50 Essential Java Collections Interview Questions To Ace Your Interview

The Java Collections Framework is a powerful tool for storing and manipulating data. It provides a wide variety of collection classes, such as lists, sets, queues, and maps. Each of these collection classes has its own strengths and weaknesses, so it is important to choose the right collection type for your data. You can also check out some Java online courses to help you excel in the field of programming. This article will provide you with the top 50 Java collections interview questions that you must know to prepare. With these interview questions, we will understand the different types of collections, and some of the best practices to use them.

This Story also Contains

1. Intermediate Java Collections Interview Questions and Answers
2. Explore Java Certification Courses by Top Providers
3. Advanced Java Collections Interview Questions

Java Collections interview questions and answers are divided into three categories – basic, intermediate, and advanced. So, let us get started with the top Java collection framework interview questions with answers.

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Basic Java Collections Interview Questions

Given below are some Java collections interview questions for freshers to prepare.

1. What is the Java Collections Framework?

This is one of the most common Java collections interview questions for freshers. The Java Collections Framework is a set of classes and interfaces that provide a unified way to store and manipulate collections of objects in Java. It includes a wide variety of collection classes, such as lists, sets, queues, and maps, each with its own strengths and weaknesses.

2. What are the basic interfaces of the Java Collections Framework?

This is another one of the most common java collection framework interview questions to consider. The basic interfaces of the Java Collections Framework are collection, list, set, queue, and map.

3. What is the difference between a set and a list?

The main difference between a set and a list is that a set does not allow duplicate elements, while a list does. Additionally, sets are unordered, while lists are ordered. This means that the order in which elements are added to a set is not preserved, while the order in which elements are added to a list is preserved.

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4. What is the difference between an array and an ArrayList?

An array is a fixed-size data structure, while an ArrayList is a variable-size data structure. An array cannot be resized, while an ArrayList can be resized as needed. Additionally, arrays are not synchronised, while ArrayLists are synchronised by default. This means that only one thread can access an array at a time, while multiple threads can be accessed by an ArrayList at a time.

5. What is the difference between a HashMap and a HashSet?

This is amongst the important Java collections interview questions for freshers. A HashMap is a hash table that uses hashing to store elements. A HashSet is a set that uses hashing to store elements. The main difference between a HashMap and a HashSet is that a HashMap can store key-value pairs, while a HashSet can only store unique elements.

6. What is the difference between a priority queue and a queue?

A priority queue is a queue that orders its elements by their priority. A queue is a queue that does not order its elements. This means that the elements of a priority queue are always retrieved in order of priority, while the elements of a queue are retrieved in the order in which they were added. If you preparing for your Java interview, you must consider this kind of java collections interview questions and answers.

7. What is the difference between an iterator and an enumerator?

This one of the best Java collections interview questions and answers must be included in your preparation list. An iterator is an object that can be used to iterate over a collection. An enumerator is an older interface that is no longer recommended for use. The main difference between an iterator and an enumerator is that an iterator is more efficient than an enumerator.

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8. What are some of the best practices for using Java Collections?

Here are some of the best practices for using Java Collections:

• Use the correct collection type for your data. For example, use a list if you need to store a collection of ordered elements, or use a set if you need to store a collection of unique elements.

• Use generics to improve the type safety of your code. This will help to prevent errors when you are iterating over a collection.

• Use synchronisation if you need to access a collection from multiple threads. This will prevent race conditions.

• Use iterators to iterate over a collection. This is more efficient than using an enumerator.

• Use the Collections class to provide common operations on collections. For example, you can use the Collections.sort() method to sort a collection.

9. Why does the Collection interface not extend the Cloneable and Serializable interfaces?

The Collection interface does not extend the Cloneable and Serializable interfaces because it is not possible to clone or serialise a collection without knowing the specific implementation of the collection. For example, if you have a collection of objects that are all implemented by the String class, you can clone or serialise the collection using the Object class's clone() and writeObject() methods. However, if you have a collection of objects that are implemented by a custom class, you will need to use the clone() and writeObject() methods of the custom class.

