In this article, We will discuss about Java Coding Interview Questions and their Answers. Moreover, we will try to provide multiple approaches to solve a coding problem. Additionally, related concepts to a particular coding problem will also be discussed. Our primary focus of this article is to provide all kinds of Java Coding Interview Questions with their Answers. However, for theoretical questions & answers of Java Interview, kindly visit our another article ‘Java Interview Questions‘.

How to iterate and modify value in Java 8 in a Map?

Here is the program to update & iterate a Map in Java 8.

public class MapUpdateTest {

       public static void main(String[] args) {

              Map<String,String> map = new HashMap<String,String>(); 
              map.put("Effective Java", "Kathy Sierra");
              map.put("Spring in Action", "Craig Walls");
              map.put("Hibernate in Action", "Gavin King");
              map.put("Pro Angular", "Freeman");
              map.put("Pro Spring Boot", "Felipe Gutierrez");

      //Only modify if key already exists in the map

              map.computeIfPresent("Effective Java", (key, value) -> "Joshua Bloch");

      //Only modify if key doesn't exist in the map

              map.computeIfAbsent("Core Java", (value) -> "Kathy Sierra");

      //iterate and print the values
              map.entrySet().iterator()
               .forEachRemaining(System.out::println);

      }
}

Output

Below is the output after executing the program.

Hibernate in Action=Gavin King
Core Java=Kathy Sierra
Pro Angular=Freeman
Effective Java=Joshua Bloch
Pro Spring Boot=Felipe Gutierrez
Spring in Action=Craig Walls

How to print keys & values of a Map in Java 8 ?

As we should be aware that keySet() method returns all the keys contained in a Map as a set. The values() method returns all the values contained in a Map as a set. Hence, we should use keySet() to print all keys present in the map and values() to print all values. There are multiple ways to do that:

        map.keySet().iterator()
          .forEachRemaining(System.out::println);

        map.values().stream()
          .forEach(System.out::println);

        Stream.of(map.keySet().toArray())
             .forEach(System.out::println);

        Stream.of(map.values().toString())
             .forEach(System.out::println);

How many ways are there to print keys & values of a Map ?

There are multiple ways to print keys & values of a Map. Below is the list and examples of each approach.

1) Using Iterator
2) Using For-each loop
3) Using Java 8 – Collection.iterator() and Iterator.forEachRemaining()
4) Using Java 8 – Collection.stream() and Stream.forEach()
5) Using Java 8 – Stream.of() and  Collection.toArray() and Stream.forEach()
6) Using Java 8 – Stream.of() and  Collection.toString() and Stream.forEach()

        // 1. Using an iterator
        Iterator<Integer> itr = map.entrySet().iterator();
        while (itr.hasNext()) {
            System.out.println(itr.next());
        }

        // Using For-each loop
        for (Integer key: map.entrySet()) {
            System.out.println(key);
        }

        // Using Java 8 – Collection.iterator() and Iterator.forEachRemaining()
        map.entrySet().iterator()
          .forEachRemaining(System.out::println);

        
               // Using Java 8 – Collection.stream() and Stream.forEach()
        map.entrySet().stream()
          .forEach(System.out::println);

        
                // Using Java 8 – Stream.of() + Collection.toArray() + Stream.forEach()
        Stream.of(map.entrySet().toArray())
             .forEach(System.out::println);

        
                // Using Java 8 - Stream.of() and  Collection.toString() and Stream.forEach()
        Stream.of(map.entrySet().toString())
             .forEach(System.out::println);

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.time.LocalDate;
import java.time.LocalDateTime;
import java.time.ZonedDateTime;
import java.time.format.DateTimeFormatter;
import java.util.Date;
import java.util.Locale;

public class StringToDate {

   public static void main(String[] args) throws ParseException {

      //String to a java.util.Date
      SimpleDateFormat dateFormatter = new SimpleDateFormat("dd-MMM-yyyy hh:mm:ss a", Locale.ENGLISH);
      String dateInString = "24-May-2021 9:45:30 AM";
      Date date = dateFormatter.parse(dateInString);
      System.out.println(date);

      //String to a java.time.LocalDate
      LocalDate localDate = LocalDate.parse("2021-05-24");
      System.out.println(localDate);

      //String to a java.time.LocalDateTime
      LocalDateTime localDateTime = LocalDateTime.parse("2021-05-24T21:45:30");
      System.out.println(localDateTime);

      //String to a java.time.ZonedDateTime
      DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss z");
      ZonedDateTime zonedDateTime = ZonedDateTime.parse("2021-05-24 21:45:30 America/New_York", formatter);
      System.out.println(zonedDateTime);
  }

}

Mon May 24 09:45:30 IST 2021
2021-05-24
2021-05-24T21:45:30
2021-05-24T21:45:30-04:00[America/New_York]

Set<String> set = new HashSet<String>() {

       {  

        add("California"); 
        add("Chicago");
        add("New York"); 

       }
};

Set<String> set = new HashSet<>(Arrays.asList("California", "Chicago", "New York"));

Set<String> set = Stream.of("California", "Chicago", "New York")
    .collect(Collectors.toCollection(HashSet::new));

Set<String> set = Set.of("California", "Chicago", "New York");

How to create various collections using Factory Method of Java 9 ?

