How to find total number of different items within an arraylist.

问题

I've done some searching but I wasn't able to find a valid solution. I have an arraylist storing Strings such as gum, socks, OJ, dog food...

I am having trouble iterating the list to determine the total number of differnt types of items.

ie.

ArrayList<String> Store = new ArrayList<String>();
    this.Store.add("Gum");
    this.Store.add("Gum");
    this.Store.add("Socks");
    this.Store.add("Candy");

The list has 4 total items, but only three different kinds of items (Gum, Sucks, Candy).

How would I design a method to calculate the 3?

回答1:

What Bhesh Gurung said, but in code:

int numUnique = new HashSet<String>(Store).size();

If what you actually have is StoreItems and need to go through getName() then I would do

Set<String> itemNames = new HashSet<String>();
for (StoreItem item : Store)
    itemNames.add(item.getName());
int numUnique = itemNames.size();

回答2:

Use a Set (HashSet) whose size will give you what you are looking for.

回答3:

This looks like a homework... So, if you do not understand the HashSet solution proposed above (or doning the same with a HashMap), think about doing something like this:

Create a new ArrayList

Take an element and check to see if it exists in the new ArrayList

If it is present in the new ArrayList, do nothing. Else add it.

Do this until you have examined the last element of the ArrayList.

Then, the size of the new array list should be the number you are looking for.

回答4:

You can use the lastIndexOf method and loop through the arraylist.

char count=0;

    for(char i=0; i<araylist.size(); i++){
        if(i == araylist.lastIndexOf(araylist.get(i))){
            count++;
        }
    }

Tested.

来源：https://stackoverflow.com/questions/12085062/how-to-find-total-number-of-different-items-within-an-arraylist

标签

java

arraylist

duplicates

loops