为什么我应该重写equals和hashcode方法来跟随scnerio

Question

为什么我需要重写以直接访问Hash映射中的值。也就是说，如果按照HashMap将数据插入到hashmap中，则可以通过将Key设置为Integer来获取值，而将Object用作Value。在这种情况下，必须重写equals（）和hashCode（）方法？请提出建议。

Answer 1

No, you don't need to override anything to use an object as a value in a HashMap .

Only keys need to have a working hashCode() .

However, you need to implement these two methods (technically only equals , but these two are a set, really) if you want to use things like Map#containsValue , List#indexOf or Collection#contains (and these should not just be using reference identity).

Answer 2

hashCode() is used to search for a specific elem when you want to retrieve it from a hashTable. hashCode() doesn't have to be distinct. in fact, you could just return the same integer for all your instance, but then, elems are stored in a list instead of a hashTable, and will cause a performance problem.

• By default implementation of hashCode() (which is the implementation of Object for subClass to extents from )of JVM returns a integer according to the memory address of the object, so this should be enough, but this implement was not required by the JVM standard.

• By default(Class object ), implementation of equals() will return true and only return true when they have same reference , ie obj1 == obj2 . read this

keep in mind that:

• equal objects must have same hashCode()
• those have same hashCode() are not required to be equal to each other.

I think override of hashCode() is not needed in most situations(not extends from other Class), cause modern JVMs has done pretty good job for you.

So conclusion is:

if your super class have overwrite the hashCode() and equals() method, then you should override them, or at least take a look at the implementation, and decide whether you should override them.