Self Encapsulate Field

You are accessing a field directly, but the coupling to the field is becoming awkward.

Create getting and setting methods for the field and use only those to access the field.

private int _low, _high;
boolean includes (int arg) {
  return arg >= _low && arg <= _high;
}

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private int _low, _high;
boolean includes (int arg) {
  return arg >= getLow() && arg <= getHigh();
}
int getLow() {return _low;}
int getHigh() {return _high;}