在Java7中重写compareTo和Compare

Question

Do I need to override the equals() method when ever I override the CompareTo and Compare method in java, in order to satisfy the Comparable contract ? Will this create any issues when I do a Collections.sort or Array.sort?

Answer 1

From the Javadoc for Comparator

It is generally the case, but not 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."

This means that you don't generally need to override equals() . You probably shouldn't do this, unless you want your Comparator to return a non-zero comparison for two values which return true when compared with equals .

If you think that the existence of a comparison requires you to change the definition of what it means for two things to be equal, then you've probably designed something badly.

Answer 2

From Comparable :

The natural ordering for a class C is said to be consistent with equals if and only if e1.compareTo(e2) == 0 has the same boolean value as e1.equals(e2) for every e1 and e2 of class C . 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 .

It is strongly recommended (though not required) that natural orderings be consistent with equals . This is so because sorted sets (and sorted maps) without explicit comparators behave "strangely" when they are used with elements (or keys) whose natural ordering is inconsistent with equals. In particular, such a sorted set (or sorted map) violates the general contract for set (or map), which is defined in terms of the equals method.

^{(emphasis mine)}

So you don't need to override equals() (ie it will not cause a problem within the standard sort methods since they use only compareTo() , nor does it violate the contract of Comparable ), but it certainly wouldn't hurt.