Java HashMap

The HashMap class of the Java collections framework provides the functionality of the hash table data structure.

It stores elements in key/value pairs. Here, keys are unique identifiers used to associate each value on a map.

The HashMap class implements the Map interface.

Create a HashMap

In order to create a hash map, we must import the java.util.HashMap package first. Once we import the package, here is how we can create hashmaps in Java.

// hashMap creation with 8 capacity and 0.6 load factor
HashMap<K, V> numbers = new HashMap<>();

In the above code, we have created a hashmap named numbers. Here, K represents the key type and V represents the type of values. For example,

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

Here, the type of keys is String and the type of values is Integer.

Example 1: Create HashMap in Java

import java.util.HashMap;

class Main {
  public static void main(String[] args) {

    // create a hashmap
    HashMap<String, Integer> languages = new HashMap<>();

    // add elements to hashmap
    languages.put("Java", 8);
    languages.put("JavaScript", 1);
    languages.put("Python", 3);
    System.out.println("HashMap: " + languages);
  }
}

Output

HashMap: {Java=8, JavaScript=1, Python=3}

In the above example, we have created a HashMap named languages.

Here, we have used the put() method to add elements to the hashmap. We will learn more about the put() method later in this tutorial.

Basic Operations on Java HashMap

The HashMap class provides various methods to perform different operations on hashmaps. We will look at some commonly used arraylist operations in this tutorial:

• Add elements
• Access elements
• Change elements
• Remove elements

1. Add elements to a HashMap

To add a single element to the hashmap, we use the put() method of the HashMap class. For example,

import java.util.HashMap;

class Main {
  public static void main(String[] args) {

    // create a hashmap
    HashMap<String, Integer> numbers = new HashMap<>();

    System.out.println("Initial HashMap: " + numbers);
    // put() method to add elements
    numbers.put("One", 1);
    numbers.put("Two", 2);
    numbers.put("Three", 3);
    System.out.println("HashMap after put(): " + numbers);
  }
}

Output

Initial HashMap: {}
HashMap after put(): {One=1, Two=2, Three=3}

In the above example, we have created a HashMap named numbers. Here, we have used the put() method to add elements to numbers.

Notice the statement,

numbers.put("One", 1);

Here, we are passing the String value One as the key and Integer value 1 as the value to the put() method.

Recommended Readings

• Java HashMap put()
• Java HashMap putAll()
• Java HashMap putIfAbsent()

2. Access HashMap Elements

We can use the get() method to access the value from the hashmap. For example,

import java.util.HashMap;

class Main {
  public static void main(String[] args) {

    HashMap<Integer, String> languages = new HashMap<>();
    languages.put(1, "Java");
    languages.put(2, "Python");
    languages.put(3, "JavaScript");
    System.out.println("HashMap: " + languages);

    // get() method to get value
    String value = languages.get(1);
    System.out.println("Value at index 1: " + value);
  }
}

Output

HashMap: {1=Java, 2=Python, 3=JavaScript}
Value at index 1: Java

In the above example, notice the expression,

languages.get(1);

Here, the get() method takes the key as its argument and returns the corresponding value associated with the key.

We can also access the keys, values, and key/value pairs of the hashmap as set views using keySet(), values(), and entrySet() methods respectively. For example,

import java.util.HashMap;

class Main {
  public static void main(String[] args) {
    HashMap<Integer, String> languages = new HashMap<>();

    languages.put(1, "Java");
    languages.put(2, "Python");
    languages.put(3, "JavaScript");
    System.out.println("HashMap: " + languages);

    // return set view of keys
    // using keySet()
    System.out.println("Keys: " + languages.keySet());

    // return set view of values
    // using values()
    System.out.println("Values: " + languages.values());

    // return set view of key/value pairs
    // using entrySet()
    System.out.println("Key/Value mappings: " + languages.entrySet());
  }
}

Output

HashMap: {1=Java, 2=Python, 3=JavaScript}
Keys: [1, 2, 3]
Values: [Java, Python, JavaScript]
Key/Value mappings: [1=Java, 2=Python, 3=JavaScript]

In the above example, we have created a hashmap named languages. Here, we are accessing the keys, values, and key/value mappings from the hashmap.

