System.out.println("o2");}}}else {synchronized (o2){System.out.println("o2");synchronized (o1){System.out.println("o1");}}}}}
}

线程通信

当多个线程共同操作共享资源的时候，线程间通过某种方式相互告知自己的状态，相互协调，避免无效的资源争夺

生产者消费者模型

生产者线程负责生产数据
消费者线程负责消费生产者生产的数据
生产者生产完数据应该等待，通知消费者消费；消费者消费完数据也应该等待，通知生产者生产

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public class ThreadTest {public static void main(String[] args) {Desk desk = new Desk();//3个生产者new Thread(()-> {while (true){desk.put();}},"厨师1").start();new Thread(()-> {while (true){desk.put();}},"厨师2").start();new Thread(()-> {while (true){desk.put();}},"厨师3").start();//2个消费者new Thread(()-> {while (true){desk.get();}},"吃货1").start();new Thread(()-> {while (true){desk.get();}},"吃货2").start();}
}import java.util.ArrayList;
import java.util.List;public class Desk {private final List<String>list = new ArrayList<>();//这个五个人是同一把锁public synchronized void put(){try {String name = Thread.currentThread().getName();if (list.isEmpty()){list.add(name+"做的肉包子");System.out.println(name+"做的肉包子");Thread.sleep(500);}//等待自己 唤醒别人 先唤醒后等待//只能线程对象调用this.notify();this.wait();} catch (Exception e) {throw new RuntimeException(e);}}public synchronized void get(){try {String name = Thread.currentThread().getName();if (!list.isEmpty()){System.out.println(name + "吃了"+list.remove(0));Thread.sleep(500);}//等待自己 唤醒别人 先唤醒后等待//只能线程对象调用this.notify();this.wait();} catch (Exception e) {throw new RuntimeException(e);}}
}

线程池

概念

可以复用线程的技术

**不使用线程池：**用户每发起一个请求，后台就需要创建一个新线程来处理，下次新任务来了肯定又要创建新线程处理的，而创建新线程的开销是很大的，并且请求过多时，肯定会产生大量的线程出来，这样会严重影响系统的性能。

使用`ExecutorService`创建线程池

使用ExecutorService的实现类ThreadPoolExecutor创建一个线程池对象（JDK5.0之后提供代表线程池的接口：ExecutorService）

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corePoolSize：指定线程池的核心线程的数量
maximumPoolSize：指定线程池的最大线程的数量
keepAliceTime：指定临时线程的存活时间
unit：指定临时线程存货时间的单位（秒、分、时、天）
workQueue：指定线程池的任务队列
threadFactory：指定线程池的线程工厂
handler：指定线程池的任务拒绝策略（线程都在忙，任务队列也满了的时候，新任务来了该怎么处理）

        /*ThreadPoolExecutor(int corePoolSize,int maximumPoolSize,long keepAliveTime,TimeUnit unit,BlockingQueue<Runnable> workQueue,ThreadFactory threadFactory,RejectedExecutionHandler handler)*/ExecutorService poolExecutor = new ThreadPoolExecutor(3, 5, 8, TimeUnit.SECONDS, new ArrayBlockingQueue<>(4), Executors.defaultThreadFactory(), new ThreadPoolExecutor.AbortPolicy());

什么时候创建临时对象？

新任务提交时发现核心线程都在忙，任务队列也满了，并且还可以创建临时线程，才会创建

什么时候会开始拒绝新任务？

核心线程和临时线程都在忙，任务队列也满了

新任务拒绝策略

在这里插入图片描述

处理Runnable任务

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import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.RejectedExecutionHandler;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;public class _ThreadPool {public static void main(String[] args) {/*ThreadPoolExecutor(int corePoolSize,int maximumPoolSize,long keepAliveTime,TimeUnit unit,BlockingQueue<Runnable> workQueue,ThreadFactory threadFactory,RejectedExecutionHandler handler)*/ExecutorService poolExecutor = new ThreadPoolExecutor(3, 5,8, TimeUnit.SECONDS, new ArrayBlockingQueue<>(4),Executors.defaultThreadFactory(), new ThreadPoolExecutor.CallerRunsPolicy());MyRunnable myRunnable1 = new MyRunnable();MyRunnable myRunnable2 = new MyRunnable();MyRunnable myRunnable3 = new MyRunnable();//三个核心线程在忙poolExecutor.execute(myRunnable1);poolExecutor.execute(myRunnable2);poolExecutor.execute(myRunnable3);//任务队列占满poolExecutor.execute(myRunnable3);poolExecutor.execute(myRunnable3);poolExecutor.execute(myRunnable3);poolExecutor.execute(myRunnable3);//可以创建两个临时线程poolExecutor.execute(myRunnable3);poolExecutor.execute(myRunnable3);//拒绝新任务poolExecutor.execute(myRunnable3);//        poolExecutor.shutdown();//等任务执行完后关闭线程池
//        poolExecutor.shutdownNow();//立刻关闭线程池}
}class MyRunnable implements Runnable {@Overridepublic void run() {String name = Thread.currentThread().getName();System.out.println(name + "666");try {Thread.sleep(100000);} catch (InterruptedException e) {throw new RuntimeException(e);}}
}

