Write a program that uses a HashMap to compute a histogram of positive numbers entered by the user. The HashMap’s key should be the number that is entered, and the value should be a counter of the number of times the key has been entered so far. Use -1 as a sentinel value to signal the end of user input. For example, if the user inputs:

5
12
3
5
5
3
21
-1
Then the program should output the following (not necessarily in this order):
The number 3 occurs 2 times.
The number 5 occurs 3 times.
The number 12 occurs 1 times.
The number 21 occurs 1 times.

The solution is a fairly straightforward implementation of a HashMap to map a number to a counter. It may be worth mentioning that equals and hashcode must be overwritten if the student wants to use HashMap with their own set. Note that Java will automatically unbox/box between an int and Integer. This solution takes advantage of this feature.

See the code in HashMapHistogram.java.