希望你没事，我开始学习java socket编程和处理客户机服务器，我在理解io库的不同类和对象时遇到了很多问题，我真的需要有人帮我总结一下。printstream，printwriter，dataoutputstream，outputstream之间的区别是什么，我猜还有outputstreamwriter。我们什么时候使用每一个（举例说明会很好）？如果我要求太多，请回复我一个youtube视频或一些外部链接。请不要引用eclipse的话来回复我。下面是我一直在编写的一个代码，它使客户机的消息变成大写：
◙ 服务器代码：

package clientServer;
import java.net.*;
import java.io.*;

public class Server{
    public static void main(String[] args) throws IOException{
        ServerSocket incoming = new ServerSocket(9999);
        System.out.println("Connecting...");
        Socket client = incoming.accept();
        System.out.println("Client is connected");
        String messageIn,messageOut;
        BufferedReader in = new BufferedReader(new InputStreamReader(client.getInputStream()));
        messageIn = in.readLine();
        messageOut = messageIn.toUpperCase();
        PrintStream outnet = new PrintStream(client.getOutputStream());
        outnet.print(messageOut + "\n");
        outnet.flush();
        outnet.close();
        client.close();
        incoming.close();

    }
}

◙ 客户代码：

package clientServer;
import java.net.*;
import java.io.*;

public class Client{
    public static void main(String[] args) throws IOException{
        Socket client = new Socket("localhost",9999);
        System.out.println("Connected to the Server");
        PrintStream outnet = new PrintStream(client.getOutputStream());
        String message;
        BufferedReader input = new BufferedReader(new InputStreamReader(System.in));
        BufferedReader in = new BufferedReader(new InputStreamReader(client.getInputStream()));
        System.out.print("Veillez tappez quelque chose: ");
        message = input.readLine();
        outnet.print(message + "\n");
//      outnet.flush();
//      outnet.close();
        message = in.readLine();
        System.out.println("Server: " + message);
        client.close();

    }
}

当我使用printstream或dataoutputstream时，代码运行良好，但当我使用printwriter或其他东西时，代码运行不好。