处理Callable任务

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import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;public class _ThreadPool {public static void main(String[] args) throws  Exception {/*ThreadPoolExecutor(int corePoolSize,int maximumPoolSize,long keepAliveTime,TimeUnit unit,BlockingQueue<Runnable> workQueue,ThreadFactory threadFactory,RejectedExecutionHandler handler)*/ExecutorService poolExecutor = new ThreadPoolExecutor(3, 5,8, TimeUnit.SECONDS, new ArrayBlockingQueue<>(4),Executors.defaultThreadFactory(), new ThreadPoolExecutor.CallerRunsPolicy());Future<String>f1 =  poolExecutor.submit(new MyCallable(100));Future<String>f2 =  poolExecutor.submit(new MyCallable(200));Future<String>f3 =  poolExecutor.submit(new MyCallable(300));Future<String>f4 =  poolExecutor.submit(new MyCallable(400));System.out.println(f1.get());System.out.println(f2.get());System.out.println(f3.get());System.out.println(f4.get());}
}class MyCallable implements Callable<String> {private  int n;public MyCallable(int n) {this.n = n;}@Overridepublic String call() throws Exception {int sum = 0;for (int i = 1; i <= n; i++) {sum+=i;}return Thread.currentThread().getName()+"计算出1-"+n+"的和为"+sum;}
}

使用`Executors`创建线程池（大型并发系统不建议）

（线程池的工具类）调用方法返回不同特点的线程池对象

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FixedThreadPool、SingleThreadExecutor允许请求队列长度为Integer.MAX_VALUE
CachedThreadPool允许创建线程数量为Integer.MAX_VALUE

这些方法的底层，都是通过线程池的实现类ThreadPoolExecutor创建的线程池对象

ExecutorService pool = Executors.newFixedThreadPool(3);

核心线程配置数量

计算密集型的任务：CPU核数+1
IO密集型的任务：CPU核数*2

并发和并行

并发的含义

进程中的线程是由CPU负责调度执行的，但是CPU能同时处理线程的数量是有限的。

为了保证全部线程都能往前执行，CPU会轮询为系统的每个线程服务，由于CPU切换速度很快，给我们的感觉就是这些线程在同时执行，这就是并发

并行的含义

同一时刻上，同时有多个线程在被CPU调度执行

线程生命周期

也就是线程从生到死的过程，经历的各种状态以及状态转换

理解线程这些状态有利于提高并发编程的理解能力

在这里插入图片描述

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扩展：悲观锁和乐观锁

悲观锁：一开始就加锁，没有安全感，每次只能一个线程进入，访问完毕后再解锁。线程安全性能较差

乐观锁：一开始不上锁，认为没问题，等出现线程安全的时候才开始控制。线程安全性能较好

//乐观锁
import java.util.concurrent.atomic.AtomicInteger;public class Demo7 {//一个静态变量，100个线程，每个线程对其加100次public static void main(String[] args) {Runnable mRunnable = new MRunnable2();for (int i = 0; i < 100; i++) {//100个线程执行相同的任务new Thread(mRunnable).start();}}
}class MRunnable2 implements Runnable {
//    private int count;//整数修改的乐观锁：原子类，private AtomicInteger count = new AtomicInteger();@Overridepublic void run() {for (int i = 0; i < 100; i++) {System.out.println("count====>" + (count.incrementAndGet()));}}
}

