Java Hashmap: Discover the Basics & Learn How to Leverage Them

Athena Ozanich
Athena Ozanich

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Java is amazing, the beverage and programming language. Today's topic is the programming language, the hashmap class.

Woman Studying how to use Java hashmaps in her software development.

Java offers several ways to work with data, but the Java hashmap provides a unique way to store data in key-value pairs.

Before we dive into using Java hashmap, check out the jump links below to find exactly what you're looking for.

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What is a Hashmap in Java?

A hashmap is similar to arrays, which are used to store data collections. However, hashmaps are different because they store data in key-value pairs. This may seem like a small differentiation, but the truth is that it makes data management much more straightforward. Let’s look at the next video to better understand how the hashmap works.

Storing data in key-value pairs makes many programming tasks more manageable, from data management to creating pleasant user interfaces. Additionally, hashmap keys and values support many data types such as strings, integers, booleans, characters, and doubles.

Now that we covered what it is, let’s look deeper at its behavior and inner workings by examining the syntactic structure.

How Java Hashmap Works

A hashmap can hold multiple key-value pairs within it; these pairs use sensible data patterns like: (“First Name”, “Johnathan”). By pairing data like this, you can easily target and retrieve data, such as all usernames or even usernames that start with X.

To use the hashmap, you will need to import it from the java.util.HashMap library as seen below.

 
HashMap<String, String> userData = new HashMap<String, String>();

The code above creates a Java hashmap named userData; this hashmap can accept a key with a string type and a value with a string type. This structure is typical, especially when working with building user interfaces. The pairing of values with a user-friendly key such as a user's name with the words First Name makes building a UI much simpler.

Now let’s look at the syntax for working with Java hashmap methods.

How to Use Java Hashmap

You can interact with Hashmaps through a variety of methods; three of the most commonly used methods are the put, get, and remove methods. There are several methods available, each serving a unique and helpful function. Let’s go over the three most common methods, then briefly discuss the others.

Check out this video on how to use the put() and get() methods before seeing some code examples.

The put() method adds items to your hashmap, and with it, you can easily add new pairs as you need to. The syntax for adding a pair with the put() method can be seen in the code snippet below.

 
userData.put("First Name", "John");

This simple line of code quickly adds a key-value pair with the strings First Name and John to the userData hashmap. This hashmap could store additional user information like Last Name and Smith, allowing you to build a very organized set of data about a user.

Knowing how to retrieve that data for use is something that the get() method solves quickly. With the get method, you can easily retrieve the value of a key simply by targeting the key. Let’s look at the syntax for that next — spoiler alert; it's as simple as you think.

 
userData.get("First Name");

The line of code above looks for the key named First Name and returns the associated value. In this example, it returns the string, John. One more important thing to consider is the need to remove a pair from the hashmap. There may be a case where a specific key-value pair may need to be changed or removed entirely — you can use the remove() method to solve this problem since it does precisely that.

The remove() method looks similar to the above method. It accepts the target key and uses that to identify which pair to remove. Look at the code below to see how to use the remove method on a hashmap.

 
userData.remove("First Name");

This line above efficiently removes the pair associated with the key First Name, leaving the hashmap empty.

Let's look at one more example of using a hashmap with a for loop. In the code example below, you will see a hashmap with several pairs, then a for loop returning the key and value of each pair.

 
HashMap<String, String> userData = new HashMap<String, String>();
userData.put("First Name", "John");
userData.put("Last Name", "Smith");
userData.put("Age", "405");
userData.put("Username", "OnComingStorm");
for (String key : userData.keySet()) {
   System.out.println( key + ": " + userData.get( key ));
}

The above hashmap contains four key-value pairs, and each key is a string used to identify its value. This structure makes displaying user data more efficient and cleaner, providing a better user experience. The for loop above uses the keySet() method to return all keys from the hashmap. The get method then uses the key returned on each iteration to retrieve the correct value.

Getting Started With Java Hashmap

Java hashmaps are not only powerful and valuable, but they can also be enjoyable to work with, thanks to the simplicity of their behavior. In addition, the many methods provided to interact with hashmaps further extend their flexibility and usefulness, creating a very efficient data collection.

In this post, you’ve learned how they work, explored their syntax, and even discovered the methods to interact with them. You’ve even seen how you can use a loop to iterate through the collection to retrieve the keys and or values for use. Some more methods to explore can be seen in the list below.

  • values(): Used to return the values of the hashmap, often used in a loop.
  • size(): Used to return the size of the hashmap, this method counts a pair as one item.
  • clear(): Used to remove all items from the hashmap.

You can learn more about the other methods provided for the hashmap and how they might benefit your software. You can also explore how you can incorporate hashmaps with other Java features.

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