Java Collection: Exercises, Practice, Solution

Last update on March 12 2025 13:34:58 (UTC/GMT +8 hours)

This resource offers a total of 630 Java Collection problems for practice. It includes 126 main exercises, each accompanied by solutions, detailed explanations, and four related problems.

Java Collection refers to a framework provided by Java to store and manipulate groups of objects. It offers a set of interfaces (like List, Set, Queue, etc.) and classes (like ArrayList, HashSet, PriorityQueue, etc.) that provide different ways to organize and work with collections of elements. This framework simplifies common operations such as adding, removing, and accessing elements. It offers a wide range of data structures to suit various programming needs.

List of Java Collection Exercises :

ArrayList:

In Java, an ArrayList is a resizable array implementation of the List interface provided by the Java Collections Framework. It's part of the java.util package. Unlike arrays, which have a fixed size, ArrayList can dynamically grow and shrink in size as elements are added or removed. This makes it a more flexible data structure for handling collections of objects.

Key features and characteristics of ArrayList:

Dynamic Size: As mentioned, an ArrayList can dynamically increase or decrease its size based on the number of elements it contains.
Random Access: Elements within an ArrayList can be accessed using their index position. This allows for fast retrieval of elements based on their position in the list.
Generics: ArrayList supports generics, which means it can hold elements of a specified type. This ensures type safety and avoids explicit type casting when retrieving elements from the list.
Iterable: ArrayList implements the Iterable interface, which means it can be easily traversed using iterators or enhanced for loops.
Not Synchronized: This class is roughly equivalent to Vector, except it is unsynchronized.

LinkedList:

In Java, LinkedList is another implementation of the List interface provided by the Java Collections Framework. It's part of the java.util package. Unlike ArrayList, which is backed by an array, LinkedList is implemented as a doubly-linked list.

Key features and characteristics of LinkedList:

Internal Structure: LinkedList utilizes a doubly linked list internally for element storage, facilitating efficient insertion and deletion operations.
Data Manipulation: LinkedList excels in data manipulation tasks due to its efficient insertion and removal operations.
List and Queue Operations: The LinkedList class can serve as both a list and a queue, as it implements both the List and Deque interfaces.
Performance Advantage: Manipulating elements in a LinkedList tends to be faster compared to ArrayList due to its employment of a doubly linked list structure, eliminating the need for bit shifting in memory operations.
Optimal Use Cases: LinkedList is ideal for scenarios where frequent addition and removal of items occurs at the beginning or middle of the list, and where random access to elements is not essential.

HashSet:

In Java, HashSet is an implementation of the Set interface provided by the Java Collections Framework. It's part of the java.util package.

Key features and characteristics of HashSet:

Support for Null Values: HashSet permits null values within its collection.
Hashing Mechanism: Elements in a HashSet are organized and stored based on a hashing mechanism, optimizing insertion, deletion, and search operations.
Non-Synchronized Implementation: HashSet is a non-synchronized class, meaning it's not inherently thread-safe and requires external synchronization for concurrent access.
Hash Code-based Element Management: Elements within a HashSet are inserted and identified using their respective hash codes, facilitating efficient retrieval and manipulation.
Unique Element Constraint: HashSet enforces uniqueness among its elements, ensuring no duplicate elements within the collection.
Primarily Suited for Search Operations: HashSet is particularly advantageous for search operations due to its constant-time complexity for basic operations, such as retrieval and verification of element existence.
Default Capacity and Load Factor: By default, HashSet initializes with a capacity of 16 and a load factor of 0.75, which can be adjusted as needed to optimize performance and memory usage.
Hash Table Data Structure: HashSet utilizes the hash table data structure internally to manage its elements efficiently, supporting rapid access and modification operations.

TreeSet:

In Java, TreeSet is an implementation of the SortedSet interface provided by the Java Collections Framework. It's part of the java.util package.

Here are the key features and characteristics of TreeSet:

Uniqueness of Elements: TreeSet ensures that only unique elements are stored within its collection, discarding duplicate elements.
Absence of Insertion Order Preservation: Unlike certain collection types, such as lists, TreeSet does not maintain the order in which elements are inserted.
Ascending Order Sorting: Elements within a TreeSet are automatically sorted in ascending order according to their natural ordering or a custom comparator.
Lack of Thread Safety: TreeSet is not inherently thread-safe, meaning it does not provide built-in mechanisms to handle concurrent access by multiple threads. External synchronization is required for thread safety.

PriorityQueue:

In Java, PriorityQueue is an implementation of the Queue interface provided by the Java Collections Framework. It's part of the java.util package.

Here are the key features and characteristics of PriorityQueue:

Priority-Based Ordering: Elements are dequeued based on their priority, not in the order they were inserted.
Heap-Based Data Structure: Internally uses a binary heap for efficient element management.
No Guaranteed Order: Order of elements with equal priority is not guaranteed.
Element Priority: Determined by natural ordering or a specified comparator.
Not Synchronized: Requires external synchronization for thread safety.
Performance: Offers logarithmic time complexity for insertion and removal operations.
Usage: Commonly used in algorithms requiring prioritized element processing, like graph algorithms and scheduling.

HashMap:

In Java, HashMap is an implementation of the Map interface provided by the Java Collections Framework. It's part of the java.util package.

Here are the key features and characteristics of HashMap:

Key-Value Storage: Stores data in key-value pairs for efficient retrieval.
Unordered Collection: Does not maintain the order of elements.
Unique Keys: Each key must be unique; duplicate keys are not allowed.
Null Keys and Values: Allows null keys and multiple null values.
Hash Table Structure: Uses a hash table internally for fast retrieval, insertion, and deletion.
Not Synchronized: Not inherently thread-safe; external synchronization is needed for concurrent access.
Good Performance: Offers constant-time performance for most operations.
Iteration Order: The order of iteration is not guaranteed.

TreeMap:

In Java, TreeMap is an implementation of the SortedMap interface provided by the Java Collections Framework. It's part of the java.util package.

Here are the key features and characteristics of TreeMap:

Key-Value Storage: Stores data in key-value pairs for efficient retrieval.
Sorted Collection: Maintains elements in sorted order based on keys.
Balanced Binary Search Tree: Uses a Red-Black Tree internally for efficient organization.
Unique Keys: Each key must be unique; duplicates are prohibited.
Null Keys: Does not allow null keys.
Null Values: Allows null values to be associated with keys.
Not Synchronized: Not inherently thread-safe; external synchronization needed for concurrent access.
Performance: Offers guaranteed logarithmic time complexity for most operations.
Navigable Map Operations: Provides additional operations for navigating elements based on their order.

More to Come !

Do not submit any solution of the above exercises at here, if you want to contribute go to the appropriate exercise page.