问题

我正在尝试通过异或位运算符进行消息的编码和解码：

class Search {
    public static void main(String[] args) {
        String msg = "This is a test";
        String encmsg = "";
        String decmsg = "";
        int key = 88;

        System.out.print("Original message: ");
        System.out.println(msg);

        // 编码消息。
        for (int i = 0; i < msg.length(); i++)
            encmsg = encmsg + (char) (msg.charAt(i) ^ key);

        System.out.print("Encoded message: ");
        System.out.println(encmsg);

        // 解码消息
        for (int i = 0; i < encmsg.length(); i++)
            decmsg = decmsg + (char) (encmsg.charAt(i) ^ key);

        System.out.print("Decoded message: ");
        System.out.println(decmsg);
    }
}

输出应该是这样的：
原始消息：This is a test
编码后消息：01+x1+x9x,=+,
解码后消息：This is a test

这是我收到的内容：

原始消息：This is a test
异常信息："main" java.lang.StringIndexOutOfBoundsException: String index out of range: 0
at java.base/java.lang.StringLatin1.charAt(StringLatin1.java:48)
at java.base/java.lang.String.charAt(String.java:711)
at Package/Package.Search.main(Search.java:15)

<details>
<summary>英文:</summary>

I am trying to encode and decode a message through a XOR bitwise operator:

class Search {
public static void main(String[] args) {
String msg = "This is a test";
String encmsg = "";
String decmsg = "";
int key = 88;

System.out.print("Original message: ");
System.out.println(msg);


// Encode the message.
for(int i=0; i < msg.length(); i++)
	encmsg = encmsg + (char) (encmsg.charAt(i) ^ key);

System.out.print("Encoded message: ");
System.out.println(encmsg);

// Decode the message
for(int i=0; i < msg.length(); i++)
	decmsg = decmsg + (char) (decmsg.charAt(i) ^ key);

System.out.print("Decoded message: ");
System.out.println(decmsg);

}
}

The output should be this: 
Original message: This is a test
Encoded message: 01+x1+x9x,=+,
Decoded message: This is a test
 
This is what a receive:

Original message: This is a test
Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 0
at java.base/java.lang.StringLatin1.charAt(StringLatin1.java:48)
at java.base/java.lang.String.charAt(String.java:711)
at Package/Package.Search.main(Search.java:15)

</details>


# 答案1
**得分**: 1

你将原始消息和加密后的消息弄混了。解密也是同样的道理。例如 `encmsg = encmsg + (char) (encmsg.charAt(i) ^ key);` 应该改为 `msg.charAt(i)` 而不是 `encmsg.charAt(i)`，因为你应该从原始字符串中获取字符，对它们进行编码，然后添加到已经编码的部分中。

以下是编写逻辑的正确方式：

```java
// 对消息进行编码
for(int i=0; i < msg.length(); i++)
    encmsg = encmsg + (char) (msg.charAt(i) ^ key);

System.out.print("Encoded message: ");
System.out.println(encmsg);

// 对消息进行解码
for(int i=0; i < msg.length(); i++)
    decmsg = decmsg + (char) (encmsg.charAt(i) ^ key);

// Encode the message.
for(int i=0; i < msg.length(); i++)
    encmsg = encmsg + (char) (msg.charAt(i) ^ key);

System.out.print("Encoded message: ");
System.out.println(encmsg);

// Decode the message
for(int i=0; i < msg.length(); i++)
    decmsg = decmsg + (char) (encmsg.charAt(i) ^ key);