Top 18 Java collection interview questions (2024)

July 15, 2024

Collection	Collections
1. It is an interface. The interface can also contain abstract and default methods.	1. It is a class
2. It extends the iterable interface.	2. It extends the Object class.
3. Collection has got many interface and classes in it.	3. It has lot of methods that can be used to work on the collection.
4. It doesn’t have any direct implementation as it is an interface. But you can implement the collection interface by using various Java classes, like ArrayList, HashSet, and PriorityQueue.	4. As it is a final class, meaning it can not be subclassed. It doesn’t have any public constructors, so its methods are accessed statically.

ArrayList	LinkedList
1. ArrayList acts as List	1. LinkedList acts as List & Deque.
2. The underlying data-structure of ArrayList is a growable and resizable array.	2. The Underlying data-structure of LinkedList is “Doubly LinkedList” OR “Circular LinkedList”.
3. Memory efficient. It only stores the object in the list.	3. Memory is inefficient. It stores the object and the pointers to next and previous nodes.
4. ArrayList are good for retrieval operations.	4. LinkedList are good for insertion or deletion operations.
5. ArrayList are worst for insertion or deletion operations.	5. LinkedList is the worst for retrieval operations.

Feature	ArrayList	Vector
Synchronization	Not synchronized	Synchronized
Thread Safe?	Not thread-safe	Thread-safe
Performance	Faster due to no synchronization	Slower due to synchronization overhead
Growth Factor	Increases its size by 50%	Doubles its size
Legacy	Part of collection framework	Part of the original version of Java (1.0)
Enumeration	Uses Iterator	Uses Enumeration and Iterator

List	Set
1. List is an Index based data Structure.	1. Set is not an index based data Structure. It stores the data according to hashcode values.
2. List Can Store Duplicate Elements.	2. Set does not allow storing duplicate elements.
3. List can store any number of null values.	3. Set can store only one null value in it.
4. List follows the insertion order.	4. Set does not follow the insertion order.
5. We can iterate (get) the list element by Iterator and ListIterator.	5. We can Iterate the set elements by Iterator only.
6. ArrayList, LinkedList, Vector and Stack are implemented classes of List.	6. HashSet, LinkedHashSet, TreeSet are implemented classes of Set.

Iterator	ListIterator
1. We can get an Iterator cursor by iterator() method. Iterator itr=l.iterator();	1. We can get ListIterator cursor by listIterator() method. ListIterator litr=l.listIterator();
2. Iterator cursor can be used with any Collection object.	2. ListIterator cursor can be used with only List implemented classes. i.e. ArrayList, LinkedList, Vector & Stack.
3. Iterator Methods are: hasNext(), next(), remove().	3. ListIterator Methods are: hasNext(), next(), hasPrevious(), previous(), remove(), set().
4. By using the Iterator cursor, We can retrieve the elements only in forward direction.	4. By using ListIterator cursor, We can retrieve the elements in forward & backward direction.
5. By using the Iterator cursor, We can read & remove the elements.	5.By using ListIterator cursor, We can read, remove, replace & add the elements.

Feature	Array	ArrayList
Size	Fixed size	Dynamic size
Type	Can hold primitives and objects	Can hold only objects
Syntax	int[] arr = new int[10];	ArrayList list = new ArrayList();
Memory Allocation	Allocated at compile-time	Allocated at runtime
Type Safety	Type safe	Type safe with generics
Iterator	No built-in iterator	Supports Iterator and ListIterator
Usage	Used when the size of the array is known and fixed	Used when the size is dynamic and can change

We can add null elements in a HashSet but we cannot add null elements in a TreeSet.

Collection Type	Class	Thread Safe	Synchronized
List
	ArrayList	No	No
	Vector	Yes	Yes
	LinkedList	No	No
	Stack	Yes	Yes
Set
	HashSet	No	No
	LinkedHashSet	No	No
	TreeSet	No	No
Map
	HashMap	No	No
	Hashtable	Yes	Yes
	LinkedHashMap	No	No
	TreeMap	No	No
	Dictionary	Yes	Yes
	WeakHashMap	No	No
	IdentityHashMap	No	No
	Properties	Yes	Yes
Queue
	LinkedList	No	No
	PriorityQueue	No	No
	ArrayDeque	No	No

Remove(Object)->It Removes the particular object from the Collection.

RemoveAll(Collection)->It removes the entire collection from the given Collection.

