The HashMap class of the Java collections framework provides the hash table implementation of the Map interface.

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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<Key, Value> numbers = new HashMap<>(8, 0.6f);

In the above code, we have created a hashmap named numbers.

Here,

• Key – a unique identifier used to associate each element (value) in a map
• Value – elements associated by keys in a map

 

Notice the part new HashMap<>(8, 0.6). Here, the first parameter is capacity and the second parameter is loadFactor.

• capacity – The capacity of this hash map is 8. Meaning, it can store 8 entries.
• loadFactor – The load factor of this hashmap is 0.6. This means, whenever our hash table is filled by 60%, the entries are moved to a new hash table of double the size of the original hash table.

 

Default capacity and load factor

It’s possible to create a hashmap without defining its capacity and load factor. For example,

// HashMap with default capacity and load factor
HashMap<Key, Value> numbers1 = new HashMap<>();

By default,

• the capacity of the hash map will be 16
• the load factor will be 0.75

 

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Creating HashMap from Other Maps

Here is how we can create a hashmap containing all the elements of other maps.

import java.util.HashMap;

class Main{
    public static void main(String[] args){
        // Creating a hashmap of even numbers
        HashMap<String, Integer> evenNumbers = new HashMap<>();
        evenNumbers.put("Two", 2);
        evenNumbers.put("Four", 4);
        System.out.println("HashMap1: " + evenNumbers);

        // Creating a hash map from other hashmap
        HashMap<String, Integer> numbers = new HashMap<>(evenNumbers);
        numbers.put("Three", 3);
        System.out.println("HashMap2: " + numbers);
    }
}

Output

HashMap1: {Four=4, Two=2}
HashMap2: {Two=2, Three=3, Four=4}

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Methods of HashMap

The HashMap class provides various methods that allow us to perform various operations on the map.

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Insert Elements to HashMap

• put() – inserts the specified key/value mapping to the map
• putAll() – inserts all the entries from specified map to this map
• putIfAbsent() – inserts the specified key/value mapping to the map if the specified key is not present in the map

 

For example,

import java.util.HashMap;

class Main{
    public static void main(String[] args){
        // Creating HashMap of even numbers
        HashMap<String, Integer> evenNumbers = new HashMap<>();

        // Using put()
        evenNumbers.put("Two", 2);
        evenNumbers.put("Four", 4);

        // Using putIfAbsent()
        evenNumbers.putIfAbsent("Six", 6);
        System.out.println("HashMap of even numbers: " + evenNumbers);

        //Creating HashMap of numbers
        HashMap<String, Integer> numbers = new HashMap<>();
        numbers.put("One", 1);

        // Using putAll()
        numbers.putAll(evenNumbers);
        System.out.println("HashMap of numbers: " + numbers);
    }
}

Output

HashMap of even numbers: {Six=6, Four=4, Two=2}
HashMap of numbers: {Six=6, One=1, Four=4, Two=2}

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Access HashMap Elements

1. Using entrySet(), keySet() and values()

• entrySet() – returns a set of all the key/value mapping of the map
• keySet() – returns a set of all the keys of the map
• values() – returns a set of all the values of the map

 

For example,

import java.util.HashMap;

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

        numbers.put("One", 1);
        numbers.put("Two", 2);
        numbers.put("Three", 3);
        System.out.println("HashMap: " + numbers);

        // Using entrySet()
        System.out.println("Key/Value mappings: " + numbers.entrySet());

        // Using keySet()
        System.out.println("Keys: " + numbers.keySet());

        // Using values()
        System.out.println("Values: " + numbers.values());
    }
}

Output

HashMap: {One=1, Two=2, Three=3}
Key/Value mappings: [One=1, Two=2, Three=3]
Keys: [One, Two, Three]
Values: [1, 2, 3]

2. Using get() and getOrDefault()

• get() – Returns the value associated with the specified key. Returns null if the key is not found.
• getOrDefault() – Returns the value associated with the specified key. Returns the specified default value if the key is not found.

