T - the type of objects that may be compared by this comparatorpublic interfaceComparator<T>Collections.sort orArrays.sort) to allow precise control over the sort order. Comparators can also be used to control the order of certain data structures (such assorted sets orsorted maps), or to provide an ordering for collections of objects that don't have anatural ordering.The ordering imposed by a comparatorc on a set of elementsS is said to beconsistent with equals if and only ifc.compare(e1, e2)==0 has the same boolean value ase1.equals(e2) for everye1 ande2 inS.
Caution should be exercised when using a comparator capable of imposing an ordering inconsistent with equals to order a sorted set (or sorted map). Suppose a sorted set (or sorted map) with an explicit comparatorc is used with elements (or keys) drawn from a setS. If the ordering imposed byc onS is inconsistent with equals, the sorted set (or sorted map) will behave "strangely." In particular the sorted set (or sorted map) will violate the general contract for set (or map), which is defined in terms ofequals.
For example, suppose one adds two elementsa andb such that(a.equals(b) && c.compare(a, b) != 0) to an emptyTreeSet with comparatorc. The secondadd operation will return true (and the size of the tree set will increase) becausea andb are not equivalent from the tree set's perspective, even though this is contrary to the specification of theSet.add method.
Note: It is generally a good idea for comparators to also implementjava.io.Serializable, as they may be used as ordering methods in serializable data structures (likeTreeSet,TreeMap). In order for the data structure to serialize successfully, the comparator (if provided) must implementSerializable.
For the mathematically inclined, therelation that defines theimposed ordering that a given comparatorc imposes on a given set of objectsS is:
{(x, y) such that c.compare(x, y) <= 0}. Thequotient for this total order is: {(x, y) such that c.compare(x, y) == 0}. It follows immediately from the contract forcompare that the quotient is anequivalence relation onS, and that the imposed ordering is atotal order onS. When we say that the ordering imposed byc onS isconsistent with equals, we mean that the quotient for the ordering is the equivalence relation defined by the objects'equals(Object) method(s): {(x, y) such that x.equals(y)}.UnlikeComparable, a comparator may optionally permit comparison of null arguments, while maintaining the requirements for an equivalence relation.
This interface is a member of the Java Collections Framework.
Comparable,Serializableint compare(T o1,T o2)
In the foregoing description, the notationsgn(expression) designates the mathematicalsignum function, which is defined to return one of-1,0, or1 according to whether the value ofexpression is negative, zero or positive.
The implementor must ensure thatsgn(compare(x, y)) == -sgn(compare(y, x)) for allx andy. (This implies thatcompare(x, y) must throw an exception if and only ifcompare(y, x) throws an exception.)
The implementor must also ensure that the relation is transitive:((compare(x, y)>0) && (compare(y, z)>0)) impliescompare(x, z)>0.
Finally, the implementor must ensure thatcompare(x, y)==0 implies thatsgn(compare(x, z))==sgn(compare(y, z)) for allz.
It is generally the case, butnot strictly required that(compare(x, y)==0) == (x.equals(y)). Generally speaking, any comparator that violates this condition should clearly indicate this fact. The recommended language is "Note: this comparator imposes orderings that are inconsistent with equals."
o1 - the first object to be compared.o2 - the second object to be compared.NullPointerException - if an argument is null and this comparator does not permit null argumentsClassCastException - if the arguments' types prevent them from being compared by this comparator.boolean equals(Object obj)
Object.equals(Object). Additionally, this method can returntrueonly if the specified object is also a comparator and it imposes the same ordering as this comparator. Thus,comp1.equals(comp2) implies thatsgn(comp1.compare(o1, o2))==sgn(comp2.compare(o1, o2)) for every object referenceo1 ando2.Note that it isalways safenot to overrideObject.equals(Object). However, overriding this method may, in some cases, improve performance by allowing programs to determine that two distinct comparators impose the same order.
equals in class Objectobj - the reference object with which to compare.true only if the specified object is also a comparator and it imposes the same ordering as this comparator.Object.equals(Object),Object.hashCode()