多线程练习

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Random;public class Test1 {public static void main(String[] args) throws InterruptedException {/*** 目标:有100份礼品，小红，小明两人同时发送，当剩下的礼品小于10份的时候则不再送出，* 利用多线程模拟该过程并将线程的名称打印出来。并最后在控制台分别打印小红，小明各自送出多少分礼物。*/ArrayList<String> gifts = new ArrayList<>();String[] names = {"口红", "包包", "腰带", "剃须刀", "香水", "衣服"};Random r = new Random();for (int i = 0; i < 100; i++) {gifts.add(names[r.nextInt(names.length)] + (i + 1));}sendThread xm = new sendThread(gifts, "小明");sendThread xh = new sendThread(gifts, "小红");xm.start();xh.start();xm.join();xh.join();System.out.println("小明送出去" + xm.getCount());System.out.println("小红送出去" + xh.getCount());}
}class sendThread extends Thread {private ArrayList<String> gifts;private int count;public int getCount() {return count;}public void setCount(int count) {this.count = count;}public sendThread() {}public sendThread(ArrayList<String> gifts, String name) {super(name);this.gifts = gifts;}@Overridepublic void run() {Random r = new Random();String name = Thread.currentThread().getName();while (true) {synchronized (gifts) {int length = gifts.size();if (length < 10)break;String s = gifts.remove(r.nextInt(length));System.out.println(name + "送出礼物" + s);++count;}}}
}

网络编程基础

可以让设备中的程序与网络上其他设备中的程序进行数据交互(实现网络通信的)

java.net.*的包下

网络通信三要素

IP地址：设备在网络中的地址，是唯一的标识
端口号：应用程序在设备中唯一的标识
协议：连接和数据在网络中传输的规则

java获取Ip地址：InetAddress

![在这里插入图片描述](https://img-blog.csdnimg.cn/direct/fe8b9baf7b3b42619311c8ac4ff54acf.png)

import java.net.InetAddress;
import java.net.UnknownHostException;public class GetIP {public static void main(String[] args) throws Exception {//本机InetAddress ip = InetAddress.getLocalHost();System.out.println(ip.getHostName());System.out.println(ip.getHostAddress());//指定InetAddress ipBaiDu = InetAddress.getByName("www.baidu.com");System.out.println(ipBaiDu.getHostName());System.out.println(ipBaiDu.getHostAddress());//本机ping 百度System.out.println(ipBaiDu.isReachable(6000));}
}

UDP通信

java.net.DatagramSocket实现UDP通信

在这里插入图片描述

一发一收

Client

import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;public class Client {public static void main(String[] args) throws Exception {//创建客户端 以及客户端端口DatagramSocket socket  = new DatagramSocket(6666);String data = "我是客户端，哈哈哈";byte[]bytes = data.getBytes();//创建数据包DatagramPacket packet = new DatagramPacket(bytes,bytes.length, InetAddress.getLocalHost(),5555);//发送数据socket.send(packet);System.out.println("客户端数据发送完毕");//释放资源socket.close();}
}

Serve

import java.net.DatagramPacket;
import java.net.DatagramSocket;public class Serve {public static void main(String[] args) throws Exception {System.out.println("===服务端启动===");//创建服务端 注册服务端端口DatagramSocket socket = new DatagramSocket(5555);byte[] buffer = new byte[1024*64];//64KB  UDP一个数据包最大为64KB//创建一个用来接收数据的数据包对象DatagramPacket packet = new DatagramPacket(buffer, buffer.length);//接受数据socket.receive(packet);//从字节数组中获取接受的数据int len = packet.getLength();String data = new String(buffer,0,len);System.out.println(data);//获取客户端的IP 端口System.out.println(packet.getAddress().getHostAddress());System.out.println(packet.getPort());//释放资源socket.close();}
}

多发多收

可以多个用户同时发送

Client

import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
import java.util.Scanner;public class Client {public static void main(String[] args) throws Exception {//创建客户端 以及客户端端口（默认随机分配）DatagramSocket socket  = new DatagramSocket();Scanner sc = new Scanner(System.in);while (true) {System.out.println("请输入消息：//exit是退出");String msg = sc.nextLine();if (msg.equals("exit")){System.out.println("欢迎下次光临");break;}byte[]bytes = msg.getBytes();//创建数据包DatagramPacket packet = new DatagramPacket(bytes,bytes.length, InetAddress.getLocalHost(),5555);//发送数据socket.send(packet);}socket.close();}
}

Serve

import java.net.DatagramPacket;
import java.net.DatagramSocket;public class Serve {public static void main(String[] args) throws Exception {System.out.println("===服务端启动===");//创建服务端 注册服务端端口DatagramSocket socket = new DatagramSocket(5555);byte[] buffer = new byte[1024*64];//64KB  UDP一个数据包最大为64KB//创建一个用来接收数据的数据包对象DatagramPacket packet = new DatagramPacket(buffer, buffer.length);while (true) {//接受数据socket.receive(packet);//从字节数组中获取接受的数据int len = packet.getLength();String data = new String(buffer,0,len);System.out.println(data);//获取客户端的IP 端口System.out.println(packet.getAddress().getHostAddress());System.out.println(packet.getPort());System.out.println("----------------");}}
}