 

For example,

import java.util.HashMap;

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

        HashMap<String, Integer> numbers = new HashMap<>();
        numbers.put("One", 1);
        numbers.put("Two", 2);
        numbers.put("Three", 3);
        System.out.println("HashMap: " + numbers);

        // Using get()
        int value1 = numbers.get("Three");
        System.out.println("Returned Number: " + value1);

        // Using getOrDefault()
        int value2 = numbers.getOrDefault("Five", 5);
        System.out.println("Returned Number: " + value2);
    }
}

Output

HashMap: {One=1, Two=2, Three=3}
Returned Number: 3
Returned Number: 5

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Remove Elements

• remove(key) – returns and removes the entry associated with the specified key from the map
• remove(key, value) – removes the entry from the map only if the specified key mapped to the specified value and return a boolean value

 

For example,

import java.util.HashMap;

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

        HashMap<String, Integer> numbers = new HashMap<>();
        numbers.put("One", 1);
        numbers.put("Two", 2);
        numbers.put("Three", 3);
        System.out.println("HashMap: " + numbers);

        // remove method with single parameter
        int value = numbers.remove("Two");
        System.out.println("Removed value: " + value);

        // remove method with two parameters
        boolean result = numbers.remove("Three", 3);
        System.out.println("Is the entry Three removed? " + result);

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

Output

HashMap: {One=1, Two=2, Three=3}
Removed value: 2
Is the entry Three removed? True
Updated HashMap: {One=1}

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Replace Elements

• replace(key, value) – replaces the value associated with the specified key by a new value
• replace(key, old, new) – replaces the old value with the new value only if old value is already associated with the specified key
• replaceAll(function) – replaces each value of the map with the result of the specified function

 

For example,

import java.util.HashMap;

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

        HashMap<String, Integer> numbers = new HashMap<>();
        numbers.put("First", 1);
        numbers.put("Second", 2);
        numbers.put("Third", 3);
        System.out.println("Original HashMap: " + numbers);

        // Using replace()
        numbers.replace("Second", 22);
        numbers.replace("Third", 3, 33);
        System.out.println("HashMap using replace(): " + numbers);

        // Using replaceAll()
        numbers.replaceAll((key, oldValue) -> oldValue + 2);
        System.out.println("HashMap using replaceAll(): " + numbers);
    }
}

Output

Original HashMap: {Second=2, Third=3, First=1}
HashMap using replace: {Second=22, Third=33, First=1}
HashMap using replaceAll: {Second=24, Third=35, First=3}

In the above program notice the statement

numbers.replaceAll((key, oldValue) -> oldValue + 2);

Here, the method accesses all the entries of the map. It then replaces all the values with the new values provided by the lambda expression.

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Recompute Values

1. Using compute() Method

• compute() – Computes a new value using the specified function. It then associates the computed value to the specified key.
• computeIfAbsent() – If the specified key is not mapped to any value, the method will compute a new value using the specified function. It then associates the new value with the key.
• computeIfPresent() – If the specified key is already mapped to any value, this method will compute a new value using the specified function. It then associates the new value with the key.

 

For example,

import java.util.HashMap;

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

        HashMap<String, Integer> numbers = new HashMap<>();
        numbers.put("First", 1);
        numbers.put("Second", 2);
        System.out.println("Original HashMap: " + numbers);

        // Using compute()
        numbers.compute("First", (key, oldValue) -> oldValue + 2);
        numbers.compute("Second", (key, oldValue) -> oldValue + 1);
        System.out.println("HashMap using compute(): " + numbers);

        // Using computeIfAbsent()
        numbers.computeIfAbsent("Three", key -> 5);
        System.out.println("HashMap using computeIfAbsent(): " + numbers);

        // Using computeIfPresent()
        numbers.computeIfPresent("Second", (key, oldValue) -> oldValue * 2);
        System.out.println("HashMap using computeIfPresent(): " + numbers);
    }
}

Output

Original HashMap: {Second=2, First=1}
HashMap using compute(): {Second=3, First=3}
HashMap using computeIfAbsent(): {Second=3 First=3, Three=5}
HashMap using computeIfPresent(): {Second=6, First=3, three=5}

In the above example, we have recomputed the values of the map using the compute() method.

