为什么这个 String.equals() 不起作用？

Question

我正在尝试构建一个简单的 UDPServer 和 UDPClient。字符串比较不起作用。
这是我到目前为止得到的：

import java.io.*;
import java.net.*;
import java.util.concurrent.TimeUnit;

public class UDPSender
{
   public static void main(String args[]) throws Exception
      {
         DatagramSocket serverSocket = new DatagramSocket(9877);
            byte[] receiveData = new byte[1024];
            byte[] sendData = new byte[1024];
            boolean weiter = true;

             do {
                  DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
                  serverSocket.receive(receivePacket);
                  String sentence = new String( receivePacket.getData());
                  System.out.println("RECEIVED: " + sentence);
                  InetAddress IPAddress = receivePacket.getAddress();
                  int port = receivePacket.getPort();

                  /*I'm trying to make the if()-statement true, but the program always enters the else()-clause, no matter what I do.*/
                  if("Shutdown".equals(sentence)) {
                      weiter = false;
                      String bye = ("Auf Wiedersehen! Verbindung wird gekappt...");
                      sendData = bye.getBytes();
                      DatagramPacket sendPacket =
                              new DatagramPacket(sendData, sendData.length, IPAddress, port);
                      serverSocket.send(sendPacket);
                      serverSocket.close();
                  } else {                    
                      String capitalizedSentence = sentence.toUpperCase();
                      sendData = capitalizedSentence.getBytes();
                      DatagramPacket sendPacket =
                              new DatagramPacket(sendData, sendData.length, IPAddress, port);
                      serverSocket.send(sendPacket);                      
                  }
               } while(weiter);
      }
}

这是客户：

import java.io.*;
import java.net.*;
import java.util.ArrayList;

class UDPClient{

    public static void main(String args[]) throws Exception
    {
       BufferedReader inFromUser =
          new BufferedReader(new InputStreamReader(System.in));
       DatagramSocket clientSocket = new DatagramSocket();
       InetAddress IPAddress = InetAddress.getByName("localhost");
       byte[] sendData = new byte[1024];
       byte[] receiveData = new byte[1024];

       //Look, I'm explicitly sending Shutdown, too!            
       String sentence = "Shutdown";
       sendData = sentence.getBytes();
       DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, 9877);
       clientSocket.send(sendPacket);
       DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
       clientSocket.receive(receivePacket);
       String modifiedSentence = new String(receivePacket.getData());
       System.out.println("FROM SERVER:" + modifiedSentence);
       clientSocket.close();
    }
}

如果现在两者都可以在同一台计算机上工作，我会很高兴，但是在解决这个字符串问题之后，我需要让更多的 PC 参与进来。尝试了关于 Whosebug 的其他类似问题中建议的每个解决方案以及前三页上列出的任何 google，但对我没有任何帮助。你能看到我看不到的东西吗？

Answer 1

这里的问题是您依赖固定大小的缓冲区 byte[] ... = new byte[1024] 来发送和接收数据包。

我的建议是只发送您真正需要的数据：

String sentence = "Shutdown";
byte[] sendData = sentence.getBytes();
DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, 9877);

...并根据从receivePacket.getLength()方法获得的长度在服务器端创建String实例：

String sentence = new String(receivePacket.getData(), 0, receivePacket.getLength());

另见 public String(byte[] bytes, int offset, int length)

为什么这个 String.equals() 不起作用？

Why does this String.equals() not work?

java

string

udp

equals