10. What are the advantages of using generics with Java Collections?

This type of Java collection framework interview questions are frequently asked by the interviewers. Generics can be used to improve the type safety of your code. This will help to prevent errors when you are iterating over a collection. For example, if you have a collection of String objects, you can be sure that the elements of the collection are all of type String. This will prevent you from accidentally trying to add an object of a different type to the collection, which could cause a ClassCastException.

Intermediate Java Collections Interview Questions and Answers

Given below are the intermediate level Java collection framework interview questions to prepare for.

11. What are the new features of the Java Collections Framework in Java 8?

Some of the new features of the Java Collections Framework in Java 8 include:

• Streams: Streams are a new way to iterate over collections. They provide a more functional approach to processing collections of data.

• Collectors: Collectors are a new way to aggregate data from collections. They can be used to create summary statistics, group data, and perform other common operations.

• Optional: It is a new type that represents the possibility of a value. Optional can be used to avoid NullPointerExceptions when working with collections.

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12. Which collection is the most commonly used in Java?

This is one of the frequently asked Java collections interview questions and answers for both freshers and experienced professionals alike. The most commonly used collection in Java is the ArrayList. This is because it is a versatile collection that can be used to store a variety of data types. Additionally, ArrayLists are relatively easy to use and efficient.

13. How do you convert an array to an ArrayList?

This is amongst the important Java collection framework interview questions you should prepare for. You can convert an array to an ArrayList using the Arrays.asList() method. This method takes an array as its argument and returns an ArrayList that contains the elements of the array.

14. How do you convert an ArrayList to an array?

This is another one of the most aksed Java collections interview questions for experienced professionals as well as freshers. You can convert an ArrayList to an array using the toArray() method. This method returns an array that contains the elements of the ArrayList.

15. How do you make an ArrayList read-only?

This is one of the common Java collections interview questions and answers you must practice. You can make an ArrayList read-only by calling the setReadOnly() method. This method prevents the elements of the ArrayList from being modified.

16. How do you synchronise an ArrayList?

You can synchronise an ArrayList by calling the synchronizedList() method. This method returns a synchronised version of the ArrayList. This means that only one thread can access the ArrayList at a time.

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17. How do you iterate over a collection?

This is one of the most asked Java collections interview questions that you can include in your Java interview preparation. You can iterate over a collection using a for-each loop or an iterator. A for-each loop is a simple way to iterate over a collection, but it does not give you as much control as an iterator. An iterator is a more powerful tool that allows you to access the elements of a collection in a more controlled way.

18. What is the purpose of the Comparable interface?

With this one of the Java collection framework interview questions, the interviewer tests your understanding of the Java framework. The Comparable interface is used to define the natural ordering of objects. It provides a single method, compareTo(), which allows objects to be compared and sorted based on their natural order.

19. What is the purpose of the Comparator interface?

The Comparator interface is used to define custom sorting orders for objects. It provides a compare() method that allows you to specify how two objects should be compared and ordered.

20. What is the difference between fail-fast and fail-safe iterators?

This is one of the important Java collections interview questions for experienced professionals as well as freshers. Fail-fast iterators immediately throw a ConcurrentModificationException if a collection is modified while being iterated. Fail-safe iterators, on the other hand, do not throw exceptions and instead work on a snapshot of the collection taken at the beginning of iteration.

21. How does a LinkedHashMap differ from a HashMap?

This one of the Java collections interview questions for freshers as well as experienced professionals must include in your preparation. A LinkedHashMap is similar to a HashMap but maintains the order of elements based on their insertion order. In a LinkedHashMap, elements are stored in the order they were added, allowing for predictable iteration.

22. What is a WeakHashMap used for?

Through this Java collection framework interview questions, an interviewer will try to test your understanding of WeakHashMap. A WeakHashMap is a special type of map where the keys are weak references. This allows the keys to be garbage collected if there are no strong references to them, making it useful for scenarios where the keys are associated with objects that can be released when not in use.