Remove Method:

Remove All Method:

Code Snippet:

				
					package automation_batch42;

import java.util.ArrayList;

public class ArrayListProgram {

public static void main(String[] args) {

ArrayList a1=new ArrayList();

a1.add("java");

a1.add("python");

a1.add("c++");

a1.add("c#");

System.out.println("Before Removal-> "+a1);

a1.remove("java");

System.out.println("After Removal-> "+a1);

ArrayList a2=new ArrayList();

a2.addAll(a1);

a2.add("ruby");

System.out.println("Before Removal-> "+a2);

a2.removeAll(a1);//we are removing the entire a1 collection from a2 collection

System.out.println("After Removal-> "+a2);

}

}

Some of the ArrayList Methods are:

add(E e)

addAll(Collection c)

contains(Object o)

containsAll(Collection c)

isEmpty()

remove(Object o)

removeAll(Collection c)

size()

clear()

Code Snippet:

				
					
public class ArrayDemo {

public static void main(String[] args) 

{

ArrayList al=new ArrayList();

al.add(100);

al.add(200);

al.add(300);

System.out.println(al); // output will be [100, 200, 300]

}

}

Code Snippet:

				
					public class ArrayDemo {
	public static void main(String[] args) 
	{	ArrayList al=new ArrayList();
		al.add(100);
		al.add(200);
		System.out.println(al); // output:- [100, 200]
		ArrayList al2=new ArrayList();
		al2.add("aa");
		al2.add("bb");
		System.out.println(al2); // output:- [aa, bb]
		al.addAll(al2);
		System.out.println(al); // output:- [100, 200, aa, bb]
	}
}

Code Snippet:

				
					public class ArrayDemo {

	public static void main(String[] args) 
	{
		ArrayList al=new ArrayList();
		al.add("aa");
		al.add("bb");
		System.out.println(al); // Output: [aa, bb]
		al.clear();
		System.out.println("After Clearing the data "+al); // Output: []
	}

}

Code Snippet:

				
					public class ArrayDemo {

	public static void main(String[] args) 
	{
		ArrayList al=new ArrayList();
		al.add("aa");
		al.add("bb");
		System.out.println(al.contains("aa")); //  Output: true
	}

}

Code Snippet:

				
					public class ArrayDemo {

	public static void main(String[] args) 
	{
		ArrayList al=new ArrayList();
		al.add("aa");
		al.add("bb");
		ArrayList al2=new ArrayList();
		al2.add("aa");
		al2.add("bb");
		System.out.println(al.containsAll(al2)); //Output: true
	}

Code Snippet:

				
					public class ArrayDemo {

	public static void main(String[] args) 
	{
		ArrayList al=new ArrayList();
		al.add("aa");
		al.add("bb");
		System.out.println(al.isEmpty()); // Output: false
	}

}

Code Snippet:

				
					public class ArrayDemo {

	public static void main(String[] args) 
	{
		ArrayList al=new ArrayList();
		al.add("aa");
		al.add("bb");
		System.out.println(al); //output: [aa, bb]
		al.remove("aa");
		System.out.println(al); // Output: [bb]
	}

}

Code Snippet:

				
					public class ArrayDemo {

	public static void main(String[] args) 
	{
		ArrayList al=new ArrayList();
		al.add("aa");
		al.add("bb");
		System.out.println(al);  // Output: [aa, bb]
		ArrayList al2=new ArrayList();
		al2.add("aa");
		al2.add("cc");
		al.removeAll(al2);
		System.out.println(al); // Output: [bb]
	}

}

Code Snippet:

				
					public class ArrayDemo {

	public static void main(String[] args) 
	{
		ArrayList al=new ArrayList();
		al.add("aa");
		al.add("bb");
		System.out.println(al.size());  // Output: 2
		
	}

}

A Java Cursor is an Iterator, which is used to iterate or traverse or retrieve a Collection or Stream object’s elements one by one. There are three cursors in Java.

1) Iterator 2) ListIterator & 3) Enumeration.

Enumeration is the cursor which is used to retrieve Collection objects one by one.
Enumeration was introduced in JDK 1.0 version.
Enumeration cursor can be used only with Legacy Classes. i.e. Vector & Stack.
Enumeration Cursor can be get by elements() method.

Enumeration e = v.elements();

Methods of Enumeration cursor are:-

hasMoreElements(), nextElement()

Enumeration cursor can be used to retrieve the elements only in forward direction.
Enumeration cursor can be used only for read operations.

Code Snippet:

				
					public class Test
{
	public static void main(String[] args)
	{
		Vector v=new Vector();
		v.add(10);
		v.add(“Jyoti”);
		v.add(15.2);
		Enumeration e= v.elements();
		while(e.hasMoreElements())
		{
			System.out.println(e.nextElement());
		}
	}
}

In JDK 1.0 version java provides classes and interfaces in which we can store the data/Objects.

for e.g: Vector, Stack, Hashtable, Properties, Dictionary.

In JDK 1.2 version collection Framework was Introduced.

Legacy Classes:

Some classes i.e. Vector, Stack, Hashtable etc. was introduced in JDK 1.0 version but when collection framework was introduced in JDK 1.2 version these classes were modified or re-engineered so that they can be adjusted in new collection hierarchy, so these older classes are known as Legacy classes.

Code Snippet:

				
					package automation_batch42;

import java.util.ArrayList;
import java.util.Collections;

public class ArrayClassMethods 
{
public static void main(String[] args) {
	ArrayList a1=new 	ArrayList();
	a1.add("java");
	a1.add("python");
	a1.add("ruby");
	a1.add("basic");
	System.out.println(a1);
	Collections.sort(a1);
	System.out.println("After Sorting in Ascending order Result will be-> "+ a1);
	Collections.sort(a1, Collections.reverseOrder());
	System.out.println("After Sorting in descending order result will be-> "+a1);
	
}
}