The error "unmappable character" in Java typically indicates that there is a character in your source code that cannot be represented in the character encoding that you are using.
To fix this error, you will need to ensure that your source code is saved in a character encoding that supports all of the characters used in your code. For example, if you are using the ASCII character encoding and your code contains non-ASCII characters, you will see this error.
To change the character encoding of your source code, you can use a text editor that supports character encoding options, such as Notepad++ or Sublime Text. Simply open your source code file in the editor, go to the "File" menu, and choose "Save As...". In the "Save As" dialog, choose a different character encoding from the "Encoding" dropdown menu, such as UTF-8 or ISO-8859-1, and save the file.
Alternatively, you can specify the character encoding of your source code when compiling it using the -encoding option of the javac compiler. For example:
javac -encoding UTF-8 MyClass.java
This will compile the MyClass.java file using the UTF-8 character encoding.
By ensuring that your source code is saved in a character encoding that supports all of the characters used in your code, you can avoid the "unmappable character" error.