Question

In Item 71 in ‘Effective Java, Second Edition’ the Double-check idiom and the single-check idiom are introduced for lazily instantiating instance fields.

Double-check idiom

private volatile FieldType field;
FieldType getField() {
  FieldType result = field;
  if (result == null) {
    synchronized(this) {
      result == field;
      if (result == null)
        field = result = computeFieldValue();
    }
  }
  return result;
}

Single-check idiom

private volatile FieldType field;
FieldType getField() {
  FieldType result = field;
  if (result == null) {
    field = result = computeFieldValue();
  }
  return result;
}

In the double-check idiom Joshua states, that the result variable is used to make sure that the volatile field is only read once, which improves performance. This I understand, however I don’t see why we need it in the single-check idiom, since we only read field once anyway.

Answer 1

In the single-check idiom, without the result variable you’d still be reading it twice; once for the null check and once for the return value.