Question

My first instinct is to say each key is an object, and has a hash code, which is what is used to determine if a duplicate is being inserted. However, I can’t find anything to back that up for sure. Can someone provide a link that says that, or provide the real answer here? Thanks!

Answer 1

The Map interface specifies that if two keys are null they are duplicates, otherwise if there’s a key k such that key.equals(k), then there is a duplicate. See the contains or get method here:

http://java.sun.com/javase/6/docs/api/java/util/Map.html#containsKey(java.lang.Object)

However, it’s up to the Map implementation how to go about performing that check, and a HashMap will use a hash code to narrow the potential keys it will check with the equals method. So in practice, for a typical hash based map, to check for duplicates a map will take the hashcode (probably mod some size), and use equals to compare against any keys whose hashcode mod the same size gives the same remainder.