Java LinkedHashMap

The LinkedHashMap class of the Java collections framework provides the hash table and linked list implementation of the Map interface.

The LinkedHashMap class extends the HashMap class to store its entries in a hash table. It internally maintains a doubly-linked list among all of its entries to order its entries.

Creating a LinkedHashMap

In order to create a linked hashmap, we must import the java.util.LinkedHashMap package first. Once we import the package, here is how we can create linked hashmaps in Java.

// LinkedHashMap with initial capacity 8 and load factor 0.6
LinkedHashMap<Key, Value> numbers = new LinkedHashMap<>(8, 0.6f);

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

Here,

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

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

• capacity - The capacity of this linked hashmap is 8. Meaning, it can store 8 entries.
• loadFactor - The load factor of this linked hashmap is 0.6. This means, whenever our hash map 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 linked hashmap without defining its capacity and load factor. For example,

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

By default,

• the capacity of the linked hashmap will be 16
• the load factor will be 0.75

Note: The LinkedHashMap class also allows us to define the order of its entries. For example

// LinkedHashMap with specified order
LinkedHashMap<Key, Value> numbers2 = new LinkedHashMap<>(capacity, loadFactor, accessOrder);

Here, accessOrder is a boolean value. Its default value is false. In this case entries in the linked hashmap are ordered on the basis of their insertion order.

However, if true is passed as accessOrder, entries in the linked hashmap will be ordered from least-recently accessed to most-recently accessed.

Creating LinkedHashMap from Other Maps

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

import java.util.LinkedHashMap;

class Main {
    public static void main(String[] args) {
        // Creating a LinkedHashMap of even numbers
        LinkedHashMap<String, Integer> evenNumbers = new LinkedHashMap<>();
        evenNumbers.put("Two", 2);
        evenNumbers.put("Four", 4);
        System.out.println("LinkedHashMap1: " + evenNumbers);

        // Creating a LinkedHashMap from other LinkedHashMap
        LinkedHashMap<String, Integer> numbers = new LinkedHashMap<>(evenNumbers);
        numbers.put("Three", 3);
        System.out.println("LinkedHashMap2: " + numbers);
    }
}

Output

LinkedHashMap1: {Two=2, Four=4}
LinkedHashMap2: {Two=2, Four=4, Three=3}

Methods of LinkedHashMap

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

Insert Elements to LinkedHashMap

• put() - inserts the specified key/value mapping to the map
• putAll() - inserts all the entries from the 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.LinkedHashMap;

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

        // Using put()
        evenNumbers.put("Two", 2);
        evenNumbers.put("Four", 4);
        System.out.println("Original LinkedHashMap: " + evenNumbers);

        // Using putIfAbsent()
        evenNumbers.putIfAbsent("Six", 6);
        System.out.println("Updated LinkedHashMap(): " + evenNumbers);

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

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

Output

Original LinkedHashMap: {Two=2, Four=4}
Updated LinkedHashMap: {Two=2, Four=4, Six=6}
New LinkedHashMap: {One=1, Two=2, Four=4, Six=6}

Access LinkedHashMap 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.LinkedHashMap;

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

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

LinkedHashMap: {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. If the key is not found, it returns null.
• getOrDefault() - Returns the value associated with the specified key. If the key is not found, it returns the specified default value.

For example,

import java.util.LinkedHashMap;

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

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

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

Removed LinkedHashMap 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 be the specified value and return a boolean value

For example,

import java.util.LinkedHashMap;

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

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

Output

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

Other Methods of LinkedHashMap

LinkedHashMap Vs. HashMap

Both the LinkedHashMap and the HashMap implements the Map interface. However, there exist some differences between them.

• LinkedHashMap maintains a doubly-linked list internally. Due to this, it maintains the insertion order of its elements.
• The LinkedHashMap class requires more storage than HashMap. This is because LinkedHashMap maintains linked lists internally.
• The performance of LinkedHashMap is slower than HashMap.