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Java的一些知识2

2020-05-20 编程语言

杂项

国际化

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package com.poorpool.demo;

import java.text.MessageFormat;
import java.util.Locale;
import java.util.ResourceBundle;

public class HelloDemo {
    public static void main(String[] args) throws Exception {
        ResourceBundle resourceBundle = ResourceBundle.getBundle("com.poorpool.resource.Ds", Locale.US);
        // properties: hello.msg={0}, hello!
        String str = resourceBundle.getString("hello.msg");
        System.out.println(MessageFormat.format(str, "poorpool"));
    }
}

ThreadLocal

先来看一段明显错误

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package com.poorpool.demo;

class Message {
    private String content;

    public String getContent() {
        return content;
    }

    public void setContent(String content) {
        this.content = content;
    }
}

class Resource {
    public static Message message;
}

class MessagePrint {
    public static void print() {
        System.out.println(Resource.message.getContent());
    }
}

public class HelloDemo {
    public static void main(String[] args) throws Exception {
        String[] vals = new String[]{"poorpool", "thank", "you"};
        for(String val : vals) {
            new Thread(()->{
                Resource.message = new Message();
                Resource.message.setContent(val);
                MessagePrint.print();
            }).start();
        }
    }
}

输出全是you。

用ThreadLocal每个线程就能保存自己的东西：

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package com.poorpool.demo;

class Message {
    private String content;

    public String getContent() {
        return content;
    }

    public void setContent(String content) {
        this.content = content;
    }
}

class Resource {
    public static final ThreadLocal<Message> MESSAGES = new ThreadLocal<>();

}

class MessagePrint {
    public static void print() {
        System.out.println(Resource.MESSAGES.get().getContent());
    }
}

public class HelloDemo {
    public static void main(String[] args) throws Exception {
        String[] vals = new String[]{"poorpool", "thank", "you"};
        for(String val : vals) {
            new Thread(()->{
                Resource.MESSAGES.set(new Message());
                Resource.MESSAGES.get().setContent(val);
                MessagePrint.print();
            }).start();
        }
    }
}

Timer类

干守护线程类似的任务。

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package com.poorpool.demo;

import java.util.Timer;
import java.util.TimerTask;

class TimeThread extends TimerTask {

    @Override
    public void run() {
        System.out.println("定时任务");
    }
}
public class HelloDemo {
    public static void main(String[] args) throws Exception {
        Timer timer = new Timer();
        timer.schedule(new TimeThread(), 1000, 2000);
    }
}

schedule方法有挺多，可以看看。

流

OutputStream字节输出流

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package com.poorpool.demo;

import java.io.File;
import java.io.FileOutputStream;
import java.io.OutputStream;

public class HelloDemo {
    public static void main(String[] args) throws Exception {
        File file = new File("oicode" + File.separator + "20200515" + File.separator + "qwq.txt");
        if(!file.getParentFile().exists())
            file.getParentFile().mkdirs();
        OutputStream output = new FileOutputStream(file);
        output.write("yxchen.net\n".getBytes());
        output.close();
    }
}

InputStream字节输入流

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package com.poorpool.demo;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.InputStream;

public class HelloDemo {
    public static void main(String[] args) {
        File file = new File("qwq.cpp");
        try(InputStream inputStream = new FileInputStream(file)) {
            StringBuffer buffer = new StringBuffer();
            byte[] bytes = new byte[10];
            int len = 0;
            while((len=inputStream.read(bytes, 0, 10))>=0)
                buffer.append(new String(bytes, 0, len));
            System.out.println(buffer);
        } catch (Exception e) {
            e.printStackTrace();
        }

    }
}

Writer字符输出流

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package com.poorpool.demo;

import java.io.*;

public class HelloDemo {
    public static void main(String[] args) {
        File file = new File("qwq.txt");
        try(Writer writer = new FileWriter(file)) {
            writer.write("poorpool\n");
            writer.write("hello!\n");
            writer.append("qwq\n").append("233\n");//或者writer.append()。write()返回void，append()返回writer
        }
        catch (Exception e) {
            e.printStackTrace();
        }