TCP通信

客户端：java.net.Socket

在这里插入图片描述

一发一收

client

import java.io.DataOutputStream;
import java.io.OutputStream;
import java.net.InetAddress;
import java.net.Socket;public class ClientTCP {public static void main(String[] args) throws Exception {//创建socket对象Socket socket = new Socket(InetAddress.getLocalHost(), 5555);//从socket通信管道中得到一个字节输出流OutputStream os = socket.getOutputStream();//封装成数据输出流DataOutputStream dataOutputStream = new DataOutputStream(os);//写入数据dataOutputStream.writeUTF("你好呀！");//关闭数据流dataOutputStream.close();//关闭socketsocket.close();}
}

serve

import java.io.DataInputStream;
import java.io.InputStream;
import java.net.ServerSocket;
import java.net.Socket;public class ServeTCP {public static void main(String[] args) throws Exception{System.out.println("--服务端启动--");//创建服务端对象 绑定端口ServerSocket serverSocket = new ServerSocket(5555);//等待连接Socket socket = serverSocket.accept();//接受数据InputStream ds = socket.getInputStream();//封装DataInputStream dataInputStream = new DataInputStream(ds);//接受数据String s = dataInputStream.readUTF();System.out.println(s);//客户端ip地址System.out.println(socket.getRemoteSocketAddress());dataInputStream.close();socket.close();}
}

多发多收

client

import java.io.DataOutputStream;
import java.io.OutputStream;
import java.net.InetAddress;
import java.net.Socket;
import java.util.Objects;
import java.util.Scanner;public class ClientTCP {public static void main(String[] args) throws Exception {//创建socket对象Socket socket = new Socket(InetAddress.getLocalHost(), 5555);//从socket通信管道中得到一个字节输出流OutputStream os = socket.getOutputStream();//封装成数据输出流DataOutputStream dataOutputStream = new DataOutputStream(os);Scanner sc = new Scanner(System.in);while (true) {//写入数据System.out.println("请说：");String s = sc.nextLine();if (Objects.equals(s, "exit")){System.out.println("欢迎下次光临");break;  }dataOutputStream.writeUTF(s);dataOutputStream.flush();}//关闭数据流dataOutputStream.close();//关闭socketsocket.close();}
}

serve

import java.io.DataInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.ServerSocket;
import java.net.Socket;public class ServeTCP {public static void main(String[] args) throws Exception{System.out.println("--服务端启动--");//创建服务端对象 绑定端口ServerSocket serverSocket = new ServerSocket(5555);//等待连接Socket socket = serverSocket.accept();//接受数据InputStream ds = socket.getInputStream();//封装DataInputStream dataInputStream = new DataInputStream(ds);//接受数据while (true) {try {String s = dataInputStream.readUTF();System.out.println(s);//客户端ip地址
//                System.out.println(socket.getRemoteSocketAddress());} catch (IOException e) {System.out.println(socket.getRemoteSocketAddress()+"离线");break;}}dataInputStream.close();socket.close();}
}

多个客户端连接一个服务端

服务端：

主线程负责接受客户端连接
子线程负责具体每一个客户端

client

import java.io.DataOutputStream;
import java.io.OutputStream;
import java.net.Socket;
import java.util.Scanner;public class ClientTCP {public static void main(String[] args) throws Exception {Socket socket = new Socket("127.0.0.1", 8888);OutputStream os = socket.getOutputStream();DataOutputStream dos = new DataOutputStream(os);Scanner sc = new Scanner(System.in);while (true){String s = sc.nextLine();if (s.equals("exit")){System.out.println("欢迎下次光临");dos.close();socket.close();break;}dos.writeUTF(s);dos.flush();}}
}

serve

import java.io.DataInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.ServerSocket;
import java.net.Socket;
import java.net.SocketAddress;public class ServeTCP {public static void main(String[] args) throws Exception {System.out.println("服务端开启...");ServerSocket serverSocket = new ServerSocket(8888);while (true) {Socket socket = serverSocket.accept();System.out.println(socket.getRemoteSocketAddress()+"上线了");new Thread(new SocketThread(socket)).start();}}
}class SocketThread implements Runnable{private Socket socket;public SocketThread(Socket socket){this.socket = socket;}@Overridepublic void run() {SocketAddress remoteSocketAddress = socket.getRemoteSocketAddress();try {InputStream is = socket.getInputStream();DataInputStream dis = new DataInputStream(is);while (true) {try {String s = dis.readUTF();System.out.println(remoteSocketAddress+"发送："+s);} catch (Exception e) {System.out.println(remoteSocketAddress+"下线了");socket.close();dis.close();break;}}} catch (Exception e) {e.printStackTrace();}}
}

