What should int compareTo() return when the parameter string is null?
Solution 1
Yes, there is no problem allowing null
for instance fields - just make sure its sorting order is defined. Most natural would be putting it either before or after all real strings, but you could do anything here, just do it consistently. (For example, you could sort null
like "null"
.)
Here is an example implementation for a single member:
class Example implements Comparable<Example> {
@Nullable
private String member;
// TODO: getter, setter, constructor, ...
public int compareTo(Example that) {
if(this.member == null)
if(that.member == null)
return 0; //equal
else
return -1; // null is before other strings
else // this.member != null
if(that.member == null)
return 1; // all other strings are after null
else
return this.member.compareTo(that.member);
}
}
Please note that the specification of Comparable.compareTo() only has a constraint for o.compareTo(null)
(which should behave just like - null.compareTo(o)
, i.e. throw a NullPointerException), but not about how null
fields are handled (it doesn't mention fields at all, so a class could return whatever it wants, as long as the antisymmetry, reflexivity and transitivity is ensured).
Solution 2
From javadoc for Comparable
Note that null is not an instance of any class, and e.compareTo(null) should throw a NullPointerException even though e.equals(null) returns false.
Solution 3
It would be a bad practice to not throw an exception because it violates the transitive antisymmetric nature of compareTo.
From Comparable.compareTo documentation:
The implementor must ensure sgn(x.compareTo(y)) == -sgn(y.compareTo(x)) for all x and y. (This implies that x.compareTo(y) must throw an exception iff y.compareTo(x) throws an exception.)
The implementor must also ensure that the relation is transitive: (x.compareTo(y)>0 && y.compareTo(z)>0) implies x.compareTo(z)>0.
Finally, the implementor must ensure that x.compareTo(y)==0 implies that sgn(x.compareTo(z)) == sgn(y.compareTo(z)), for all z.
More importantly, it's a bad idea to use compareTo on your objects to compare them with strings, for the same reason: sign(obj.compareTo(str)) != -sign(str.compareTo(obj))
. Implement a custom Comparator and do whatever you want in it.
Solution 4
You need to decide if null is greater-than or less-than to a non-null value. You can design compareTo
to meets the needs of your class's natural ordering, thus it is not bad practice.
Solution 5
Because the documentation of compareTo
states that it should throw a NullPointerException
, you should follow those guidelines so your implementation is consistent with the interface documentation. This also handles the questions of whether or not non-null strings are lexicographically smaller or bigger than null
.
You have a couple of options on how to handle this. If empty and not-applicable are different, then you should probably wrap the string field in your own field class. For example, say you could create a type of MyField that might have an isApplicable
method, which indicates if the field is applicable to this case (or something similar). Or you could rethink your design and be sure that an empty string and N/A really are two different things. If they are, you do need a way to differentiate between the two.
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zw324
Ex Computer Science Student at NYU "The sleeper must awaken."
Updated on July 09, 2022Comments
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zw324 almost 2 years
It is said that when input parameter is null, compareTo() should throw a NullPointerException. However, I am implementing a class which needs to compare fields with the type of String. These fields need not to be mandatory. I wonder in this case,
1) What should I return when the input is null? Should any not-null strings lexicographically bigger or smaller than null?
and
2) If this is considered bad practice, is there any supporting arguments? Should I force the user to use empty strings instead? If using empty string, won't that confuse the case in which the field is not applicable and the case in which the field is empty? And if exception must be thrown, then except from warning the user in the manual, what else could/shall I do?
EDIT: I might not express myself clearly here, but in the program I am implementing, the strings that could be null are all fields or a class, which should not be null. In other words, the objects comparedTo() uses could not be null, just their private fields could be. So in this case, I believe if I implement compareTo() properly, it would not violate the transitive requirement since classes with null fields would be considered the same always. Am I right or am I interpreting this wrong?
Thank you all for the answers!
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Ciro Santilli OurBigBook.com almost 9 yearsHow to best implement it: stackoverflow.com/questions/481813/…
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zw324 almost 13 yearsThanks! However, in the program I am implementing, the strings that could be null are all parts or a class, which should not be null. So in this case, I believe if I implement compareTo() properly, it would not violate the transitive requirement since classes with null fields would be considered the same always. Am I right or am I interpreting this wrong?
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user1002601 almost 13 yearsIt's perfectly fine to compare two instances of a class which might or might not have some instance fields set. Just make sure that you implement it transitively. I.E. c1.compareTo(c2) == -c2.compareTo(c1)
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Paŭlo Ebermann almost 13 yearsJust for terminology: The condition
c1.compareTo(c2) == -c2.compareTo(c1)
is called antisymmetry, not transitivity. Transitivity is mainly the second condition in your quote (and maybe also the third). -
user1002601 almost 13 years@Paulo, thanks, you are correct, and also the antisymmetry requirement is weaker than what I had stated. It only has to do with signs, not actual values (subject to the transitivity constraint):
sign(c.compareTo(c2)) == -sign(c2.compareTo(c1))
. I've adjusted the answer; left previous comment for context. -
user207421 almost 13 yearsWell, there's the problem that it disagrees with the specification ;-)
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Paŭlo Ebermann almost 13 years@EJP: The specification of
compareTo
says nothing on how (or even whether) to compare instance fields of the objects. -
user207421 almost 13 yearsThe specification of compareTo() says 'e.compareTo(null) should throw a NullPointerException'.
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Paŭlo Ebermann almost 13 yearsYes, but in this question (at least after the edit before my answer) it is not about comparing an object with
null
, but about comparing an object with another object, one of which has anull
field. ThecompareTo
specification says nothing about this. -
user207421 almost 13 yearsthe question before and after editing is 'What should I return when the input is null?' The Javadoc provides the answer. Your suggestion doesn't comply.
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skelly about 10 years@EJP Paulo is correct here. The original question asks what to do when
foo1.compareTo(foo2)
finds thatfoo2 != null
butfoo2.someField == null
. The specification only dictates behavior whenfoo == null
. -
gene b. over 3 yearsOne comment: This won't work if you need to handle Asc/Desc order. I'll provide a solution with Asc/Desc.
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Paŭlo Ebermann over 3 yearsIt might be easier to just invert the existing comparator instead of having this case distinction three times, using Comparator.reversed() or Comparator.reverseOrder (based on the natural order).