Java Collections Tutorial 3: HashMap

HashMaps implement the Map interface. Now what a HashMap is, is a member of the Java collections framework that stores objects in pairs, where the first member of the pair is the key, while the second member of the pair is the value. You can have duplicate values, but you can never have duplicate keys. So let us create a HashMap that has integers as keys and strings as values. Here is how we declare it:

HashMap<Integer, String> numbers = new HashMap<Integer, String>();

Adding Items to HashMap

So to add items to the HashMap, we normally use the put() method like this:

numbers.put(3, “Lucy”);

numbers.put(5, “Joseph”);

numbers.put(10, “Anthony”);

numbers.put(4, “Wayne”);

If you want to access the values referred to by the keys, we use the get() method like this:

String name = numbers.get(10);

If we output the above operation using System.out.println, it will output “Anthony”.

 Figure 1: Adding items to a HashMap

Removing Items from HashMap

If we want to remove an item from a HashMap, we use the remove() method, in which we pass the key of the value we want to remove. So, let’s say that in the above HashMap, we want to remove the key-value pair [5, “Joseph”]. Here is how we’d do it:

numbers.remove(5);

Iterating through a HashMap

There is a specific syntax for iterating through a HashMap. For most people, including myself, the easiest way is to Google the syntax for iterating through a HashMap and then copying and pasting it. However, over time, you will get the hang of it. So here is the syntax with the above example HashMap:

for(Map.Entry<Integer, String> entry: numbers.entrySet()) {

            int key = entry.getKey();

            String value = entry.getValue();

            System.out.println(key + “: ” + value);

}

 Figure 2: Removing and iterating through items in a HashMap

That’s all for now with regards to HashMap. Just remember that we use HasMaps when we want to store data with key value pairs. However, it is vital to note that HashMaps aren’t ordered. What this means is that when you iterate through a HashMap you may get one order several times, and after several iterations, you may get another order. If you don’t require order in your data, then use HashMaps. If, however, you require the data that you place be of the same order always, you might want to use a sorted map, which we look at in the next tutorial.

In case you have any questions or comments, please leave them in the comments section below and I will make sure to address them. 

Otherwise, stay safe until next time.