100 Java Collection Interview Questions and Answers (Part 1)

Java Collection Interview Questions and Answers walks you through Java collection framework. It provides an architecture to store and manipulate a group of objects. A Java collection framework includes the following: Interfaces, Classes, Algorithm. The Java collection framework provides the developers to access prepackaged data structures as well as algorithms to manipulate data.

1. What is the difference between Collection and Collections Framework in Java?
In Java, a Collection is an object that contains multiple elements of same type in a single unit. These multiple elements can be accessed through one Collection object.
In Java Collections Framework is a library that provides common architecture for creating, updating and accessing different types of collections. In Collections framework there are common methods that are frequently used by developers for working on a Collection object.
2. What are the main benefits of Collections Framework in Java?
Main benefits of Collections Framework in Java are as follows:

  • 1. Reusability: Java Collections Framework provides common classes and utility methods than can be used with different types of collections. This promotes the reusability of the code. A developer does not have to re-invent the wheel by writing the same method again.
  • 2. Quality: Using Java Collection Framework improves the program quality, since the code is already tested and used by thousands of developers.
  • 3. Speed: Most of programmers report that their development speed increased since they can focus on core logic and use the generic collections provided by Java framework.
  • 4. Maintenance: Since most of the Java Collections framework code is open source and API documents is widely available, it is easy to maintain the code written with the help of Java Collections framework. One developer can easily pick the code of previous developer.

3. What is the root interface of Collection hierarchy in Java?
The root interface of Collection hierarchy in Java is Collection interface.
But the Collection interface extends Iterable interface. Due to this some people consider Iterable interface as the root interface.
Iterable interface is present in java.lang package but Collection interface is present in java.util package. Oracle Java API docs mention that Collection interface is a member of the Java Collections framework.
Whereas, Iterable interface is not stated as a part of Java Collections framework in Java docs.
Due to this Collection interface is the root of Collections Framework.
4. What are the main differences between Collection and Collections?
Main differences between Collection and Collections are as follows:

  • 1. Collection is an interface in Java. But Collections is a class in Java.
  • 2. Collection is a base interface. Collections is a utility class in Java.
  • 3. Collection defines methods that are used for data structures that contain the objects. Collections defines the methods that are used for operations like access, find etc. on a Collection.

5. What are the Thread-safe classes in Java Collections framework?
The Thread-safe classes in Java Collections framework are:

  • 1. Stack
  • 2. Properties
  • 3. Vector
  • 4. Hashtable
  • 5. BlockingQueue
  • 6. ConcurrentMap
  • 7. ConcurrentNavigableMap

6. How will you efficiently remove elements while iterating a Collection?
The right way to remove elements from a collection while iterating is by using ListIterator.remove() method.
E.g.

Some developers use following code to remove an element which is incorrect:

By doing so we get ConcurrentModificationException.
An iterator is first created to traverse the list. But at the same time the list is changed by remove() method.
In Java, it is not allowed for a thread to modify a collection while another thread is iterating it. ListIterator provides the capability of removing an object during traversal.
7.How will you convert a List into an array of integers like- int[]?
We can use ArrayUtils class in Apache Commons Lang library.
Sample code is:

If we use List.toArray(), it will convert List to Integer[].
Another option is:

8.How will you convert an array of primitive integers int[] to a List collection?
We can use ArrayUtils in Apache Commons Lang library for this purpose.
Sample code is:

The other option would be to use a for loop and explicitly adding integers to a List.
Sample code is:

9.How will you run a filter on a Collection?
We can use CollectionUtils of Apache for this purpose. We will have to create a Predicate that will define the condition for our filter. Then we can apply this Predicate in filter() method.
Sample code is:
In this example we filter any names that are less than 5 characters long.

We can also use Google Guava library for this.
In Java 8, we can use Predicate to filter a Collection through Stream.
10.How will you convert a List to a Set?
There are two ways to convert a List to a Set in Java.
Option 1: Use HashSet

In this case we put a list into a HashSet. Internally hashCode() method is used to identify duplicate elements.
Option 2: Use TreeSet
In this case we use our own comparator to find duplicate objects.

