Get number of duplicates from ArrayList

Question

For example, say I have an ArrayList that could contain the following values:

x
x
x
y
y

Now what I want to retrieve is the number

Answer 1

What you want to do is use a HashMap. Store the Object as the key and the Long as the occurrence count.

Here's some pseudo code which will do what you're trying to do.

for(x in list) {
 if(x in Map) {
   map.put(x, map.get(x)++);
 } else {
   map.put(x, 1);
 }
}

You can then iterate through the Map and print the value and occurrence count. I'll let you write that one up. It's easy enough.