CODEWITHSUNDEEP
javanullset-theory
M4TT4CK
M4TT4CK

Reputation: 371

How to implement the empty set - ∅?

Assuming that you want to implement set theory concepts such as element, set, collection and relation in Java: How would you represent the empty set ∅?

Do I mislead myself, if I think of the NULL concept as it is used by the three-valued logic of databases?

Upvotes: 28

Views: 53374

Answers (2)

Stephen C
Stephen C

Reputation: 719386

Using null to represent an empty set is a bad idea. A null doesn't behave like a Set because (obviously) all attempts to perform an operation on it will throw a NullPointerException. That means that if you use null to denote an empty set, you code will be littered with tests for null ... and if you miss one, you've got a bug.

The practical solution is to use Collections.emptySet() if you want an immutable empty set, or create an instance of the appropriate Set class (e.g. new HashSet<>()) if you want a mutable set that starts out empty.

On rereading the question, I realize that you may have meant something different to my original understanding.

If you were trying to implement / model mathematical set theory concepts in Java from scratch, you would probably implement Set as an immutable class. (In mathematics, you don't mutate things!) The empty set is then just a Set instance with no elements in it. No special handling required.

The NULL concept is not required ... unless you are specifically trying to incorporate a "null", "undefined sets" or some similar concept in your mathematical model of sets. (If you are, I'm not sure we can advise you without understanding your model ... from a mathematical perspective.)

Upvotes: 16

Wojciech Owczarczyk
Wojciech Owczarczyk

Reputation: 5745

Use Collections.emptySet():

Returns the empty set (immutable). This set is serializable. Unlike the like-named field, this method is parameterized. This example illustrates the type-safe way to obtain an empty set:

 Set<String> s = Collections.emptySet();

Implementation note: Implementations of this method need not create a separate Set object for each call. Using this method is likely to have comparable cost to using the like-named field. (Unlike this method, the field does not provide type safety.)

Upvotes: 55

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