question: static void checkIdentifier ( final String name , final int start , final int end , final String msg ) { if ( name == null || ( end == - 1 ? name . length ( ) <= start : end <= start ) ) { throw new IllegalArgumentException ( "Invalid " + msg + " (must not be null or empty)" ) ; } if ( ! Character . isJavaIdentifierStart ( name . charAt ( start ) ) ) { throw new IllegalArgumentException ( "Invalid " + msg + " (must be a valid Java identifier): " + name ) ; } int max = end == - 1 ? name . length ( ) : end ; for ( int i = start + 1 ; i < max ; ++ i ) { if ( ! Character . isJavaIdentifierPart ( name . charAt ( i ) ) ) { throw new IllegalArgumentException ( "Invalid " + msg + " (must be a valid Java identifier): " + name ) ; } } }
target:  Checks that the given substring is a valid Java identifier.