Difference between Set, List and Map in Java - Interview question

Set, List and Map are three important interfaces of the Java collection framework, and the difference between Set, List, and Map in Java is one of the most frequently asked Java Collection interview questions. Sometimes this question is asked as When to use List, Set and Map in Java. Clearly, the interviewer is looking to know that whether you are familiar with the fundamentals of the Java collection framework or not. In order to decide when to use List, Set or Map, you need to know what are these interfaces and what functionality they provide. List in Java provides ordered and indexed collection which may contain duplicates. 

The Set interface provides an unordered collection of unique objects, i.e. Set doesn't allow duplicates, while Map provides a data structure based on key-value pair and hashing. 

All three List, Set, and Map are interfaces in Java and there are many concrete implementations of them are available in Collection API. ArrayList and LinkedList are the two most popular used List implementations while LinkedHashSet, TreeSet, and HashSet are frequently used Set implementations. 

In this Java article, we will see the difference between Map, Set, and List in Java and learn when to use List, Set, or Map. 

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Set vs List vs Map in Java

As I said Set, List and Map are interfaces, which define core contracts e.g. a Set contract says that it can not contain duplicates. Based on our knowledge of List, Set and Map let's compare them on different metrics.

1.Duplicate Objects

The main difference between the List and Set interface in Java is that List allows duplicates while Set doesn't allow duplicates. All implementation of Set honor this contract. 

While a Map holds two objects per Entry e.g. a key and a value and It may contain duplicate values but keys are always unique. See here for more differences between List and Set data structure in Java. 

2. Order

Another key difference between List and Set is that List is an ordered collection, List's contract maintains insertion order or element. Set is an unordered collection, you get no guarantee on which order element will be stored. 

Though some of the Set implementations e.g. LinkedHashSet maintains order. Also, SortedSet and SortedMapl like TreeSet and TreeMap maintain a sorting order, imposed by using Comparator or Comparable. 

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3. Null elements

The list allows null elements and you can have many null objects in a List because it also allowed duplicates. Set just allow one null element as there is no duplicate permitted while in Map you can have null values and at most one null key. 

Worth noting is that Hashtable doesn't allow null key or values but HashMap allows null values and one null key.  This is also the main difference between these two popular implementations of Map interface, aka HashMap vs Hashtable. 

4. Useful implementations

The most popular implementations of the List interface in Java are ArrayList, LinkedList, and Vector class. ArrayList is more general-purpose and provides random access with index, while LinkedList is more suitable for frequently adding and removing elements from List.

Vector is a synchronized counterpart of ArrayList. On the other hand, the most popular implementations of the Set interface are HashSet, LinkedHashSet, and TreeSet. The first one is a general-purpose Set which is backed by HashMap, see how HashSet works internally in Java for more details.

It also doesn't provide any ordering guarantee but LinkedHashSet does provide ordering along with uniqueness offered by the Set interface.

Third implementation TreeSet is also an implementation of the SortedSet interface, hence it keeps elements in a sorted order specified by compare() or compareTo() method.

Now the last one, the most popular implementation of Map interface is HashMap, LinkedHashMap, Hashtable, and TreeMap.

The first one is the non-synchronized general-purpose Map implementation while Hashtable is its synchronized counterpart, both don't provide any ordering guarantee which comes from LinkedHashMap. Just like TreeSet, TreeMap is also a sorted data structure and keeps keys in sorted order (see Java Collections from basics to Advanced Udemy course to learn more about it)

When to use List, Set, and Map in Java

Based upon our understanding of the difference between Set, List and Map we can now decide when to use List, Set or Map in Java.

1. If you need to access elements frequently by using the index than List is a way to go. Its implementation e.g. ArrayList provides faster access if you know the index. 

2. If you want to store elements and want them to maintain an order on which they are inserted into a collection then go for List again, as List is an ordered collection and maintain insertion order.

3. If you want to create a collection of unique elements and don't want any duplicate then choosing any Set implementation e.g. HashSet, LinkedHashSet or TreeSet. 

All Set implementation follow there general contract e.g. uniqueness but also add addition feature e.g. TreeSet is a SortedSet and elements stored on TreeSet can be sorted by using Comparator or Comparable in Java. LinkedHashSet also maintains insertion order.

4) If you store data in form of key and value than Map is the way to go. You can choose from Hashtable, HashMap, TreeMap based upon your subsequent need. In order to choose between the first two see the difference between HashSet and HashMap in Java.

That's all about the difference between Set, List, and Map in Java. All three are the most fundamental interface of the Java Collection framework and any Java developer should know their distinguish feature and given a situation should be able to pick the right Collection class to use.  

It's also good to remember the difference between there implementation e.g. When to use ArrayList and LinkedList, HashMap vs Hashtable or When to use Vector or ArrayList etc. Collection API is huge and it's difficult to know every bits and piece but at the same time, there is no excuse for not knowing fundamentals like the difference between Set, List, and Map in Java.

Further Learning
Java Programming Masterclass for Beginners
Java Fundamentals: Collections
Data Structures and Algorithms: Deep Dive Using Java

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P. S. - If you are preparing for Java interviews and need more such questions for practice then you can also check out my book, Grokking the Java Interview which contains many such excellent core Java questions and covers all essential core Java topics like Collections, Multithreading, Core Java basics, design patterns, JDBC, and much more.