Java Collections Framework - The Map Interface

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In this guide, we will learn about Map interface, Map interface methods and Map interface implementation classes.

Important Key Points About Map Interface

• Map interface is a part of Java Collection Framework, but it doesn’t inherit Collection Interface.
• A Map cannot contain duplicate keys:  Each key can map to at most one value. It models the mathematical function abstraction.
• Each key at most must be associated with one value.
• Each key-value pairs of the map are stored as Map.Entry objects. Map.Entry is an inner interface of Map interface.
• The Java platform contains three general-purpose Map interface implementations: HashMap, TreeMap, and LinkedHashMap.
• Order of elements in a map is implementation dependent. HashMap doesn’t maintain any order of elements. LinkedHashMap maintains insertion order of elements. Where as TreeMap places the elements according to the supplied Comparator.
• The Map interface provides three methods, which allows map’s contents to be viewed as a set of keys (keySet() method), a collection of values (values() method), or set of key-value mappings (entrySet() method).

Map Interface with It's HashMap Implementation Class Example

Let's create a simple example to demonstrate Map interface with it's HashMap implementation class:

import java.util.HashMap; import java.util.Map; public class CreateHashMapExample { public static void main(String[] args) { // Creating a HashMap Map<String, Integer> numberMapping = new HashMap<>(); // Adding key-value pairs to a HashMap numberMapping.put("One", 1); numberMapping.put("Two", 2); numberMapping.put("Three", 3); // Add a new key-value pair only if the key does not exist in the HashMap, or is mapped to `null` numberMapping.putIfAbsent("Four", 4); System.out.println(numberMapping); } }

Output:

{One=1, Four=4, Two=2, Three=3}

Map Interface with It's LinkedHashMap Implementation Class Example

Let's create a simple example to demonstrate Map interface with it's LinkedHashMap implementation class:

// Creating a LinkedHashMap LinkedHashMap<String, Integer> dayNumberMapping = new LinkedHashMap<>(); // Adding new key-value pairs to the LinkedHashMap dayNumberMapping.put("Mon", 1); dayNumberMapping.put("Tus", 2); dayNumberMapping.put("Wen", 3); dayNumberMapping.put("Thu", 4); dayNumberMapping.put("Fri", 5); dayNumberMapping.put("Sat", 6); // Add a new key-value pair only if the key does not exist  // in the LinkedHashMap, or is mapped to `null` dayNumberMapping.putIfAbsent("Sun", 7); System.out.println(dayNumberMapping);

Output:

{Mon=1, Tus=2, Wen=3, Thu=4, Fri=5, Sat=6, Sun=7}

Map Interface with It's TreeMap Implementation Class Example

Let's create a simple example to demonstrate Map interface with it's TreeMap implementation class:

// Creating a TreeMap TreeMap<String, String> fileExtensions = new TreeMap<>(); // Adding new key-value pairs to a TreeMap fileExtensions.put("python", ".py"); fileExtensions.put("c++", ".cpp"); fileExtensions.put("kotlin", ".kt"); fileExtensions.put("golang", ".go"); fileExtensions.put("java", ".java"); // Printing the TreeMap (Output will be sorted based on keys) System.out.println(fileExtensions);

Output:

{c++=.cpp, golang=.go, java=.java, kotlin=.kt, python=.py}

Map Interface APIs/Methods

The class diagram shows a list of APIs/Methods Map interface provides.

Read more about each API with an example in Guide to HashMap Class

Map Interface Implementations

General-purpose Map Implementations:

• Collections Framework - HashMap Class
• Collections Framework - LinkedHashMap Class
• Collections Framework - TreeMap class

Special-Purpose Map Implementations:

• Collections Framework - EnumMap
• Collections Framework - WeakHashMap
• Collections Framework - IdentityHashMap

Related Collections Framework Interfaces

• Collections Framework - The Collection Interface
• Collections Framework - The Set Interface
• Collections Framework - The SortedSet Interface
• Collections Framework - The List Interface
• Collections Framework - The Queue Interface
• Collections Framework - The Deque Interface
• Collections Framework - The Map Interface
• Collections Framework - The SortedMap Interface