For point 1), that's a simple method call, inlined by the JVM if "hot":

public int length() {
    return value.length;
}

For point 3), there is no computation for charAt method (except a check which is optimized by the JVM RangeCheckElemination & Co):

public char charAt(int index) {
   if ((index < 0) || (index >= value.length)) {
       throw new StringIndexOutOfBoundsException(index);
  }
   return value[index];

}