Returning a Object in a Hashset in Java

Question

Hi I have a HashSet like the following in a class called Memory:

Set<Idea> ideas = new HashSet<Idea>();

The generic type "Idea" is another class I wrote that has hashcode() and equals() overriden. I want to be able to get(and not remove) an Idea object in the HashSet ideas, then change it by adding something to it possibly changing it's hashcode() return value. I heard that this is would not work but no one explained why. I was wondering if someone could tell me how I can do this most efficiently.

Answer 1

HashSet在内部使用具有与键相同的值的HasMap。对于将对象放入Hashset，jvm fill首先计算对象的哈希码，然后基于该哈希码选择相应的存储桶并放入对象，因此如果您在更改哈希码之后将对象放入哈希集中，您将无法正确获取其位置。因此，如果您确实要删除该元素，最好从哈希集中删除该对象，更改其值，然后再次放回

Answer 2

It wont work because the hashcode is the key to find the object, if you modify the object in such a way you change it's hashcode you won't be able to find it again.

It is like the entry in a dictionary, if you change it you won't find it again. Does it makes sense?

Answer 3

If you want to perform a lookup, you should be using a Map. If you want to change the key (or an element of a Set) you have to remove it first and add it again. For this reason your key should only have immutable fields.