案例：群聊

client

import java.io.DataInputStream;
import java.io.DataOutputStream;import java.io.InputStream;
import java.io.OutputStream;
import java.net.InetAddress;
import java.net.Socket;
import java.util.Scanner;public class ClientChat {public static void main(String[] args) {try {Socket socket = new Socket("127.0.0.1", 8888);new ClientThread(socket).start();OutputStream os = socket.getOutputStream();DataOutputStream dos = new DataOutputStream(os);Scanner sc = new Scanner(System.in);while (true) {String s = sc.nextLine();if (s.equals("exit")) {System.out.println("欢迎下次光临");socket.close();dos.close();break;}dos.writeUTF(s);dos.flush();}} catch (Exception e) {e.printStackTrace();}}
}class ClientThread extends Thread {private Socket socket;public ClientThread(Socket socket) {this.socket = socket;}@Overridepublic void run() {try {InputStream is = socket.getInputStream();DataInputStream dis = new DataInputStream(is);while (true) {try {String msg = dis.readUTF();System.out.println(msg);} catch (Exception e) {dis.close();socket.close();break;}}} catch (Exception e) {e.printStackTrace();}}
}

serve

import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.ServerSocket;
import java.net.Socket;
import java.net.SocketAddress;
import java.util.ArrayList;
import java.util.List;
import java.util.Objects;public class ServeChat {public static final List<Socket> onlineUsers = new ArrayList<>();public static void main(String[] args) throws Exception{System.out.println("==服务器启动==");ServerSocket serverSocket = new ServerSocket(8888);while (true) {Socket socket = serverSocket.accept();onlineUsers.add(socket);new ServeReaderThread(socket).start();}}
}class ServeReaderThread extends Thread {private Socket socket;public ServeReaderThread(Socket socket) {this.socket = socket;}@Overridepublic void run() {try {InputStream is = socket.getInputStream();DataInputStream dis = new DataInputStream(is);while (true) {try {String msg = dis.readUTF();System.out.println(msg);sendAllOnlineUsers(socket,msg);} catch (Exception e) {ServeChat.onlineUsers.remove(socket);socket.close();dis.close();System.out.println(socket.getRemoteSocketAddress() + "下线");break;}}} catch (Exception e) {e.printStackTrace();}}private void sendAllOnlineUsers(Socket socket,String msg) throws Exception {for (Socket onlineUser : ServeChat.onlineUsers) {SocketAddress remoteSocketAddress = socket.getRemoteSocketAddress();if (Objects.equals(onlineUser.getRemoteSocketAddress(),remoteSocketAddress)){continue;}OutputStream os = onlineUser.getOutputStream();DataOutputStream dos = new DataOutputStream(os);dos.writeUTF(remoteSocketAddress+"说："+msg);dos.flush();}}
}

案例：简易BS架构

import java.io.DataOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.io.PrintStream;
import java.net.ServerSocket;
import java.net.Socket;public class Serve {public static void main(String[] args) throws Exception {ServerSocket serverSocket = new ServerSocket(8080);while (true){Socket socket = serverSocket.accept();System.out.println(socket.getRemoteSocketAddress()+"上线了");new CThread(socket).start();}}
}class CThread extends Thread{private Socket socket;public CThread(Socket socket){this.socket=socket;}@Overridepublic void run() {try {OutputStream os = socket.getOutputStream();PrintStream ps = new PrintStream(os);/*服务器必须给浏览器相应Http协议规定的格式*/ps.println("HTTP/1.1 200 OK");ps.println("Content-Type:text/html;charset=UTF-8");ps.println();//必须换行ps.println("<div style='color:red;font-size:120px;'>java666</div>");ps.close();socket.close();} catch (Exception e) {e.printStackTrace();}}
}

