您的代码期望使用char，但是您在此处返回int:

Your code is expecting a char, but you return an int here:

public static int fgetc(InputStream stream)
//            ↑ tells method will return an int

您可以

• 更改方法签名以返回char.

public static char fgetc(InputStream stream)
//            ↑ tells method will return a char

message[index] = (char) fgetc(System.in);
//               ↑ cast returning value to a char