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Java Collection Map

~1 min read · Java
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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 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) } }
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LinkedHashMap:

import java.util.LinkedHashMap; import java.util.Map; public class Main { public static void main(String[] args) { Map 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) } }
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TreeMap:

import java.util.TreeMap; import java.util.Map; public class Main { public static void main(String[] args) { Map 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) } }
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Iterating over a map:

import java.util.LinkedHashMap; import java.util.Map; public class Main { public static void main(String[] args) { Map map = new LinkedHashMap<>(); map.put(1, "Dog"); map.put(2, "Cat"); map.put(3, "Elephant"); map.forEach((key, value) -> System.out.println(key + " -> " + value)); } }
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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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