 

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2. Using merge() Method

The merge() method associates the specified value to the specified key if the specified key is not already associated.

However, if the specified key is already associated with a value, it will merge the new specified value with the existing old value. For example,

import java.util.HashMap;

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

        HashMap<String, Integer> numbers = new HashMap<>();
        numbers.put("First", 1);
        numbers.put("Second", 2);
        System.out.println("Original HashMap: " + numbers);

        // Using merge() Method
        numbers.merge("First", 4, (oldValue, newValue) -> oldValue + newValue);
        System.out.println("New HashMap: " + numbers);
    }
}

Output

Original HashMap: {Second=2, First=1}
New HashMap: {Second=2, First=5}

In the above example, the merge() method takes 3 parameters: key, newValue and a lambda expression (that computes the new merged value).

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Other Methods of HashMap

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Iterate Through a HashMap

In a HashMap, we can

• iterate through its keys
• iterate through its values
• iterate through its keys/values

 

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1. Using the forEach loop

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

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

        // Creating a HashMap
        HashMap<String, Integer> numbers = new HashMap<>();
        numbers.put("One", 1);
        numbers.put("Two", 2);
        numbers.put("Three", 3);
        System.out.println("HashMap: " + numbers);

        // Accessing the key/value pair
        System.out.print("Entries: ");
        for(Entry<String, Integer> entry: numbers.entrySet()) {
            System.out.print(entry);
            System.out.print(", ");
        }

        // Accessing the key
        System.out.print("nKeys: ");
        for(String key: numbers.keySet()) {
            System.out.print(key);
            System.out.print(", ");
        }

        // Accessing the value
        System.out.print("nValues: ");
        for(Integer value: numbers.values()) {
            System.out.print(value);
            System.out.print(", ");
        }
    }
}

Output

HashMap: {One=1, Two=2, Three=3}
Entries: One=1, Two=2, Three=3
Keys: One, Two, Three,
Values: 1, 2, ,3,

In the above program, note that we have imported the java.util.Map.Entry package. Here, Map.Entry is the nested class of the Map interface.

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

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2. Using iterator() Method

It is also possible to iterate a HashMap using the iterator() method. In order to use this method, we must import the java.util.Iterator package.

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

class Main{
    public static void main(String[] args){
        // Creating a HashMap
        HashMap<String, Integer> numbers = new HashMap<>();
        numbers.put("One", 1);
        numbers.put("Two", 2);
        numbers.put("Three", 3);
        System.out.println("HashMap: " + numbers);

        // Creating an object of Iterator
        Iterator<Entry<String, Integer>> iterate1 = numbers.entrySet().iterator();

        // Accessing the Key/Value pair
        System.out.print("Entries: ");
        while(iterate1.hasNext()) {
            System.out.print(iterate1.next());
            System.out.print(", ");
        }

        // Accessing the key
        Iterator<String> iterate2 = numbers.keySet().iterator();
        System.out.print("nKeys: ");
        while(iterate2.hasNext()) {
            System.out.print(iterate2.next());
            System.out.print(", ");
        }

        // Accessing the value
        Iterator<Integer> iterate3 = numbers.values().iterator();
         System.out.print("nValues: ");
        while(iterate3.hasNext()) {
            System.out.print(iterate3.next());
            System.out.print(", ");
        }
    }
}

Output

HashMap: {One=1, Two=2, Three=3}
Entries: One=1, Two=2, Three=3
Keys: One, Two, Three,
Values: 1, 2, 3,

In the above program, note that we have imported the java.util.Map.Entry package. Here, Map.Entry is the nested class of the Map interface.

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