Java Collection Interview Questions and Answers Archives

Java Collections is a framework that provides several classes and interfaces for storing and manipulating groups of objects. These classes and interfaces are collectively known as the Java Collections Framework. It provides various classes, like ArrayList, HashSet, etc. and interfaces, like List, Set, etc., to handle different types of data.

In an interview, a Java developer may be asked about their knowledge and experience with the Java Collections Framework. They may be asked about the different classes and interfaces, how they are used, and their differences, as well as how they would implement specific collection-related tasks.

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What is the Java Collections Framework and what are some of the main classes and interfaces it provides?

“The Java Collections Framework is a set of classes and interfaces that provide a unified way to store and manipulate groups of objects. Some of the main classes it provides include ArrayList, HashSet, and HashMap. Some of the main interfaces it provides include List, Set, and Map.”

How do you decide which collection class to use for a particular task?

“When deciding which collection class to use for a particular task, I consider factors such as the size of the collection, the type of data it will hold, and the operations that will be performed on it. For example, if I need a collection that will hold a large number of elements and allow fast lookups, I would use a HashMap. If I need a collection that will hold unique elements and maintain their order, I would use a LinkedHashSet.”

Can you explain the difference between an ArrayList and a LinkedList?

“An ArrayList is a dynamic array that provides fast random access to its elements. A LinkedList is a doubly-linked list that provides fast insertion and deletion of elements but slower random access. In general, an ArrayList is better suited for situations where you will be frequently accessing elements by index, whereas a LinkedList is better suited for situations where you will be frequently inserting or deleting elements.”

How do you iterate over the elements in a collection?

“There are several ways to iterate over the elements in a collection, but one of the most common ways is to use the for-each loop. For example, for an ArrayList of Strings, I would use the following code: for (String element : myArrayList) { // do something with element } Another way is to use Iterator, for example, Iterator<String> it = myArrayList.iterator(); while(it.hasNext()){ String element = it.next(); // do something with element }”

How do you sort the elements of a collection?

“To sort the elements of a collection, I would use the Collections.sort() method. This method sorts the elements of a List in ascending order. If the elements of the List implement the Comparable interface, the sort() method uses their natural order. If the elements of the List do not implement the Comparable interface, the sort() method requires a Comparator to be passed as an argument.

For example, Collections.sort(myList);”

How do you remove duplicates from a collection?

“To remove duplicates from a collection, I would use a Set. A Set is a collection that cannot contain duplicate elements. I would add all elements of the original collection to a Set, and then add the elements of the Set back to the original collection.

For example, Set<String> set = new HashSet<>(myList); myList.clear(); myList.addAll(set);”

How do you search for an element in a collection?

“To search for an element in a collection, I would use the contains() method for List and Set collections or the get() method for Map collections.

For example, myList.contains(element); or myMap.get(key);”

How do you perform a deep copy of a collection?

“To perform a deep copy of a collection, I would use the clone() method for the collection, or create a new collection and add all the elements of the original collection to the new collection.

For example, ArrayList<String> copy = (ArrayList<String>) originalList.clone(); or ArrayList<String> copy = new ArrayList<>(originalList);

What is the difference between a HashMap and a Hashtable?

“Both HashMap and Hashtable are implementations of the Map interface, but they have some key differences. HashMap is not thread-safe and allows null values, while Hashtable is thread-safe and does not allow null values. HashMap is generally preferred in a single-threaded environment, while Hashtable is preferred in a multi-threaded environment.

How do you perform a binary search on a collection?

“To perform a binary search on a collection, I would use the Collections.binarySearch() method. This method requires that the collection is sorted and returns the index of the specified element.

For example, Collections.binarySearch(myList, “element”);

How do you use a PriorityQueue in Java?

“A PriorityQueue is a collection that orders its elements based on their natural ordering or based on a provided comparator. It is typically used when elements need to be processed in some priority order.

For example, PriorityQueue<Integer> pq = new PriorityQueue<>(); pq.add(5); pq.add(2); pq.add(3); int smallest = pq.poll();

How do you use a Deque in Java?

“A Deque, short for Double Ended Queue, is a collection that allows elements to be added or removed at both ends. It can be used as a Stack or a Queue, depending on the use case.

For example, Deque<Integer> dq = new ArrayDeque<>(); dq.addFirst(1); dq.addLast(2); int first = dq.removeFirst(); int last = dq.removeLast();

How do you convert a collection to an array in Java?

“To convert a collection to an array in Java, I would use the toArray() method of the collection.

For example, String[] myArray = myList.toArray(new String[0]);

How do you convert an array to a collection in Java?

“To convert an array to a collection in Java, I would use the Arrays.asList() method.

For example, List<String> myList = Arrays.asList(myArray);

How do you use the Enumeration interface in Java?

“The Enumeration interface provides a way to access the elements of a collection one at a time. It is typically used with legacy classes that do not implement the Iterator interface. To use the Enumeration interface, I would call the elements() method on the collection to get an Enumeration object, and then use the hasMoreElements() and nextElement() methods to iterate through the elements of the collection.

For example,

Vector<String> v = new Vector<String>();

Enumeration<String> e = v.elements();

while (e.hasMoreElements()) {

String element = e.nextElement();

// do something with element

}

It’s important to note that Enumeration is considered less functional and less efficient than Iterator, so it’s not recommended to use it in new code.