改进：线程池

import java.io.DataOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.io.PrintStream;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.Executors;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;public class Serve {public static void main(String[] args) throws Exception {ServerSocket serverSocket = new ServerSocket(8080);ThreadPoolExecutor pool = new ThreadPoolExecutor(16 * 2, 16 * 2, 0, TimeUnit.SECONDS, new ArrayBlockingQueue<>(8), Executors.defaultThreadFactory(), new ThreadPoolExecutor.AbortPolicy());while (true){Socket socket = serverSocket.accept();System.out.println(socket.getRemoteSocketAddress()+"上线了");pool.execute(new CThread(socket));}}
}class CThread implements Runnable{private Socket socket;public CThread(Socket socket){this.socket=socket;}@Overridepublic void run() {try {OutputStream os = socket.getOutputStream();PrintStream ps = new PrintStream(os);/*服务器必须给浏览器相应Http协议规定的格式*/ps.println("HTTP/1.1 200 OK");ps.println("Content-Type:text/html;charset=UTF-8");ps.println();//必须换行ps.println("<div style='color:red;font-size:120px;'>java666</div>");ps.close();socket.close();} catch (Exception e) {e.printStackTrace();}}
}

案例：多用户即时通信系统

需求分析

用户登录
拉取在线用户
无异常退出
私聊
群聊
发文件
服务器推送新闻

java高级

单元测试

就是针对最小的功能单元（方法），编写测试代码对其进行正确性测试

junit单元测试框架

可以灵活的编写测试代码，可以针对某个方法执行测试，也支持一键完成对全部的方法自动化测试
不需要程序员去分析测试结果，会自动生成测试报告

具体使用

public class Demo {public static void printNumber(String name){if (name==null)return;System.out.println("名字长度："+name.length());}public static int getMaxIndex(String data){if (data==null)return -1;return data.length();}
}

import org.junit.After;
import org.junit.AfterClass;
import org.junit.Assert;
import org.junit.Before;
import org.junit.BeforeClass;
import org.junit.Test;/*
测试类*/
public class DemoTest {@Beforepublic void test1(){System.out.println("---------Before---------");}@Afterpublic void test2(){System.out.println("---------After---------");}@AfterClasspublic static void test3(){System.out.println("---------AfterClass---------");}@BeforeClasspublic static void test4(){System.out.println("---------BeforeClass---------");}/*公开 无返回值*/@Test //测试方法public void testPrintNumber(){Demo.printNumber("admin");Demo.printNumber(null);}@Test //测试方法public void testGetMaxIndex(){//断言机制：可以通过预测业务方法的结果来测试 bugSystem.out.println(Demo.getMaxIndex("admin"));System.out.println(Demo.getMaxIndex(null));//断言机制：可以通过预测业务方法的结果来测试 bugAssert.assertEquals("有bug",4,Demo.getMaxIndex("admin"));}
}

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以下是学习框架源码的时候会用到，开发几乎不会用

反射

认识反射

加载类，并允许以编程的方式解剖类中的各个成分（成员变量、方法、构造器等）

步骤

加载类，获取类的字节码：Class对象
获取类的构造器：Constructor对象
获取类成员变量：Field对象
获取类成员方法：Method对象

获取类的字节码

Class c1 = 类名.class
调用Class提供的方法 public static Class forName(String package); 全类名
Object的方法对象.getClass()

获取类的构造器

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import java.lang.reflect.Constructor;public class Demo1 {public static void main(String[] args) throws Exception {Class c = Cat.class;Constructor constructor = c.getDeclaredConstructor();System.out.println(constructor.getName()+"--"+constructor.getParameterCount());Cat o = (Cat) constructor.newInstance();System.out.println(o);Constructor declaredConstructor = c.getDeclaredConstructor(String.class, int.class);System.out.println(declaredConstructor.getName()+"--"+declaredConstructor.getParameterCount());declaredConstructor.setAccessible(true);//打破修饰符的限制Cat o1 = (Cat)declaredConstructor.newInstance("学习", 5);}
}   class Cat{private String name;private int age;public String getName() {return name;}public void setName(String name) {this.name = name;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}public Cat() {}private Cat(String name, int age) {this.name = name;this.age = age;}public Cat(String name) {this.name = name;}
}

获取类的成员变量

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import java.lang.reflect.Field;public class Demo1 {public static void main(String[] args) throws Exception {Class c = Cat.class;Field[] fields = c.getDeclaredFields();for (Field field : fields) {System.out.println(field.getName()+"--"+field.getType());}Field name = c.getDeclaredField("name");System.out.println(name.getName()+"--"+name.getType());Cat cat = new Cat();name.setAccessible(true);name.set(cat,"猫猫");System.out.println(name.get(cat));}
}class Cat{public static int a;public static final String COUNTRY ="中国";private String name;private int age;public String getName() {return name;}public void setName(String name) {this.name = name;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}public Cat() {}private Cat(String name, int age) {this.name = name;this.age = age;}public Cat(String name) {this.name = name;}
}

