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为什么 Collection接口(interface)有 equals(Object o)
和 hashCode()
,假设任何实现都默认有那些(继承自 Object
)?
最佳答案
While the
Collection
interface adds no stipulations to the general contract for theObject.equals
, programmers who implement theCollection
interface "directly" (in other words, create a class that is aCollection
but is not aSet
or aList
) must exercise care if they choose to override theObject.equals
. It is not necessary to do so, and the simplest course of action is to rely on Object's implementation, but the implementor may wish to implement a "value comparison" in place of the default "reference comparison." (TheList
andSet
interfaces mandate such value comparisons.)The general contract for the
Object.equals
method states that equals must be symmetric (in other words,a.equals(b)
if and only ifb.equals(a)
). The contracts forList.equals
andSet.equals
state that lists are only equal to other lists, and sets to other sets. Thus, a custom equals method for a collection class that implements neither theList
norSet
interface must return false when this collection is compared to any list or set. (By the same logic, it is not possible to write a class that correctly implements both the Set and List interfaces.)
和
While the Collection interface adds no stipulations to the general contract for the
Object.hashCode
method, programmers should take note that any class that overrides theObject.equals
method must also override theObject.hashCode
method in order to satisfy the general contract for theObject.hashCode
method. In particular,c1.equals(c2)
implies thatc1.hashCode()==c2.hashCode()
.
关于java - 为什么Collection接口(interface)有equals()和hashCode()?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/45630264/
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