    }
}

Reader字符输入流

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package com.poorpool.demo;

import java.io.*;

public class HelloDemo {
    public static void main(String[] args) {
        File file = new File("qwq.txt");
        try(Reader reader = new FileReader(file)) {
            char []data = new char[5];
            StringBuffer buffer = new StringBuffer();
            int len = 0;
            while((len=reader.read(data, 0, 5))>=0)
                buffer.append(data, 0, len);
            System.out.println(buffer);
        }
        catch (Exception e) {
            e.printStackTrace();
        }

    }
}

字节流和字符流的区别

因为字节流是字节，字节输出流便可以直接与目标介质进行输出控制。字符流要有一个内存缓冲区，flush一下才会写入目标介质。具体地，写了不close，字节输出流的文件有东西，字符输出流的文件没东西。中文主要用字符输出流。

转换流

zhuanhuanliu

顺带提一下transferTo。这样能很轻松实现文件复制。

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package com.poorpool.demo;

import java.io.*;

public class HelloDemo {
    public static void main(String[] args) throws Exception {
        InputStream inputStream = new FileInputStream(new File("cpt1-1.png"));
        OutputStream outputStream = new FileOutputStream(new File("qwq.png"));
        inputStream.transferTo(outputStream);
    }
}

内存流

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package com.poorpool.demo;

import java.io.*;

public class HelloDemo {
    public static void main(String[] args) throws Exception {
        InputStream input = new ByteArrayInputStream("poorpool".getBytes());
        OutputStream output = new ByteArrayOutputStream();
        int data = 0;
        while((data = input.read())>=0) {
            output.write(Character.toUpperCase(data));
        }
        System.out.println(output);
        input.close();
        output.close();
    }
}

还有管道流，用于多线程or进程之间的输入输出。这个不写了。

RandomAccessFile

实现文件的随机读取。

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package com.poorpool.demo;

import java.io.*;

public class HelloDemo {
    public static final int MAX_LENGTH=10;
    public static String addEscape(String x) {//随机读取的密宝（等长）
        StringBuffer buffer = new StringBuffer(x);
        while(buffer.length()<MAX_LENGTH)
            buffer.append(" ");
        return buffer.toString();
    }
    public static void writes() {
        File file = new File("qwq.txt");
        try(RandomAccessFile raf = new RandomAccessFile(file, "rw")) {
            String[] names = new String[]{"poorpool", "qwq", "qaq"};
            int[] ages = new int[]{12, 2, 999};
            for(int i=0; i<names.length; i++) {
                raf.write(addEscape(names[i]).getBytes());
                raf.writeInt(ages[i]);
            }
        }
        catch (Exception e) {
            e.printStackTrace();
        }
    }
    public static void reads() {
        File file = new File("qwq.txt");
        try(RandomAccessFile raf = new RandomAccessFile(file, "r")) {
            raf.skipBytes(MAX_LENGTH+4);//随机读取的密宝
            byte[] data = new byte[MAX_LENGTH];
            raf.read(data);
            System.out.println(new String(data).trim());
            System.out.println(raf.readInt());
        }
        catch (Exception e) {
            e.printStackTrace();
        }
    }
    public static void main(String[] args) throws Exception {
        writes();
        reads();
    }
}

打印流

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package com.poorpool.demo;

import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.PrintWriter;

public class HelloDemo {
    public static void main(String[] args) throws Exception {
        File file = new File("qwq.txt");
        PrintWriter pw = new PrintWriter(new FileOutputStream(file));
        pw.println(10);
        pw.printf("hello %s, your age is %d\n", "porp", 16);
        pw.close();
    }
}