获取类的成员方法

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作用、应用场景

基本作用：可以得到一个类全部成分对其操作；可以破坏封装性；适合做java的框架

import java.io.FileOutputStream;
import java.io.PrintStream;
import java.lang.reflect.Field;public class Test {public static void main(String[] args)throws Exception {Student stu = new Student("小明", 18, 82.5);Teacher tea = new Teacher("大强", 58);saveObject(stu);saveObject(tea);}public static void saveObject(Object obj) throws Exception {Class o = obj.getClass();String cname = o.getSimpleName();PrintStream ps = new PrintStream(new FileOutputStream("./out/obj.txt",true));ps.println("---------"+cname+"---------");Field[] fields = o.getDeclaredFields();for (Field field : fields) {field.setAccessible(true);String name = field.getName();String value = field.get(obj)+"";ps.println(name+":"+value);}ps.close();}
}
class Student{private String name;private int age;private double sorce;public Student(String name, int age, double sorce) {this.name = name;this.age = age;this.sorce = sorce;}
}class Teacher{private String name;private int age;public Teacher(String name, int age) {this.name = name;this.age = age;}
}

注解

就是java中特殊的标记，比如@override、@Test等

作用：让其他程序根据注解信息来决定怎么执行程序

注解可以用在类、方法、构造器、成员变量、参数等等

自定义注解

public @interface 注解名称{public 属性类型 属性名() default 默认值;
}

只有一个注解且为 value 可以省略不写value

注解原理

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注解本质就是一个接口，java中所有的注解都是继承了Annotation的接口

@注解(…)其实就是一个实现类对象，实现了该注解以及Annotation的接口

元注解

修饰注解的注解

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注解的解析

就是判断类上、方法上、成员变量上是否存在注解，并把注解里的内容给解析出来。

要解析谁的注解，就要先拿到谁

Class、Method、Field,Constructor、都实现了AnnotatedElement接口，它们都拥有解析注解的能力。
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package annotation;import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;@Target({ElementType.TYPE,ElementType.METHOD})//当前被修饰的注解只能使用在类上
@Retention(RetentionPolicy.RUNTIME)
public @interface MyTest{String value();double aaa() default 100;String[] bbb();
}

package annotation;@MyTest(value = "大强",aaa = 199.9,bbb={"css","java","html"})public class Demo {
@MyTest(value = "小明",aaa = 99.9,bbb={"java","html"})void test(){}
}

package annotation;import java.lang.reflect.Method;
import java.util.Arrays;public class AnnotationTest {public static void main(String[] args) throws Exception {Class c = Demo.class;Method test = c.getDeclaredMethod("test");if (c.isAnnotationPresent(MyTest.class)) {MyTest myTest = (MyTest) c.getDeclaredAnnotation(MyTest.class);System.out.println(myTest.value());System.out.println(myTest.aaa());System.out.println(Arrays.toString(myTest.bbb()));}if (test.isAnnotationPresent(MyTest.class)) {MyTest myTest = test.getDeclaredAnnotation(MyTest.class);System.out.println(myTest.value());System.out.println(myTest.aaa());System.out.println(Arrays.toString(myTest.bbb()));}}
}

应用场景

模拟junit

package annotation;import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;public class TestTest {@MyTest2public void test1(){System.out.println("==test1==");}public void test2(){System.out.println("==test2==");}public void test3(){System.out.println("==test3==");}public static void main(String[] args) throws Exception {Class c = TestTest.class;Method[] methods = c.getDeclaredMethods();for (Method method : methods) {if (method.isAnnotationPresent(MyTest2.class)){method.invoke(new TestTest());}}}
}

