Top 30 Collection Programs in Java for Interview

My first priority always will be to listen to my readers and understand what their pain points are. This article is about addressing one of their pain point which is collection programs in java for interview or java collections coding questions.

Where you can expect to get bombarded with such kind of questions? The answer is you can be asked to write java collection programs in written round or in face to face round of interview. Make sure you are well prepared whenever the opportunity arises.

Let me explain one thing before deep diving into the questions, you can solve one question in many ways. For example, if you need to traverse ArrayList then you can traverse through for loop, while loop, advance for loop or iterator. In this scenario, you should know at least one way to traverse the ArrayList. In short, produce the output whatever interviewer demands.

Read Also : Java Collections Interview Questions

I have divided this post into 3 categories :

1. Beginner's level (0-1 year) freshers
2. Intermediate level (2-7 years) experienced
3. Advance level (7 or above) years experienced

Beginner's Level (0-1 year experience) Collection Programs in Java for Interview


Q1. Write a program to traverse (or iterate) ArrayList? (Solution)

As I already mentioned,you can traverse ArrayList using for loop, while loop, advance for loop and iterator. This question tests knowledge of add() method of ArrayList and looping concept. Below example traverses ArrayList using advance for loop:

import java.util.*;
public class ArrayListLoopExample {
    public static void main(String args[]) {
        // initialize ArrayList  
        ArrayList<Integer> al = new ArrayList<Integer>();
        // add elements to ArrayList object
        al.add(3);
        al.add(17);
        al.add(6);
        al.add(9);
        al.add(7);
        System.out.println("Using Advanced For Loop");   
        // printing ArrayList 
        for (Integer num : al) {         
            System.out.println(num);   
        }
    }
}

Q2 Write a program to convert List to Array.  (Solution)

List for e.g ArrayList or LinkedList can be converted to Array using toArray() method.

Q3 Write a program to traverse(or iterate) HashSet? (Solution)

You can traverse the HashSet using iterator or without using iterator as well.

Q4 Given an element write a program to check if element(value) exists in ArrayList?

You can check if element(value) exists in ArrayList using ArrayList contains() method. Example is given below :

import java.util.*;
public class ArrayListContainsExample {
    public static void main(String args[]) {
        // initialize ArrayList  
        ArrayList<Integer> al = new ArrayList<Integer>();
        // add elements to ArrayList object
        al.add(3);
        al.add(17);
        al.add(6);
        al.add(9);
        al.add(7);
        // check if ArrayList contains element
        if (al.contains(7)) {
            System.out.println("7 was found in the list");
        } else {
            System.out.println("7 was not found in the list");
        }
    }
}


Q5 Given an element write a program to check if element exists in HashSet?

You can check if element(value) exists in HashSet using the contains() method. Example is given below :

import java.util.*;
public class HashSetContainsExample {
    public static void main(String args[]) {
        // initialize HashSet  
        HashSet<Integer> hs = new HashSet<Integer>();
        // add elements to HashSet object
        hs.add(3);
        hs.add(17);
        hs.add(6);
        hs.add(9);
        hs.add(7);
        // check if HashSet contains element
        if (hs.contains(7)) {
            System.out.println("7 was found in the list");
        } else {
            System.out.println("7 was not found in the list");
        }
    }
}

Q6 Write a program to initialize a HashMap in java ?

You can initialize a HashMap in java using below code :

        // initialize HashMap  
        HashMap<String,String> hashmap = new HashMap<String,String>();


Q7 Write a program to initialize an ArrayList in java?

        // initialize ArrayList  
        ArrayList<Integer> al = new ArrayList<Integer>();


Q8  Write a program to convert Array to List? (Solution)

Array can be converted to list using Arrays.asList() method.

Q9 Write a program to find the length of the ArrayList? (Solution)

Length of the ArrayList can be found using size() method.

Q10 Write a program to add elements to the HashMap given the key and value data type is String?

    // Declaring a HashMap of String keys and String values
    HashMap<String, String> hashmap = new HashMap<String, String>();
    // Adding key-value pairs to HashMap
    hashmap.put("1", "Value1");
    hashmap.put("2", "Value2");
    hashmap.put("3", "Value3");
    hashmap.put("4", "Value4");
    hashmap.put("5", "Value5");


Q11 Write a program to initialize a HashSet in java?

        // initialize HashSet  
        HashSet<Integer> al = new HashSet<Integer>();


Q12 Write a program to add elements to ArrayList ?

        ArrayList<Integer> al = new ArrayList<Integer>();
        // add elements to ArrayList object
        al.add(3);
        al.add(17);
        al.add(6);
        al.add(9);
        al.add(7);


Q13 Write a program to add elements to HashSet? 

import java.util.*;
public class HashSetAddExample {
    public static void main(String args[]) {
        // initialize HashSet  
        HashSet<Integer> hs = new HashSet<Integer>();
        // add elements to HashSet object
        hs.add(3);
        hs.add(17);
        hs.add(6);
        hs.add(9);
        hs.add(7);
        System.out.println("Using Advanced For Loop");   
        // printing HashSet 
        for (Integer num : hs) {         
            System.out.println(num);   
        }
    }
}


Q14 Write a program to get size of HashMap? (Solution)

You can use the size() method of HashMap class.

Q15  How to check if HashMap is empty? (Solution)

You can check if HashMap is empty using isEmpty() method.

Intermediate level(2-7 year experience) Collection Programs in Java

Q16 Write a program to iterate the HashMap ? (Solution)

There are many ways to iterate the HashMap.The best way to iterate the HashMap is using iterator.

Q17 Write a program to sort HashMap by keys ? (Solution)

You can sort HashMap by keys using TreeMap object.

Q18 Write a program to sort ArrayList using Comparable and Comparator? (Solution)

You can sort ArrayList of Objects using Comparable and Comparator.

Q19 Write a program to sort ArrayList in descending order? (Solution)

You can sort the ArrayList in descending order using Collections.reverseOrder() method.

Q20 Write a program to add element at particular index of ArrayList? (Solution)

You can add element at particular index of ArrayList using method add(int index, Object element).

Q21 Write a program to remove element from specified index of ArrayList? (Solution)

You can remove element at particular index of ArrayList using remove(int index) method.

Q22 Write a program to convert LinkedList to ArrayList? (Solution)

Q23 Write a program to convert HashSet to Array? (Solution)

Q24 Write a program to reverse ArrayList in java? (Solution)

Q25 Write a program to iterate TreeMap in java? (Solution)

Advance Level(7+ years experience) Collection Programs in Java


Q26 Write a program to sort HashMap by value? (Solution)

Q27 How to serialize a HashMap in java? (Solution)

Q28 How to synchronize a HashMap in java? (Solution)

Q29 How to serialize an ArrayList in java? (Solution)

Q30 How to synchronize an ArrayList in java? (Solution)

Please mention in the comments if you have face any collection programs interview questions which are not in the above list.

About The Author

Subham Mittal has worked in Oracle for 3 years .
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