11. How will you remove duplicate elements from an ArrayList?
The trick in this question is to use a collection that does not allow duplicate elements. So we use a Set for this purpose.
Option 1: Use Set
If ordering of elements is not important then we just put the elements of ArrayList in a HashSet and then add them back to the ArrayList.
Sample Code is:

Option 2: Use LinkedHashSet
If ordering of elements is important then we put the elements of ArrayList in a LinkedHashSet and then add them back to the ArrayList.
Sample Code is:

12.How can you maintain a Collection with elements in Sorted order?
In Java, there are many ways to maintain a Collection with elements in sorted order. Some collections like TreeSet store elements in the natural ordering. In case of natural ordering we have to implement Comparable interface for comparing the elements.
We can also maintain custom ordering by providing a custom Comparator to a Collection.
Another option is to use the utility method Collections.sort() to sort a List. This sorting gives nlog(n) order of performance. But if we have to use this method multiple times then it will be costly on performance.
Another option is to use a PriorityQueue that provides an ordered queue. The main difference between PriorityQueue and Collections.sort() is that PriorityQueue maintains a queue in Order all the time, but we can only retrieve head element from queue. We cannot access the elements of PriorityQueue in Random order.
We can use TreeSet to maintain sorted order of elements in collection if there are no duplicate elements in collection.
13.What are the differences between the two data structures: a Vector and an ArrayList?
An ArrayList is a newer class than a Vector. A Vector is considered a legacy class in Java. The differences are:

  • 1. Synchronization: Vector is synchronized, but the ArrayList is not synchronized. So an ArrayList has faster operations than a Vector.
  • 2. Data Growth: Internally both an ArrayList and Vector use an array to store data. When an ArrayList is almost full it increases its size by 50% of the array size. Whereas a Vector increases it by doubling the underlying array size.

14.What are the differences between Collection and Collections in Java?
Main differences between Collection and Collections are:

  • 1. Type: Collection is an interface in Java. Collections is a class.
  • 2. Features: Collection interface provides basic features of data structure to List, Set and Queue interfaces. Collections is a utility class to sort and synchronize collection elements. It has polymorphic algorithms to operate on collections.
  • 3. Method Type: Most of the methods in Collection are at instance level. Collections class has mainly static methods that can work on an instance of Collection.


15. In which scenario, LinkedList is better than ArrayList in Java?
ArrayList is more popular than LinkedList in Java due to its ease of use and random access to elements feature. But LinkedList is better in the scenario when we do not need random access to elements or there are a lot of insertion, deletion of elements.
16.What are the differences between a List and Set collection in Java?
Main differences between a List and a Set are:

  • 1. Order: List collection is an ordered sequence of elements.
  • A Set is just a distinct collection of elements that is unordered.
  • 2. Positional Access: When we use a List, we can specify where exactly we want to insert an element. In a Set there is no order, so we can insert element anywhere without worrying about order.
  • 3. Duplicate: In a List we can store duplicate elements. A Set can hold only unique elements.


17.What are the differences between a HashSet and TreeSet collection in Java?
Main differences between a HashSet and TreeSet are:

  • 1. Ordering: In a HashSet elements are stored in a random order. In a TreeSet, elements are stored according to natural ordering.
  • 2. Null Value Element: We can store null value object in a HashSet. A TreeSet does not allow to add a null value object.
  • 3. Performance: HashSet performs basic operations like add(), remove(), contains(), size() etc in a constant size time. A TreeSet performs these operations at the order of log(n) time.
  • 4. Speed: A HashSet is better than a TreeSet in performance for most of operations like add(), remove(), contains(), size() etc .
  • 5. Internal Structure: a HashMap in Java internally backs a HashSet. A NavigableMap backs a TreeSet internally.
  • 6. Features: A TreeSet has more features compared to a HashSet. It has methods like pollFirst(), pollLast(), first(), last(), ceiling(), lower() etc.
  • 7. Element Comparison: A HashSet uses equals() method for comparison. A TreeSet uses compareTo() method forcomparison to maintain ordering of elements.

18. In Java, how will you decide when to use a List, Set or a Map collection?

  • 1. If we want a Collection that does not store duplicate values, then we use a Set based collection.
  • 2. If we want to frequently access elements operations based on an index value then we use a List based collection. E.g. ArrayList
  • 3. If we want to maintain the insertion order of elements in a collection then we use a List based collection.
  • 4. For fast search operation based on a key, value pair, we use a HashMap based collection.
  • 5. If we want to maintain the elements in a sorted order, then we use a TreeSet based collection.

19.What are the differences between a HashMap and a Hashtable in Java?
Main differences between a HashMap and a Hashtable are:

  • 1. Synchronization: HashMap is not a synchronized collection. If it is used in multi-thread environment, it may not provide thread safety. A Hashtable is a synchronized collection. Not more than one thread can access a Hashtable at a given moment of time. The thread that works on Hashtable acquires a lock on it and it makes other threads wait till its work is completed.
  • 2. Null values: A HashMap allows only one null key and any number of null values. A Hashtable does not allow null keys and null values.
  • 3. Ordering: A HashMap implementation by LinkedHashMap maintains the insertion order of elements. A TreeMap sorts the mappings based on the ascending order of keys. On the other hand, a Hashtable does not provide guarantee of any kind of order of elements. It does not maintain the mappings of key values in any specific order.
  • 4. Legacy: Hashtable was not the initial part of collection framework in Java. It has been made a collection framework member, after being retrofitted to implement the Map interface. A HashMap implements Map interface and is a part of collection framework since the beginning.
  • 5. Iterator: The Iterator of HashMap is a fail-fast and it throws ConcurrentModificationException if any other Thread modifies the map by inserting or removing any element except iterator’s own remove() method. Enumerator of the Hashtable is not fail-fast.