动态代理

概念

对象做的事情太多的话，可以通过代理来转移部分职责

![外链图片转存失败,源站可能有防盗链机制,建议将图片保存下来直接上传](https://img-blog.csdnimg.cn/direct/867cad296ec44d6385e47ae55f719acc.png)

package proxy;import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;public class ProxyUtil {public static Star createProxy(BigStar bigStar) {/*参数1：指定一个类加载器参数2：指定生成的代理是什么样子，也就是有什么方法参数3：指定生成的代理对象要干什么事情*/return (Star) Proxy.newProxyInstance(ProxyUtil.class.getClassLoader(), new Class[]{Star.class}, new InvocationHandler() {@Overridepublic Object invoke(Object proxy, Method method, Object[] args) throws Throwable {//代理对象要做的事情 会在这里写代码if (method.getName().equals("sing")){System.out.println("准备话筒，收钱20w");}else if (method.getName().equals("dance")){System.out.println("准备场地，收钱1000w");}return method.invoke(bigStar,args);}});}
}

package proxy;public class BigStar implements Star {private String name;public BigStar() {}public BigStar(String name) {this.name = name;}@Overridepublic String sing(String name) {System.out.println(this.name+"正在唱"+name+"歌~~~");return "谢谢！谢谢~";}@Overridepublic void dance() {System.out.println(name+"正在跳舞~~~");}
}

package proxy;public interface Star {public String sing(String name);public void dance();
}

package proxy;public class Test {public static void main(String[] args) {BigStar s = new BigStar("杨超越");Star starProxy = ProxyUtil.createProxy(s);String rs = starProxy.sing("好日子");System.out.println(rs);System.out.println("--------------------------");starProxy.dance();}
}

坦克大战

java坐标系

下图说明了Java坐标系。坐标原点位于左上角，以像素为单位。在Java坐标系中,第一个是x坐标,表示当前位置为水平方向，距离坐标原点x个像素;第二个是y坐标，表示当前位置为垂直方向，距离坐标原点y个像素。

外链图片转存失败,源站可能有防盗链机制,建议将图片保存下来直接上传

package tankeGame;import javax.swing.*;
import java.awt.*;public class Draw extends JFrame {//JFrame 对应窗口 可以理解为一个画框private MyPanel mp =null;//定义一个画板public Draw(){//初始化画板mp = new MyPanel();//画板放入窗口this.add(mp);//设置窗口大小this.setSize(1000,800);//可以显示this.setVisible(true);//点窗口的× 程序退出this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);}public static void main(String[] args) {new Draw();}
}//1.定义一个MyPanel继承JPanel，这个就是画板  画图形
class MyPanel extends JPanel{/*MyPanel：画板（面板）对象Graphics g：画笔paint调用时机：1.组件第一次在屏幕中显示的时候，系统自动调用2.窗口最小化 再最大化3.窗口大小发生变化4.repaint函数被调用*/@Overridepublic void paint(Graphics g) {//绘图的方法super.paint(g);//调用父类的方法完成初始化//画一个圆g.drawOval(10,10,100,100);//画直线g.drawLine(10,10,60,60);//画矩形g.drawRect(10,10,100,100);//填充矩形//设置画笔颜色g.setColor(Color.BLUE);g.fillRect(50,50,100,100);g.fillOval(200,200,50,60);//画图片//1.加载图片资源Image image = Toolkit.getDefaultToolkit().getImage("d:/shangan.png");//2.画图片g.drawImage(image,300,300,300,300,this);//画字符串g.setColor(Color.cyan);g.setFont(new Font("隶书",Font.BOLD,50));//位置是字体的左下角g.drawString("yb0os1",500,100);}
}

事件处理机制

委派事件模型

import javax.swing.*;
import java.awt.*;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;//事件控制 键盘控制小球的移动
//画笔
public class BallMove extends JFrame {private DrawBall ball = null;public BallMove() {ball = new DrawBall();this.add(ball);this.setVisible(true);this.setSize(500, 400);this.addKeyListener(ball);//JFame对象可以监听ball上面发生的键盘事件this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);}public static void main(String[] args) {new BallMove();}
}//画板
//KeyListener 监听器 监听键盘事件
class DrawBall extends JPanel implements KeyListener {int x = 10;int y = 10;@Override//有字符输出时 该方法会触发public void keyTyped(KeyEvent e) {}@Override//当某个键被按下时 该方法会触发public void keyPressed(KeyEvent e) {
//        System.out.println((char) e.getKeyChar() + "被按下");//根据用户按下的不同键，来处理小球的移动//java中给每一个键分配一个值switch (e.getKeyCode()){case KeyEvent.VK_DOWN://向下的箭头++y;break;case KeyEvent.VK_UP://向上的--y;break;case KeyEvent.VK_LEFT://向左--x;break;case KeyEvent.VK_RIGHT://向右++x;break;}//重绘面板this.repaint();}@Override//当某个键被松开时 该方法会触发public void keyReleased(KeyEvent e) {}@Overridepublic void paint(Graphics g) {super.paint(g);g.fillOval(x, y, 20, 20);}
}