public static void checkState(boolean expression,@Nullable Object errorMessage){
  if (!expression) {
    throw new IllegalStateException(String.valueOf(errorMessage));
  }
}

public static void checkState ( boolean expression , @ Nullable Object errorMessage ) { if ( ! expression ) { throw new IllegalStateException ( String . valueOf ( errorMessage ) ) ; } }
ensures the truth of an expression involving the state of the calling instance , but not involving any parameters to the calling method .
