Java Collections Cheat Sheet

Java Collections Cheat Sheet is a quick reference guide that helps developers understand and use the powerful Java Collections Framework (JCF) efficiently. Collections in Java provide a set of classes and interfaces to store, manage, and manipulate groups of objects, making development faster and more flexible compared to using arrays..

Java Collections Cheat Sheet : Step-by-Step

So let's start with an overview of the cheat sheet: 

Step 1: Introduction: Java Collections

What is Java Collections?

Java Collections is a framework in Java that provides a set of interfaces, classes, and algorithms to store, manage, and process groups of objects efficiently. It is part of the Java Collections Framework (JCF), introduced in Java 2 (JDK 1.2).

• Imagine that you had a box that had many tools, one for each specific task.
• The Java collection framework is quite similar to such a box because it offers a common design for representing and manipulating collections.

Why Java Collections Framework ?

The Java Collections Framework (JCF) was introduced to overcome the limitations of arrays and to provide a standardized, flexible, and efficient way of handling groups of objects.

• Dynamic & Flexible: Collections can grow and shrink in size, unlike fixed-size arrays.
• Standardized API: Common interfaces (List, Set, Map, Queue) with consistent methods.
• Code Reusability: Pre-built classes like ArrayList, HashSet, HashMap save development time.
• Efficient Algorithms: Built-in methods for sorting, searching, and manipulation.
• Interchangeability: Easy to switch between implementations (e.g., ArrayList ↔ LinkedList) with minimal code changes.

Step 2: Java Collections Hierarchy

The collection framework contains multiple interfaces where every interface is used to store a specific type of data. The following are the interfaces present in the framework.

Key Interfaces:

• Collection: Base interface for List, Set, and Queue.
• List: Ordered, index-based collection allowing duplicates.
• Set: Unordered collection with no duplicates.
• Queue: FIFO (or priority-based) collection for processing elements.

Step 3: Core Interfaces and Implementations

1. List

List is an ordered collection (also known as a sequence). Lists can contain duplicate elements. Elements can be inserted or removed or retrieved from any arbitrary position using an integer index.

Popular Implementations :

• ArrayList, Vector And LinkedList

Key Features of List

• Null Elements: All implementations allow storing null.
• Duplicates: Duplicate elements are allowed.
• Ordering: Maintains insertion order strictly.

Synchronization: In Java, synchronization means making a collection thread-safe, i.e., safe to use by multiple threads at the same time without corrupting data

• ArrayList → Not synchronized.
• LinkedList → Not synchronized.
• Vector → Synchronized (thread-safe).

import java.util.ArrayList;
import java.util.List;

public class ListExample {
    public static void main(String[] args) {
        List<Integer> numbers = new ArrayList<>();
        numbers.add(1);
        numbers.add(2);
        numbers.add(2); // Allows duplicates
        System.out.println(numbers); 
    }
}

Output

[1, 2, 2]

Explaination:

• This code creates a List using ArrayList to store integers.
• Elements are added with add(), and duplicates are allowed (2 is added twice).
• Lists maintain insertion order, so elements are stored as [1, 2, 2].
• Finally, System.out.println() prints the list contents.

Internal Structure :

• ArrayList : Internally uses re-sizable array which grows or shrinks as we add or delete elements.
• Vector : Same as ArrayList but it is synchronized.
• LinkedList : Elements are stored as Nodes where each node consists of three parts – Reference To Previous Element, Value Of The Element and Reference To Next Element.

2. Set

• HashSet, LinkedHashSet and TreeSet

Key Features of Set

• Null Elements: HashSet and LinkedHashSet permit only one null element in the collection. TreeSet does not allow null, since null cannot be compared for sorting.
• Duplicates: All Set implementations strictly prohibit duplicate elements, ensuring uniqueness.
• Order: HashSet does not guarantee any order of elements. LinkedHashSet preserves the insertion order. TreeSet maintains sorted order (natural or comparator-based).
• Synchronization: By default, none of the Set implementations are synchronized (thread-safe). Thread safety can be enabled using Collections.synchronizedSet(set).

import java.util.HashSet;
import java.util.Set;

public class SetExample {
    public static void main(String[] args) {
        Set<String> uniqueNames = new HashSet<>();
        uniqueNames.add("Alice");
        uniqueNames.add("Bob");
        uniqueNames.add("Alice"); // Does not allow duplicates
        System.out.println(uniqueNames); 
    }
}

Output

[Bob, Alice]

• This code creates a Set using HashSet to store unique names.
• Elements are added with add(), but duplicates (like "Alice") are ignored.
• Sets do not maintain insertion order, so the output order may vary.
• Finally, System.out.println() prints the set contents (e.g., [Alice, Bob])

3. Queue

• PriorityQueue, ArrayDeque and LinkedList (implements List also)

Key Features of Queue:

• Null Elements: Not allowed in PriorityQueue and ArrayDeque to prevent ambiguity in ordering.
• Duplicates: Allowed across all Queue implementations since queues don’t enforce uniqueness.
• Order: PriorityQueue arranges by priority (natural/comparator), ArrayDeque supports FIFO/LIFO, LinkedList maintains insertion order.
• Synchronization: Not synchronized by default; thread safety can be achieved using wrappers (Collections.synchronizedCollection) or concurrent queues.

import java.util.LinkedList;
import java.util.Queue;

public class QueueExample {
    public static void main(String[] args) {
        Queue<String> queue = new LinkedList<>();
        queue.add("Alice");
        queue.add("Bob");
        System.out.println(queue.poll()); // Retrieves and removes the head
        System.out.println(queue); 
    }
}

Output

Alice
[Bob]

• This code creates a Queue using LinkedList to store elements in FIFO order.
• Elements "Alice" and "Bob" are added to the queue.
• poll() retrieves and removes the head element ("Alice").
• Finally, the queue prints remaining elements ([Bob]). 

