Why does Comparator declare equals?

问题

The Comparator interface has its own equals() method. Any class will get equals() by default through Object class. What is the need to have equals() method inside an interface?

回答1:

Comparator refines the contract of Object.equals: It has to satisfy the constraints set out by Object.equals and then some.

Additionally, this method can return true only if the specified object is also a comparator and it imposes the same ordering as this comparator. Thus, comp1.equals(comp2) implies that sgn(comp1.compare(o1, o2))==sgn(comp2.compare(o1, o2)) for every object reference o1 and o2.

Declaring an equals inside Comparator allows you to document this in the form of javadoc.

Note that the documentation of the API also serves as the contract, so it's not just cosmetics here. It's explicit constraints that other code and your code can rely on.

In similar situations where you have less established methods, it may also serve as documenting an intent. I.e., Interface.method should be there, regardless of how its super interfaces evolves.

回答2:

From the Java documentations, the reason why Comparator has it's own equals() method:

However, overriding this method may, in some cases, improve performance by allowing programs to determine that two distinct comparators impose the same order.

回答3:

Read its javadoc. It's there only to explain what equals() must return if you choose to override it in a class implementing Comparator. You might think that no comparator can be equal to any other, but it's not the case. You might think that two comparators are equal if they return the same thing for any arguments, but it's not the case. The javadoc explains that two comparators are equal if they impose the same ordering, whatever the arguments given. The javadoc also says:

Note that it is always safe not to override Object.equals(Object)

Most of the time, you don't override equals() in comparators.

回答4:

from the docs:

it is always safe not to override Object.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.

回答5:

Technically, the declaration of the method is redundant (the compiler does not care), but...

Declaring the equals method in this interface makes it part of the contract between caller and different Comparators and allows it to specify/extend its semantics.

It specifies that two Comparators are equal only if they impose the same ordering with their compare() method. This extends the semantics of Object.equals() and must therefore be documented in the interface.

回答6:

The equals() method in Comparator is provided to force a user implementing the Comparator interface to implement equals() with some additional rules and constraints in addition to the ones already applied on equals() from Object.

The additional rule being:

This method must obey the general contract of Object.equals(Object). Additionally, this method can return true only if the specified object is also a comparator and it imposes the same ordering as this comparator. Thus, comp1.equals(comp2) implies that sgn(comp1.compare(o1, o2))==sgn(comp2.compare(o1, o2)) for every object reference o1 and o2.

回答7:

Putting an Object method in an interface declaration allows Javadoc declaration of the meaning equals is required to have in classes that implement the interface.

回答8:

Comparator interface have their own equals() method

Well,. first of all, it should be clear that whenever you implement Comparable interface you should provide your program to decide when objects are equal, less or greater.

I am quite confuse about have equals() inside Comparator. Any class will get equals() by default through Object class.

The equals() method implementation u inherit from Object class only checks whether the two referances point to the same object or not. It dosn't apply any comparison. It's you who will provide in your class (or possibly in your Interface) the criteria for objects to be equal.

Then what is need to have equals() method inside an interface?

Obviously , whenever you implement when objects are less , greater than you must implement when they are equal. so rather than relying on default Object equals() method you should provide your logic to check equality

来源：https://stackoverflow.com/questions/11013850/why-does-comparator-declare-equals