Scanner

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package com.poorpool.demo;

import java.util.Scanner;

public class HelloDemo {
    public static void main(String[] args) throws Exception {
        Scanner scanner = new Scanner(System.in);//System.in是一个InputStream常量。构造函数参数也可以是File之类的。
        /*
        if(scanner.hasNext()) {//输入字符串
            String qwq = scanner.next();
            System.out.println(qwq);
        }
        if(scanner.hasNextInt()) {
            int qwq = scanner.nextInt();
            System.out.println(qwq);
        }
        if(scanner.hasNext("\\w+@\\w+\\.\\w+")) {//正则匹配的
            String qwq = scanner.next("\\w+@\\w+\\.\\w+");
            System.out.println(qwq);
        }*/
        scanner.useDelimiter("\n");//分隔符
        while(scanner.hasNext())
            System.out.println(scanner.next());
    }
}

这个最好使。不行就BufferReader。

输出可以考虑PrintWriter。

序列化

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package com.poorpool.demo;

import java.io.*;

class Book implements Serializable {
    private String name;
    private String author;
    private double price;

    public Book(String name, String author, double price) {
        this.name = name;
        this.author = author;
        this.price = price;
    }

    @Override
    public String toString() {
        return "Book{" +
                "name='" + name + '\'' +
                ", author='" + author + '\'' +
                ", price=" + price +
                '}';
    }
}

public class HelloDemo {
    public static void main(String[] args) throws Exception {
        ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream(new File("qwq.txt")));
        oos.writeObject(new Book("qwq", "orz", 2.33));
        oos.close();

        ObjectInputStream ois = new ObjectInputStream(new FileInputStream(new File("qwq.txt")));
        Book book = (Book)ois.readObject();
        System.out.println(book);
    }
}

不过，像是总价=单价*数量就没有必要序列化，可以用transient关键字。

网络编程

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package com.poorpool.server;

import java.io.PrintStream;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.Scanner;

public class ServerDemo {
    public static void main(String[] args) throws Exception {
        ServerSocket sers = new ServerSocket(9999);
        Socket sock = sers.accept();
        PrintStream ps = new PrintStream(sock.getOutputStream());
        Scanner scanner = new Scanner(sock.getInputStream());
        while(true) {
            if(scanner.hasNext()) {
                String qwq = scanner.next();
                System.out.println("Get " + qwq);
                if(qwq.equalsIgnoreCase("exit")) {
                    break;
                }
                ps.println(qwq);
            }
        }
        sock.shutdownOutput();
        sers.close();
    }
}
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package com.poorpool.client;

import java.io.PrintStream;
import java.net.Socket;
import java.util.Scanner;

public class ClientDemo {
    public static void main(String[] args) throws Exception {
        Socket sock = new Socket("localhost", 9999);
        Scanner scanner = new Scanner(sock.getInputStream());
        PrintStream out = new PrintStream(sock.getOutputStream());
        Scanner scanner1 = new Scanner(System.in);
        while(true) {
            String qwq = scanner1.nextLine();
            out.println(qwq);
            System.out.println("waiting...");
            if (scanner.hasNext()) {
                String ret = scanner.next();
                System.out.println("Echo: " + ret);
            }
            if(qwq.equalsIgnoreCase("exit"))
                break;
        }
        sock.close();
    }
}

Java里头有ServerSocket和Socket，挺方便的。

多线程也好处理，每次accept一个client的socket就把它扔进一个Runnable里头跑起来。

也可以使用udp的socket。

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package com.poorpool.server;

import java.net.DatagramPacket;
import java.net.DatagramSocket;

public class ServerDemo {
    public static void main(String[] args) throws Exception {
        DatagramSocket client = new DatagramSocket(9999);
        byte[] data = new byte[1024];
        DatagramPacket packet = new DatagramPacket(data, data.length);
        client.receive(packet);
        System.out.println(new String(data, 0, packet.getLength()));
        client.close();
    }
}
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package com.poorpool.client;

import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;

public class ClientDemo {
    public static void main(String[] args) throws Exception {
        DatagramSocket server = new DatagramSocket(9998);
        String msg = "poorpool";
        DatagramPacket packet = new DatagramPacket(msg.getBytes(), 0, msg.length(), InetAddress.getByName("localhost"), 9999);
        server.send(packet);
        server.close();
    }
}

更新于 2022-06-28

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