Check if element exists in HashSet by its hash

Question

Can I check if an object is in a HashSet if I have only the object's hash code, but not the object itself?

Answer 1

You can't, since the hashCode is only used to locate the bin in which the object is stored within the HashSet. There may be multiple elements in the same bin, and HashSet uses equals to determine if the searched element exists in the bin that the hashCode was mapped to.

The bin may contain multiple elements having the same hashCode, so knowing only the hashCode is not enough.

Answer 2

You can make a special object, something like this

int hashCode = 1;  // your 
new Object() {
    @Override
    public boolean equals(Object obj) {
        return true;
    }
    @Override
    public int hashCode() {
        return hashCode;
    }
};

Answer 3

No you cant. Both equals and hashcode are required for finding an object in a hashed collection. The hashcode method tells which bucket to find the object in and the equals method finds the object in the bucket.

A real world example would be a house with many people in it. If you tell me to go find someone in some house, I am going to ask you two questions :

1. What house do I go to?
2. Who do I find?

Here (1) will be answered by hashcode and (2) will be answered by equals method.