java - 我打印出来都是相同的，这个字符串比较为什么会是False？

Question

我写的就是UDP协议的很简单的本机两个端口下的网络通信，一直不能理解为什么在类UDPSender中接收到UDPReciver发送的ACK消息时，receiverRes.equals("ACK")的返回值是False，真的是不理解啊，我特意还打印了receiverRes的值，就是ACK，错在哪了，求指教
我不知道大家看懂我的代码没有，就是发送方向接收方发送字符，接收方正确接收后，会发送ACK给接收方，接收方收到ACK后才能确认，实现的是停等协议！

import java.net.*;

import java.util.*;

public class UDPReciver{
public static void main(String[] args) throws Exception{
    DatagramSocket ds = null;
    DatagramPacket dp1 = null;
    DatagramPacket dp2 = null;
    byte[] buf1 = new byte[1024];
    byte[] buf2 = new byte[1024];
    ds = new DatagramSocket(9000);
    while(true){
        System.out.println("开始接收数据");
        dp1 = new DatagramPacket(buf1,1024);
        ds.receive(dp1);
        String sendWhat =new String(buf1);
        System.out.println("请进行以下选择：a.成功接收该字符  b.丢失  c.无反应、超时");
        Scanner scan = new Scanner(System.in);
        String choose = scan.next();
        if(choose.equals("a")){
            System.out.println("成功接收" + sendWhat);
            String ACKSend = new String("ACK");
            buf2 = ACKSend.getBytes();
            dp2 = new DatagramPacket(buf2,ACKSend.length(),InetAddress.getByName("localhost"),3000);
            ds.send(dp2);
            System.out.println("dada");

        }
        if(choose.equals("b")){
            String NAKSend = new String("NAK");
            buf2 = NAKSend.getBytes();
            dp2 = new DatagramPacket(buf2,NAKSend.length(),InetAddress.getByName("localhost"),3000);
            ds.send(dp2);
        }
        if(choose.equals("c")){
            System.out.println("超时，请重传！");
        }

    }
}
}




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

public class UDPSender{
//private static final int TIMEOUT=3000;  
public static void main(String[] args) throws Exception{
        int TIMEOUT=3000;  
        DatagramSocket ds = null;
        DatagramPacket dp1 = null;
        DatagramPacket dp2 = null;
        byte[] buf1 = new byte[1024];
        byte[] buf2 = new byte[1024];
        ds = new DatagramSocket(3000);
        System.out.println("开始发送数据");
        int i = 0;
        String send =new String();
        while(true){
            Scanner scan = new Scanner(System.in);
            System.out.println("请输入传送的字符");
            send = ""; 
            send += i;
            send += scan.next();
            buf1 = send.getBytes();
            dp1 = new DatagramPacket(buf1,send.length(),InetAddress.getByName("localhost"),9000);
            ds.send(dp1);
             //设置receive()方法的最长阻塞时间  
            ds.setSoTimeout(TIMEOUT);
            dp2 = new DatagramPacket(buf2,1024);
            try{
            ds.receive(dp2);
            String receiverRes = new String(buf2);
            System.out.println(receiverRes);
            System.out.println(receiverRes.equals("ACK"));
            //System.out.println("aaaaa");
            if(receiverRes.equals("ACK")){
                System.out.println("aaaaa");
                i = (i+1) % 2;
                //System.out.println(i);
                continue;
            }
            else if(receiverRes.equals("NAK")){
                ds.send(dp1);

            }
            }catch(InterruptedIOException e){
                ds.send(dp1);
            }
            //else{
                //ds.send(dp1);
                //System.out.println(i);
            //}

        }
}

}

Answer 1

byte[] buf2 = new byte[1024];
...
String receiverRes = new String(buf2);
System.out.println(receiverRes);
System.out.println(receiverRes.equals("ACK"));

因为你这里直接用buf2，new String了，所以此时receiverRes其实是除了你接收到的和数据外，后面还多加了很多个空字符。总长度就是buf2的长度。两个长度和内容都不同的字符串，怎么可能相同呢