23. How can you sort elements in a List in reverse order?

You can use the Collections.reverse() method to reverse the order of elements in a List. This method modifies the original List in place and is an easy way to achieve reverse sorting. You must include this one of the Java collection framework interview questions in your preparation list.

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24. What is the purpose of the java.util.concurrent package?

This is one of the must know Java collections interview questions and answers. The java.util.concurrent package provides classes and interfaces for creating thread-safe concurrent applications. It includes collections, locks, and other utilities designed to handle multi-threaded scenarios efficiently.

25. What is the purpose of the CopyOnWriteArrayList class?

This type of Java collections interview questions for freshers as well as experienced professionals must be in your preparation list. The CopyOnWriteArrayList is a thread-safe variant of the ArrayList. It ensures thread safety by creating a new copy of the internal array whenever an element is added, modified, or removed. This makes it suitable for scenarios with a high number of reads and fewer writes.

26. What is the difference between HashSet and LinkedHashSet?

This one of the Java collection framework interview questions will test your understanding of Java tools. A HashSet is an unordered collection that does not allow duplicate elements. A LinkedHashSet, on the other hand, maintains the order of insertion and prevents duplicates. It is implemented as a hash table with a linked list running through it to maintain insertion order.

27. What is the purpose of the IdentityHashMap class?

The IdentityHashMap class uses reference equality (rather than the equals() method) to determine key equality. This means that two keys are considered equal only if they reference the same memory location, making it suitable for scenarios where object identity matters.

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28. How do you search for an element in a collection?

This is another one of the best Java collections interview questions. You can search for an element in a collection using the contains() method. This method returns True if the element is found in the collection, and False otherwise.

29. How do you sort a collection?

You can sort a collection using the Collections.sort() method. This method takes a collection as its argument and sorts the elements of the collection in ascending order. You can also specify a custom comparator to sort the elements in a different order.

30. How do you compare the two collections?

This is one of the important Java collections interview questions for freshers and professionals alike. You can compare two collections using the equals() method. This method returns True if the two collections have the same elements, and False otherwise. You can also use the Collections.compare() method to compare two collections in a more specific way.

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Advanced Java Collections Interview Questions

Given below are the advanced-level Java collections interview questions for experienced professionals.

31. How do you merge two collections?

This is one of the top Java collections interview questions for experienced professionals. You can merge two collections using the Collections.merge() method. This method takes two collections as its arguments and merges them into a new collection. The new collection will contain all of the elements from the first collection, followed by all of the elements from the second collection.

32. How do you clone a collection?

This is another one of the important Java collections interview questions for experienced candidates. You can clone a collection using the clone() method. This method returns a shallow copy of the collection. A shallow copy is a copy of the collection that does not contain the actual elements of the collection. Instead, the shallow copy contains references to the original elements.

33. How do you serialise a collection?

This amongst Java collections interview questions for experienced professionals must include in your preparation. You can serialise a collection using the ObjectOutputStream class. This class can be used to write objects to a stream. The ObjectOutputStream class will serialise the collection and all of its elements.

34. How do you deserialize a collection?

You can deserialize a collection using the ObjectInputStream class. This class can be used to read objects from a stream. The ObjectInputStream class will deserialize the collection and return it to you.

35. What is the difference between a linked list and a stack?

This type of Java collections interview questions and answers are important to remember. A linked list is a collection that stores its elements in a linked list data structure. A stack is a collection that stores its elements in a stack data structure. The main difference between a linked list and a stack is that a linked list is a linear data structure, while a stack is a hierarchical data structure. This means that the elements of a linked list are stored in a sequence, while the elements of a stack are stored in a last-in, first-out (LIFO) order.

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36. What is the difference between a queue and a deque?

A queue is a collection that stores its elements in a queue data structure. A deque is a collection that stores its elements in a double-ended queue data structure. The main difference between a queue and a deque is that a queue can only be accessed from the front, while a deque can be accessed from both the front and the back.

37. What is the difference between a tree map and a hash map?

A tree map is a map that stores its elements in a tree data structure. A hash map is a map that stores its elements in a hash table data structure. The main difference between a tree map and a hash map is that a tree map is sorted, while a hash map is not sorted. This means that the elements of a tree map can be accessed in sorted order, while the elements of a hash map can only be accessed by their key.

