it SCJP book by K Siera & Berts Bates it is written that "hash map can have one null key & multiple null values" null values is alright but how can it has null key. then can values be retrieved from it.
HashMap object hold key and it's corresponding value.
See the following non-generic code example
Ouput is English not Maths.
The second put method call changes value to English. Initially null key point to Maths but now point to English.
In such a way HashMap have unique keys and multiple values. Multiple values may be same or different but keys must have unique values.