20.What are the differences between a HashMap and a TreeMap?
Main differences between a HashMap and a TreeMap in Java are:

  • 1. Order: A HashMap does not maintain any order of its keys. In a HashMap there is no guarantee that the element inserted first will be retrieved first.
  • 2. In a TreeMap elements are stored according to natural ordering of elements. A TreeMap uses compareTo() method to store elements in a natural order.
  • 3. Internal Implementation: A HashMap uses Hashing internally. A TreeMap internally uses Red-Black tree implementation.
  • 4. Parent Interfaces: A HashMap implements Map interface. TreeMap implements NavigableMap interface.
  • 5. Null values: A HashMap can store one null key and multiple null values. A TreeMap can not contain null key but it may contain multiple null values.
  • 6. Performance: A HashMap gives constant time performance for operations like get() and put(). A TreeMap gives order of log(n) time performance for get() and put() methods.
  • 7. Comparison: A HashMap uses equals() method to compare keys. A TreeMap uses compareTo() method for maintaining natural ordering.8. Features: A TreeMap has more features than a HashMap. It has methods like pollFirstEntry() , pollLastEntry() , tailMap() , firstKey() , lastKey() etc. that are not provided by a HashMap.

21. What are the differences between Comparable and Comparator?
Main differences between Comparable and Comparator are:

  • 1. Type: Comparable<T> is an interface in Java where T is the type of objects that this object may be compared to.
  • 2. Comparator<T> is also an interface where T is the type of objects that may be compared by this comparator.
  • 3. Sorting: In Comparable, we can only create one sort sequence. In Comparator we can create multiple sort sequences.
  • 4. Method Used: Comparator<T> interface in Java has method public int compare (Object o1, Object o2) that returns a negative integer, zero, or a positive integer when the object o1 is less than, equal to, or greater than the object o2. A Comparable<T> interface has method public int compareTo(Object o) that returns a negative integer, zero, or a positive integer when this object is less than, equal to, or greater than the object o.
  • 5. Objects for Comparison: The Comparator compares two objects given to it as input. Comparable interface compares “this” reference with the object given as input.
  • 6. Package location: Comparable interface in Java is defined in java.lang package. Comparator interface in Java is defined in java.util package.

22.In Java, what is the purpose of Properties file?
A Properties file in Java is a list of key-value pairs that can be parsed by java.util.Properties class. Generally a Properties file has extension .properties e.g. myapp.properties.
Properties files are used for many purposes in all kinds of Java applications. Some of the uses are to store configuration, initial data, application options etc.
When we change the value of a key in a properties file, there is no need to recompile the Java application. So it provides benefit of changing values at runtime.

23.What is the reason for overriding equals() method?
The equals() method in Object class is used to check whether two objects are same or not. If we want a custom implementation we can override this method.
For example, a Person class has first name, last name and age. If we want two Person objects to be equal based on name and age, then we can override equals() method to compare the first name, last name and age of Person objects.
Generally in HashMap implementation, if we want to use an object as key, then we override equals() method.

24.How does hashCode() method work in Java?
Object class in Java has hashCode() method. This method returns a hash code value, which is an integer.
The hashCode() is a native method and its implementation is not pure Java.
Java doesn’t generate hashCode(). However, Object generates a HashCode based on the memory address of the instance of the object.
If two objects are same then their hashCode() is also same.

25.Is it a good idea to use Generics in collections?
Yes. A collection is a group of elements put together in an order or based on a property. Often the type of element can vary. But the properties and behavior of a Collection remains same. Therefore it is good to create a Collection with Generics so that it is type-safe and it can be used with wide variety of elements.

26.What is the difference between Collections.emptyList() and creating new instance of Collection?
In both the approaches, we get an empty list. But Collections.emptyList() returns an Immutable list. We cannot add new elements to an Immutable empty list.
Collections.emptyList() works like Singleton pattern. It does not create a new instance of List. It reuses an existing empty list instance.
Therefore, Collections.emptylist() gives better performance if we need to get an emptyList multiple times.

Continue reading 100 Java Collection Interview Questions and Answers (Part 2)

References

Java Collections Framework

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