38. What is the difference between a concurrent hash map and a synchronised hash map?

You must prepare for this one of the Java collections interview questions and answers. A concurrent hash map is a hash map that can be accessed by multiple threads concurrently. A synchronised hash map is a hash map that is synchronised, which means that only one thread can access it at a time. The main difference between a concurrent hash map and a synchronised hash map is that a concurrent hash map is more efficient for multi-threaded applications.

39. What is the difference between a properties file and a configuration file?

A properties file is a text file that contains key-value pairs. A configuration file is a file that contains instructions for how to configure a program. The main difference between a properties file and a configuration file is that a properties file is typically used to store simple configuration settings, while a configuration file is basically used to store more complex configuration settings. You must prepare for this Java collections interview questions for freshers as well as experienced individuals for you next interview.

40. What are some of the common errors that Java developers make when using collections?

This is one of the frequently asked Java collections interview questions for experienced professionals. Some of the most common errors that Java developers make when using collections include:

• Using the wrong collection type for the data.

• Not using generics to improve type safety.

• Not using synchronisation when accessing a collection from multiple threads.

• Not using iterators to iterate over a collection.

• Not using the Collections class to provide common operations on collections.

41. How does the ConcurrentHashMap differ from a synchronised HashMap?

The ConcurrentHashMap is designed for concurrent access and is more efficient in multi-threaded scenarios compared to a synchronised HashMap. It divides the map into segments, allowing multiple threads to access different segments concurrently without blocking each other.

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42. What is the purpose of the EnumSet class?

The EnumSet class is used to represent a set of elements where the elements are values of an enum type. It is highly optimised for enum types and provides efficient memory usage and performance. This is one of the essential Java collections interview questions to prepare.

43. How do you create an immutable collection in Java?

To create an immutable collection, you can use the ImmutableList builder to add more elements using the addAll() method and call the build() method to create an immutable collection.

44. How can you sort a collection using custom sorting criteria?

You can use the overloaded Collections.sort() method that takes a Comparator as a second argument. This allows you to define a custom sorting order for the elements in the collection.

45. What is the purpose of the BitSet class?

This type of Java collection framework interview questions are asked frequently in interviews. The BitSet class is used to represent a collection of bits, where each bit can be set (1) or cleared (0). It provides efficient memory usage and operations for working with individual bits.

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46. What is the difference between ListIterator and Iterator?

ListIterator is a specialised iterator for lists that allows bidirectional iteration and provides methods for adding, modifying, and removing elements during iteration. The regular Iterator only supports forward iteration and does not provide modification methods.

47. How can you efficiently search for an element in a sorted collection?

You can use the Collections.binarySearch() method to efficiently search for an element in a sorted List. This method utilises binary search to find the index of the element, or a negative value if the element is not found.

48. What is the purpose of the NavigableMap interface?

This is another one of the best Java collections interview questions for experienced professionals. The NavigableMap interface extends the Map interface and provides methods for navigating through the map in a specific order. It is implemented by classes like TreeMap and allows operations like finding the next or previous key.

49. What is the purpose of the Spliterator interface?

This is amongst the top Java collections interview questions for freshers as well as experienced candidates. The Spliterator (split-iterator) interface is used to traverse and partition the elements of a collection for parallel processing. It provides methods to split the data into sub-spliterators and perform actions concurrently.

50. How can you create an unmodifiable view of a collection in Java?

This is one of the essential Java collections interview questions for freshers and experienced alike you must practice. You can use the Collections.unmodifiableCollection() method to create an unmodifiable view of a collection. Any attempt to modify the view will result in an UnsupportedOperationException.

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Conclusion
The Java Collections Framework is a tool used for storing and manipulating data. By choosing the right collection type for your data, you can improve the performance and efficiency of your code. With these Java collections interview questions and answers, you cannot only improve your understanding but also add further knowledge to your understanding of the Java collections framework.