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From: Lew on 31 May 2010 14:18 Lew wrote: >> Always override 'equals()', 'hashCode()' and 'toString()' together or >> not at all; keep them consistent with each other. If 'Foo' implements >> 'Comparable<Foo>' then it must override those three, all consistent with >> 'compareTo(Foo)'. Arne Vajhøj wrote: > There are very good reasons to override equals and hashCode together. > > I can see many cases where overriding toString alone makes sense, > because its main purpose is to make developer logging easier. And > often you will want to log something without it having to be > comparable. There's nothing in my advice to require loggable things to be Comparable. If you're logging information that identifies an instance, by definition you have to provide a string representation of the identifying attributes of the instance. If you aren't logging an instance's identifying attributes, you don't use 'toString()' in the log message. Ergo, 'toString()' has to include the identifying attributes of the instance to be useful. The identifying attributes of the instance are precisely those that figue in to 'equals()' and 'hashCode()', with the possible addition of the object identifier. Ergo, 'toString()' must be consistent with 'equals()' and 'hashCode()', in the sense that it must represent the values of the attributes that figure in to those methods. Q.E.D. -- Lew
From: Robert Klemme on 31 May 2010 15:33 On 31.05.2010 18:57, Tom Anderson wrote: > On Mon, 31 May 2010, Lew wrote: > >> Tom Anderson wrote: >>> if ((obj == null) || !(obj instanceof SearchResultHotels)) return false; >> >> The '(obj == null)' clause is superfluous. > > Good point. > > My brain, for some reason, has a hard time remembering that: > > SearchResultHotels x = (SearchResultHotels)null; > > is okay but: > > null instanceof SearchResultHotels > > is false. You just need to remember that null does not have a type. At least that's what helped me to get this straight. :-) Also, "nothing" cannot really be an instance of "something", can it? Kind regards robert -- remember.guy do |as, often| as.you_can - without end http://blog.rubybestpractices.com/
From: Tom Anderson on 31 May 2010 15:39 On Mon, 31 May 2010, Robert Klemme wrote: > On 31.05.2010 18:57, Tom Anderson wrote: >> On Mon, 31 May 2010, Lew wrote: >> >>> Tom Anderson wrote: >>>> if ((obj == null) || !(obj instanceof SearchResultHotels)) return false; >>> >>> The '(obj == null)' clause is superfluous. >> >> Good point. >> >> My brain, for some reason, has a hard time remembering that: >> >> SearchResultHotels x = (SearchResultHotels)null; >> >> is okay but: >> >> null instanceof SearchResultHotels >> >> is false. > > You just need to remember that null does not have a type. At least > that's what helped me to get this straight. :-) Yes, when i think about it, it's all perfectly clear. But i get it wrong when i'm not thinking about it. In further penance, i should point out that the instanceof test is risky if the class can have subclasses, and where those subclasses might have different ideas about equality. A more conservative form might be: if ((obj == null) || !(obj,getClass().equals(SearchResultHotels.class)) return false; > Also, "nothing" cannot really be an instance of "something", can it? Ah, but you would agree that nothing can be an instance of everything! tom -- Judge Dredd. Found dead. Face down in Snoopy's bed.
From: Mike Schilling on 31 May 2010 15:58 Tom Anderson wrote: > On Mon, 31 May 2010, Robert Klemme wrote: > >> On 31.05.2010 18:57, Tom Anderson wrote: >>> On Mon, 31 May 2010, Lew wrote: >>> >>>> Tom Anderson wrote: >>>>> if ((obj == null) || !(obj instanceof SearchResultHotels)) return >>>>> false; >>>> >>>> The '(obj == null)' clause is superfluous. >>> >>> Good point. >>> >>> My brain, for some reason, has a hard time remembering that: >>> >>> SearchResultHotels x = (SearchResultHotels)null; >>> >>> is okay but: >>> >>> null instanceof SearchResultHotels >>> >>> is false. >> >> You just need to remember that null does not have a type. At least >> that's what helped me to get this straight. :-) > > Yes, when i think about it, it's all perfectly clear. But i get it > wrong when i'm not thinking about it. Me too. Especially since the usage if (x instanceof Type) { Type t = (Type)x; ... is so common. The instanceof check (among other things) avoids a cast failure, but the cast would work perfectly well if x were null.
From: markspace on 31 May 2010 19:18
Tom Anderson wrote: > Ah, but you would agree that nothing can be an instance of everything! No, but I'd agree that a reference can point to nothing. ;-) |