List vs Set vs Map in Java

List, Set, and Map interfaces are very important part of collection framework. List and Set implements Collection interface but Map does not. Let us discuss all with examples and differences.

Collection Hierarchy:

Map Hierarchy:

List

A List in java extends the collection interface and represent an sequenced or ordered group of elements. It can contain duplicate elements. It also defines some additional methods which it inherits from Collection interface.

Note: Elements in List can be inserted, updated, or retrieved by their position or index. Index or position value starts from 0.

Example

import java.util.*;  
public class Main {  
    public static void main(String args[]){  
        //Create List object
        List<String> mySubjects = new ArrayList<>();  
 
        //Add elements to list
        mySubjects.add("Java");  
        mySubjects.add("Spring");  
        mySubjects.add("Hibernate");  
 
 
        //Add elements at specified position
        mySubjects.add(1,"SQL");  
        mySubjects.add(2,"Oracle"); 
 
        System.out.println("My Subjects:"); 
        //Print all subjects
        for(String subject : mySubjects){  
         System.out.println(subject);  
        }
 
        //Print element on 2nd index
        System.out.println("Element at 2nd index: "+mySubjects.get(2));  
    }  
}

Output

My Subjects:    
Java
SQL
Oracle
Spring
Hibernate
Element at 2nd index: Oracle

Set

A set represents a group or collection of items. Set has a special property that is unique items, it can not contain a duplicate item or element. It extends the collection interface.

Note: Set interface does not have any additional method other than methods inherited from Collection interface. With all collection interface methods it adds the restriction that it can not contain a duplicate elements.

Example

import java.util.*;  
public class Main {  
 
    public static void main(String args[]){  
        //Create Set object
        Set<String> mySubjects = new HashSet<>();  
 
        //Add elements to Set
        mySubjects.add("Java");  
        mySubjects.add("Spring");  
        mySubjects.add("Hibernate");  
 
        System.out.println("My Subjects:"); 
        //Print all subjects
        for(String subject : mySubjects){  
         System.out.println(subject);  
        }
 
    }  
}

Output

My Subjects:
Java
Hibernate
Spring

Map

A map in java, not extends the Collection interface. It represents a group of special elements or objects. Every map element or object contains key and value pair. A map can’t contain duplicate keys and one key can refer to at most one value.

Example

import java.util.*;  
public class Main {  
 
    public static void main(String args[]){  
        Map<Integer,String> mysubjects = new HashMap<Integer,String>();  
 
        //Add elements to map
        mysubjects.put(1,"Java");  
        mysubjects.put(2,"Spring");  
        mysubjects.put(3,"Oracle");  
 
        //Print map elements in key value form
        for(Map.Entry subject : mysubjects.entrySet()) 
           System.out.println(subject.getKey()+" - "+subject.getValue());  
 
    }  
}

Output

1 - Java
2 - Spring
3 - Oracle

When to use List, Set and Map in Java?

Use of a data structure or collection is depends upon the requirement.

Use Set: If you need group of unique elements.
Use List: If get operations are higher than any other operation.
Use Map: If objects contains the key and value pair.

Difference between List, Set, and Map

List	Set	Map
Allow duplicate elements.	Does not allow duplicate elements.	Does not allow duplicate key but values can be duplicate.
Allow multiple null values.	Allow single null value.	Allow single null as key and multiple null as values.
Maintains insertions order.	Set represents an unordered collection but some of its implementation classes maintains some order. LinkedHashSet maintains order, TreeSet maintains ascending order.	Like Set, Map also represents an unordered collection. Again same like Set, some of its implementation classes maintains some order. TreeMap maintains ascending order of keys.

Java interview questions on collections

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