Recommended Readings

• Java HashMap get()
• Java Hashmap getOrDefault()
• Java HashMap keySet()
• Java HashMap values()
• Java HashMap entrySet()

3. Change HashMap Value

We can use the replace() method to change the value associated with a key in a hashmap. For example,

import java.util.HashMap;

class Main {
  public static void main(String[] args) {

    HashMap<Integer, String> languages = new HashMap<>();
    languages.put(1, "Java");
    languages.put(2, "Python");
    languages.put(3, "JavaScript");
    System.out.println("Original HashMap: " + languages);

    // change element with key 2
    languages.replace(2, "C++");
    System.out.println("HashMap using replace(): " + languages);
  }
}

Output

Original HashMap: {1=Java, 2=Python, 3=JavaScript}
HashMap using replace(): {1=Java, 2=C++, 3=JavaScript}

In the above example, we have created a hashmap named languages. Notice the expression,

languages.replace(2, "C++");

Here, we are changing the value referred to by key 2 with the new value C++.

The HashMap class also provides some variations of the replace() method. To learn more, visit

• Java HashMap replace()
• Java HashMap replaceAll()

4. Remove HashMap Elements

To remove elements from a hashmap, we can use the remove() method. For example,

import java.util.HashMap;

class Main {
  public static void main(String[] args) {

    HashMap<Integer, String> languages = new HashMap<>();
    languages.put(1, "Java");
    languages.put(2, "Python");
    languages.put(3, "JavaScript");
    System.out.println("HashMap: " + languages);

    // remove element associated with key 2
    String value = languages.remove(2);
    System.out.println("Removed value: " + value);

    System.out.println("Updated HashMap: " + languages);
  }
}

Output

HashMap: {1=Java, 2=Python, 3=JavaScript}
Removed value: Python
Updated HashMap: {1=Java, 3=JavaScript}

Here, the remove() method takes the key as its parameter. It then returns the value associated with the key and removes the entry.

We can also remove the entry only under certain conditions. For example,

remove(2, "C++");

Here, the remove() method only removes the entry if the key 2 is associated with the value C++. Since 2 is not associated with C++, it doesn't remove the entry.

To learn more, visit Java HashMap remove().

Other Methods of HashMap

Iterate through a HashMap

To iterate through each entry of the hashmap, we can use Java for-each loop. We can iterate through keys only, vales only, and key/value mapping. For example,

import java.util.HashMap;
import java.util.Map.Entry;

class Main {
  public static void main(String[] args) {

    // create a HashMap
    HashMap<Integer, String> languages = new HashMap<>();
    languages.put(1, "Java");
    languages.put(2, "Python");
    languages.put(3, "JavaScript");
    System.out.println("HashMap: " + languages);

    // iterate through keys only
    System.out.print("Keys: ");
    for (Integer key : languages.keySet()) {
      System.out.print(key);
      System.out.print(", ");
    }

    // iterate through values only
    System.out.print("\nValues: ");
    for (String value : languages.values()) {
      System.out.print(value);
      System.out.print(", ");
    }
    
    // iterate through key/value entries
    System.out.print("\nEntries: ");
    for (Entry<Integer, String> entry : languages.entrySet()) {
      System.out.print(entry);
      System.out.print(", ");
    }
  }
}

Output

HashMap: {1=Java, 2=Python, 3=JavaScript}
Keys: 1, 2, 3,
Values: Java, Python, JavaScript,        
Entries: 1=Java, 2=Python, 3=JavaScript,

Note that we have used the Map.Entry in the above example. It is the nested class of the Map interface that returns a view (elements) of the map.

We first need to import the java.util.Map.Entry package in order to use this class.

This nested class returns a view (elements) of the map.

Creating HashMap from Other Maps

In Java, we can also create a hashmap from other maps. For example,

import java.util.HashMap;
import java.util.TreeMap;

class Main {
  public static void main(String[] args) {

    // create a treemap
    TreeMap<String, Integer> evenNumbers = new TreeMap<>();
    evenNumbers.put("Two", 2);
    evenNumbers.put("Four", 4);
    System.out.println("TreeMap: " + evenNumbers);

    // create hashmap from the treemap
    HashMap<String, Integer> numbers = new HashMap<>(evenNumbers);
    numbers.put("Three", 3);
    System.out.println("HashMap: " + numbers);
  }
}

Output

TreeMap: {Four=4, Two=2}
HashMap: {Two=2, Three=3, Four=4}

In the above example, we have created a TreeMap named evenNumbers. Notice the expression,

numbers = new HashMap<>(evenNumbers)

Here, we are creating a HashMap named numbers using the TreeMap. To learn more about treemap, visit Java TreeMap.

HashMap<K, V> numbers = new HashMap<>(8, 0.6f);