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Java Coding Interview Questions

Java Coding Interview QuestionsIn 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:

1) Using  Collection.iterator() and Iterator.forEachRemaining()
        map.keySet().iterator()
          .forEachRemaining(System.out::println);
2) Using Collection.stream() and Stream.forEach()
        map.values().stream()
          .forEach(System.out::println);
3) Using Stream.of() and  Collection.toArray() and Stream.forEach()
        Stream.of(map.keySet().toArray())
             .forEach(System.out::println);
4) Using Stream.of() and  Collection.toString() and Stream.forEach()
        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);

How to convert String to Date Object in Java 8 ?

Below code demonstrates the concept. Here we can convert a String to four forms of a Date.
1) String to java.util.Date
2) String to java.time.LocalDate
3) String to java.time.LocalDateTime
4) String to java.time.ZonedDateTime
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);
  }

}

Output

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]

How many ways are there to initialize a Set ?

There are multiple ways to initialize a Set collection. Below is the list of some of them with examples.

1) Using Anonymous Class

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

       {  

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

       }
};

2) Using instance of another Collection

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

3) Using Stream of Java 8

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

4) Using Factory Method of Java 9

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

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

Java 9 introduced the new factory method to create immutable collections very concisely just a one-liner approach. The method name is of(…) for all the three interfaces(List, Set, Map). They have provided static methods for List, Set, and Map interfaces which take the elements as arguments and return an instance of List, Set, and Map, respectively. For example, as we can see in below code snippets, how simple, short, and concise are they!
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");
The above example creates unmodifiable collections with no generic type specified. However, we can also specify a generic type of the Collection returned by of() which will look like below:
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");
The signature of these factory methods are as below :
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

How to retrieve values from Set in Java 8 ?

In order to retrieve values from a Set using Java 8, we can use Stream API of Java 8. Moreover, we must create a Stream from the Set and then iterate the Stream. For example, below code demonstrates the concept.

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); });
       } 
}

However, if you are using JDK 9 and above version, you can even minimize the lines of code.

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);
       } 
}

How to iterate a Map containing a List of String in Java 8 ?

Let’s assume that we are writing a program of Animal Kingdom. For example, suppose that there are three categories of Animals such as Mammals, Birds & Reptiles. We will have list of some Animals in each category. Let’s consider Categories as the keys and list of animals as the values of the Map.
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]

How to count occurrences of each character of a String in Java 8?

For example, let’s assume a string “JAVA PROGRAMMER”. Now, we have to count occurrences of each character in this string including spaces. We are including space also, so that we do this exercise for a sentence as well using this program.
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);
            }
}

Output

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?

Using Java 8 java.time.LocalDate API , we can find next/previous date. We can utilize plusDays() and minusDays() method to get the next day and the previous day, just by adding or subtracting 1 from today as shown below. We can also find the date after or before any number of days accordingly using this program.
private LocalDate getNextDay(LocalDate localdate) {

    return localdate.plusDays(1);
}

private LocalDate getPrevDay(LocalDate localdate) {

    return localdate.minusDays(1);
}
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3 thoughts on “Java Coding Interview Questions

  1. Write a program using Java 8 streams to find second highest Integer in a given array of 11,13,65,76,45,98,123,100,111,44 ?

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