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Top 15 Java Collections Interview Questions for 2022

1) What is Collections in Java?

It is a framework which is used to store and manipulate the group of objects.

2) What are the commonly used methods of Collection interface in Java?

Commonly used methods of Collection interface in java are:
  • public boolean add(Object element)
  • public boolean addAll(Collection c)
  • public boolean remove(Object element)
  • public boolean removeAll(Collection c)
  • public boolean retainAll(Collection c)
  • public int size()
  • public void clear()
  • public boolean contains(Object element)
  • public boolean containsAll(Collection c)
  • public Iterator iterator()
  • public Object[] toArray()
  • public boolean isEmpty()
  • public boolean equals(Object element)
  • public int hashCode()

3) What are the commonly used methods of Iterator interface in Java?

Commonly used methods of Iterator interface in java are:
  • public Object next()
  • public void remove()
  • public boolean hasNext()

4) What are the ways to iterate the elements of collection in Java?

Two ways to iterate the elements of collection in java are:
  • By using for-each loop
  • By using Iterator interface

5) What are the types of Constructors of Java ArrayList?

There are three types of Constructors in Java ArrayList are:
  • ArrayList()
  • ArrayList(Collection c)
  • ArrayList(int capacity)

6) What are the types of Constructors of Java LinkedList?

There are two types of Constructors in Java LinkedList are:
  • LinkedList()
  • LinkedList(Collection c)

7) What is the difference between ArrayList and LinkedList in Java?

Difference between ArrayList and LinkedList are:
ArrayList LinkedList
It uses dynamic array to store the elements It uses dynamic array to store the elements It uses doubly linked list to store the elements.
It is better for storing and accessing data. It is better for manipulating data.
Manipulation with ArrayList is slow Manipulation with LinkedList is faster

8) What are the commonly used methods of Java List Interface?

Methods of Java List Interface:
  • object get(int index)
  • object remove(int index)
  • ListIterator listIterator()
  • void add(int index,Object element)
  • object set(int index,Object element)
  • ListIterator listIterator(int index)
  • boolean addAll(int index,Collection c)

9) What are the types of Constructors of Java TreeSet class?

Constructors of Java TreeSet class are:
  • TreeSet()
  • TreeSet(Collection c)
  • TreeSet(SortedSet ss)
  • TreeSet(Comparator comp)

10) What are the types of Constructors of Java TreeMap class?

Constructors of Java TreeMap class are:
  • TreeMap()
  • TreeMap(Map m)
  • TreeMap(SortedMap sm)
  • TreeMap(Comparator comp)

11) Which methods you need to override to use any object as key in HashMap ?

equals() and hashCode() method.

12) What is the difference between HashMap and Hashtable in Java?

Difference between HashMap and Hashtable are:
HashMap Hashtable
It is fast It is slow
It inherits AbstractMap class. It inherits Dictionary class.
Non-synchronized synchronized
Traversed by Iterator Traversed by Enumerator and Iterator

13) What are the types of classes implementing List interface?

There are three types of classes:
  • ArrayList
  • LinkedList
  • Vector

14) Which design pattern Iterator follows?

The design pattern Iterator follows is Iterator design pattern.

15) Which data structure HashSet implements in java?

HashSet internally implements HashMap



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