Update Java 16:

line =  buffer.lines().toList();

One line of code using Java 8:

line =  buffer.lines().collect(Collectors.joining());

You are right, a loop would be needed here.

The usual idiom (using only plain Java) is something like this:

public String ReadBigStringIn(BufferedReader buffIn) throws IOException {
    StringBuilder everything = new StringBuilder();
    String line;
    while( (line = buffIn.readLine()) != null) {
       everything.append(line);
    }
    return everything.toString();
}

This removes the line breaks - if you want to retain them, don't use the readLine() method, but simply read into a char[] instead (and append this to your StringBuilder).

Please note that this loop will run until the stream ends (and will block if it doesn't end), so if you need a different condition to finish the loop, implement it in there.

You should mark() the starting buffer position, so that your reset() call comes back to the beginning. For example, if you are reading chars, and want to mark a position to come back:

Data sample: A B C D

System.out.println((char)buffer.read());
System.out.println((char)buffer.read());
buffer.mark(4); // save the position
System.out.println((char)buffer.read());
System.out.println((char)buffer.read());
buffer.reset() // back to 4
System.out.println((char)buffer.read());
System.out.println((char)buffer.read());

Will print:

A B C D C D

(as a char takes 2 bytes)

The readLine() method returns null at end of stream, which doesn't occur until the peer closes the connection.