4. Map

Popular Implementations :

• HashMap, LinkedHashMap And TreeMap

Key Features of Map:

• Null Elements: HashMap & LinkedHashMap allow one null key and many null values; TreeMap disallows null keys but allows null values; Hashtable allows neither.
• Duplicates: Keys must be unique, but values can be duplicated across different keys.
• Order: HashMap has no order, LinkedHashMap maintains insertion order, TreeMap keeps keys sorted, and Hashtable has no defined order.
• Synchronization: Hashtable is synchronized; others are not, but thread safety can be achieved with Collections.synchronizedMap() or ConcurrentHashMap

import java.util.HashMap;
import java.util.Map;

public class MapExample {
    public static void main(String[] args) {
        Map<String, Integer> ages = new HashMap<>();
        ages.put("Alice", 30);
        ages.put("Bob", 25);
        ages.put("Alice", 32); // Overwrites the previous value
        System.out.println(ages); 
    }
}

Output

{Bob=25, Alice=32}

Explaination:

• This code creates a Map using HashMap to store key–value pairs.
• put() adds entries, but if a key already exists ("Alice"), its value is overwritten.
• Maps do not allow duplicate keys but allow duplicate values.
• Finally, printing shows the map, e.g., {Alice=32, Bob=25}.

Step 4: Utility Classes and Methods

Collections Class:

• The Collections class in Java is part of java.util and provides static utility methods for operating on or returning collections.
• It helps perform common operations like sorting, reversing, shuffling, finding minimum/maximum values, and making collections thread-safe or unmodifiable.

Key Methods of Collections:

Step 5: Best Practices of Java Collections

1. Choose the Right Collection

• Select a collection based on your needs (e.g., List for ordered elements, Set for uniqueness, Map for key-value pairs).
• Avoid overusing generic Collection or Object references; be specific for clarity and performance.

2. Consider Performance and Time Complexity

• Use ArrayList for fast random access; LinkedList for frequent insert/delete in the middle.
• Use HashSet/HashMap for O(1) lookups; TreeSet/TreeMap for sorted elements.
• Avoid unnecessary iterations; use built-in methods like contains, addAll, removeIf.

3. Use Generics

• Always use parameterized types to avoid ClassCastException and improve type safety.

4. Handle Nulls Carefully

• Know which collections allow nulls (e.g., HashSet, HashMap) and which don’t (e.g., TreeSet, TreeMap).
• Avoid NullPointerException when performing operations on collections with nulls.

5. Make Collections Thread-Safe When Needed

• Collections like ArrayList and HashMap are not synchronized by default.
• Use Collections.synchronizedList/Set/Map() or concurrent collections (e.g., ConcurrentHashMap) for multi-threaded environments.

6. Avoid Overhead of Legacy Collections

• Avoid Vector and Hashtable; prefer ArrayList and HashMap.
• Use modern concurrent alternatives (ConcurrentHashMap, CopyOnWriteArrayList) instead of synchronized legacy classes.

Step 6: Learning Tips for Java Collections

1. Practice Daily

• Regular practice is key to mastering collections.
• Solve problems on platforms like  LeetCode, HackerRank, Scholarhat Skill test,  focusing specifically on List, Set, Queue, and Map operations.

2. Study Real-World Code

• Explore open-source projects on GitHub to see how Java Collections Framework (JCF) is applied in real applications.
• Observe design patterns, data structures choices, and performance considerations in real scenarios.

3. Use Official Documentation

• Refer to Oracle’s Java Documentation for authoritative guidance on classes and methods.
• Supplement learning with tutorials from Scholarhat for examples and explanations.

4. Build Mini-Projects

• Hands-on experience solidifies concepts.
• Examples include:
• Contact Manager → Use HashMap for storing contacts.
• Shopping Cart → Use ArrayList for items and HashSet for unique coupons.
• Priority Task Manager → Use PriorityQueue to handle tasks by priority.

5. Use Visual Aids

• Visualize collection operations with UML diagrams or IDE debugging to see how data moves internally.
• Tools like VisualVM or IntelliJ’s debugger can show memory usage, references, and insertion order in real-time.

6. Combine with Algorithm Practice

• Many collection problems are algorithm-heavy (searching, sorting, filtering).
• Implement custom algorithms with collections to reinforce understanding of time complexity and internal mechanics.

Conclusion 

The Java Collections Framework (JCF) provides a robust, flexible, and standardized way to store, access, and manipulate groups of objects. By understanding the core interfaces (List, Set, Queue, Map), their implementations, and utility classes like Collections and Arrays, developers can choose the right collection for any scenario.

Only 10% of Java coders become full-stack experts. Don’t limit yourself—Enroll now in our Java Full Stack Course for 50% more opportunities. 

Take our Java skill challenge to evaluate yourself!

In less than 5 minutes, with our skill challenge, you can identify your knowledge gaps and strengths in a given skill.

GET FREE CHALLENGE