List<String> list = List.of("USA", "Canada", "Russia"); 

Set<String> set = Set.of("USA", "Canada", "Russia");

Map<String, String> map = Map.of("USA", "Washington, D.C.", "Canada", "Ottawa", "Russia", "Moscow");

List<String> list = List.<String>of("USA", "Canada", "Russia"); 
Set<String> set = Set.<String>of("USA", "Canada", "Russia"); 
Map<String, String> map = Map.<String, String>of("USA", "Washington, D.C.", "Canada", "Ottawa", "Russia", "Moscow");

static <E> List<E> of(E e1, E e2, E e3) 

static <E> Set<E> of(E e1, E e2, E e3)

static <K,V> Map<K,V> of(K k1, V v1, K k2, V v2, K k3, V v3)   // K=Key, V=Value

public class SetValuesRetrieval {

       public static void main(String[] args) {

       // Creating a Set
          Set<String> set = new HashSet<>();
          set.add("California");
          set.add("Chicago");
          set.add("New York");

      // Retrieving values of the Set
          Stream<String> stream = set.stream();
          stream.forEach((element) -> { System.out.println(element); });
       } 
}

public class SetValuesRetrieval {

       public static void main(String[] args) {

       //Creating a Set
          Set<String> set = Set.of("California","Chicago","New York");

       //Retrieving values of the Set
          set.stream().forEach(System.out::println);
       } 
}

public class IterateMapOfList {

      public static void main(String[] args) {

          // Create HashMap of category and list of animals under the category
          Map<String, List<String>> listOfAnimals = new HashMap<String, List<String>>();

          // List #1: Creating list of Animals in Mammals Category
          List<String> listOfMammals = Arrays.asList("Cat", "Dog", "Monkey", "Cow"); 
          //Adding listOfMammals into Mammal's Category
          listOfAnimals.put("Mammals", listOfMammals);

          // List #2: Creating list of Animals in Birds Category
          List<String> listOfBirds = Arrays.asList("Crow", "Parrot", "Peacock", "Flamingo"); 
          //Adding listOfMammals into Bird's Category
          listOfAnimals.put("Birds", listOfBirds);

          // List #3: Creating list of Animals in Reptiles Category
          List<String> listOfReptiles = Arrays.asList("Lizard", "Turtle", "Crocodile", "Python"); 
          //Adding listOfMammals into Reptile's Category
          listOfAnimals.put("Reptiles", listOfReptiles);

          // Iterating Map using forEach() in Java 8
          listOfAnimals.forEach(
               (key, value)->System.out.println(
                      "Category name : " + key + "\t\t"
                      + "List of Animals under the Category : " + value));
      }
}

Output 

Category name : Reptiles                      List of Animals under the Category : [Lizard, Turtle, Crocodile, Python]
Category name : Birds                           List of Animals under the Category : [Crow, Parrot, Peacock, Flamingo]
Category name : Mammals                   List of Animals under the Category : [Cat, Dog, Monkey, Cow]

public class CharactersCountTest {

       public static void main(String[] args) {

            String someString = "JAVA PROGRAMMER";
            char[] strArray = someString.toCharArray();
            
            //getting distinct characters in strArray
            Set<Character> set = new TreeSet<>();
            for (char c : strArray){
              set.add(c);
            }
            //set.forEach(System.out::println);

            for (char c : set) {
               // Using Streams & Lambda Expressions in Java 8
               long count = someString.chars().filter(ch -> ch == c).count();
               System.out.println("Occurances of Character '" +c+ "' : " +count);
            }
}

Occurrences of Character ' ' : 1
Occurrences of Character 'A' : 3
Occurrences of Character 'E' : 1
Occurrences of Character 'G' : 1
Occurrences of Character 'J' : 1
Occurrences of Character 'M' : 2
Occurrences of Character 'O' : 1
Occurrences of Character 'P' : 1
Occurrences of Character 'R' : 3
Occurrences of Character 'V' : 1

How to find next/previous(tomorrow/yesterday) date in Java 8?

private LocalDate getNextDay(LocalDate localdate) {

    return localdate.plusDays(1);
}

private LocalDate getPrevDay(LocalDate localdate) {

    return localdate.minusDays(1);
}

How to remove all duplicates from an array of integers in Java using Java 8?

Below is the program:

import java.util.Arrays;

public class RemoveDuplicates {
    public static void main(String[] args) {
       Integer[] array = {5, 10, 3, 7, 2, 10, 5};
       Integer[] distinct = Arrays.stream(array)
                                  .distinct()
                                  .toArray(Integer[]::new);
       System.out.println("Distinct elements: " + Arrays.toString(distinct));
    }
}

Distinct elements: [5, 10, 3, 7, 2]

Write a program to count the number of occurrences of a given word in a list of strings using Java 8?

Here is the program:

import java.util.Arrays;
import java.util.List;

public class WordCount {
    public static void main(String[] args) {
       List<String> strings = Arrays.asList("java scala ruby", "java react spring java");
       String word = "java";
       long count = strings.stream()
                           .flatMap(s -> Arrays.stream(s.split(" ")))
                           .filter(w -> w.equals(word))
                           .count();
       System.out.println("Occurrences of \"" + word + "\": " + count);
    }
}

Occurrences of "java": 3

How to filter an array of strings by a given prefix using Java 8 Stream?

import java.util.Arrays;

public class StringPrefixFilter {
    public static void main(String[] args) {
       String[] strings = {"java", "scala", "javascript", "ruby","spring","angular"};
       String prefix = "j";
       String[] filtered = Arrays.stream(strings)
                                 .filter(s -> s.startsWith(prefix))
                                 .toArray(String[]::new);
       System.out.println("Filtered strings: " + Arrays.toString(filtered));
    }
}

Filtered strings: [java, javascript]