Java Collection Map

A Map in Java is a key-value pair data structure where each key is unique and maps to a single value. Unlike other collections like List or Set, Map does not extend Collection since it works on key-value pairs.

Implementation	Ordering	Performance	Null Keys/Values	Use Case
HashMap	No order	O(1) (Fastest)	✅ 1 null key, multiple null values	General-purpose, fastest
LinkedHashMap	Insertion Order	O(1)	✅ 1 null key, multiple null values	Maintain insertion order
TreeMap	Sorted (Ascending by Key)	O(log N) (Red-Black Tree)	❌ No null keys, ✅ multiple null values	Sorted data access

HashMap:

import java.util.HashMap;

import java.util.Map;

public class Main {

public static void main(String[] args) {

Map<Integer, String> map = new HashMap<>();

map.put(1, "Apple");

map.put(5, "Mango");

map.put(33, "Grape");

System.out.println(map); // Output: {1=Apple, 33=Grape, 5=Mango} (Order may vary)

}

Try it yourself

LinkedHashMap:

import java.util.LinkedHashMap;

import java.util.Map;

public class Main {

public static void main(String[] args) {

Map<Integer, String> map = new LinkedHashMap<>();

map.put(1, "Apple");

map.put(5, "Mango");

map.put(33, "Grape");

System.out.println(map); // Output: {1=Apple, 5=Mango,33=Grape} (Preserves insertion order)

}

Try it yourself

TreeMap:

import java.util.TreeMap;

import java.util.Map;

public class Main {

public static void main(String[] args) {

Map<Integer, String> map = new TreeMap<>();

map.put(1, "Apple");

map.put(33, "Grape");

map.put(5, "Mango");

System.out.println(map); // Output: {1=Apple, 5=Mango,33=Grape} (Sorted by key)

}

Try it yourself

Iterating over a map:

import java.util.LinkedHashMap;

import java.util.Map;

public class Main {

public static void main(String[] args) {

Map<Integer, String> map = new LinkedHashMap<>();

map.put(1, "Dog");

map.put(2, "Cat");

map.put(3, "Elephant");

map.forEach((key, value) -> System.out.println(key + " -> " + value));

}

Try it yourself

List Methods:

Commonly Used Methods of `List` Interface

Method	Description
put(K key, V value)	Adds a key-value pair
get(K key)	Retrieves value by key
remove(K key)	Removes an entry by key
containsKey(K key)	Checks if a key exists
containsValue(V value)	Checks if a value exists
size()	Returns number of key-value pairs
keySet()	Returns a `Set` of all keys
values()	Returns a `Collection` of all values
entrySet()	Returns a `Set` of all key-value pairs

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Implementation

Ordering

Performance

Null Keys/Values

Use Case

HashMap:

LinkedHashMap:

TreeMap:

Iterating over a map:

List Methods:

Commonly Used Methods of List Interface

Method

Description

Commonly Used Methods of `List` Interface