Java Hub Point: Java 8 - Questions & Answers

In this tutorial, We will discuss top Java 8 coding and programming interview questions and answers using Stream API functions.

1. Given a list of integers, find out all the even numbers that exist in the list using Stream functions?

import java.util.*;
import java.util.stream.*;

public class EvenNumberTest{

public static void main(String args[]) {

List<Integer> list = Arrays.asList(20,15,80,49,25,18,36);
list.stream()
.filter(n -> n%2 == 0)
.forEach(System.out::println);
}
}

Output:

20, 80, 18, 36

2. Given a list of integers, find out all the numbers starting with 1 using Stream functions?

import java.util.*;
import java.util.stream.*;

public class NumberStartingWithOneTest{

public static void main(String args[]) {

List<Integer> myList = Arrays.asList(100,99,35,20,15);
myList.stream()
.map(s -> s + "") // Convert integer to String
.filter(s -> s.startsWith("1"))
.forEach(System.out::println);
}
}

Output:

100, 15

3. How to find duplicate elements in a given integers list in java using Stream functions?

import java.util.*;
import java.util.stream.*;

public class DuplicateElements {

public static void main(String args[]) {

List<Integer> myList = Arrays.asList(20,15,18,25,98,98,32,15);
Set<Integer> set = new HashSet();
myList.stream()
.filter(n -> !set.add(n))
.forEach(System.out::println);
}
}

Output:

98, 15

4. Given the list of integers, find the first element of the list using Stream functions?

import java.util.*;
import java.util.stream.*;

public class FindFirstElement{

public static void main(String args[]) {

List<Integer> myList = Arrays.asList(15,8,98,32,18);
myList.stream()
.findFirst()
.ifPresent(System.out::println);
}
}

Output:

5. Given a list of integers, find the total number of elements present in the list using Stream functions?

import java.util.*;
import java.util.stream.*;

public class FindTheTotalNumberOfElements{

public static void main(String args[]) {

List<Integer> myList = Arrays.asList(10,25,98,15,8,49,98,32);
long count = myList.stream()
.count();
System.out.println(count);
}
}

Output:

6. Given a list of integers, find the maximum value element present in it using Stream functions?

import java.util.*;
import java.util.stream.*;

public class FindMaxElement{

public static void main(String args[]) {

List<Integer> myList = Arrays.asList(25,8,49,25,88,88,32,25);
int max = myList.stream()
.max(Integer::compare)
.get();
System.out.println(max);
}
}

Output:

7. Given a String, find the first non-repeated character in it using Stream functions?

import java.util.*;
import java.util.stream.*;
import java.util.function.Function;

public class FirstNonRepeated{

public static void main(String args[]) {

String input = "Java articles are Awesome";

Character result = input.chars() // Stream of String

// First convert to Character object and then to lowercase
.mapToObj(s -> Character.toLowerCase(Character.valueOf((char) s)))

//Store the chars in map with count
.collect(Collectors.groupingBy(Function.identity(), LinkedHashMap::new, Collectors.counting()))
.entrySet()
.stream()
.filter(entry -> entry.getValue() == 1L)
.map(entry -> entry.getKey())
.findFirst()
.get();
System.out.println(result);
}
}

Output:

8. Given a String, find the first repeated character in it using Stream functions?

import java.util.*;
import java.util.stream.*;
import java.util.function.Function;

public class FirstRepeated{

public static void main(String args[]) {

String input = "Java Articles are Awesome";
Character result = input.chars() // Stream of String
// First convert to Character object and then to lowercase
.mapToObj(s -> Character.toLowerCase(Character.valueOf((char) s)))
//Store the chars in map with count
.collect(Collectors.groupingBy(Function.identity(), LinkedHashMap::new, Collectors.counting()))
.entrySet()
.stream()
.filter(entry -> entry.getValue() > 1L)
.map(entry -> entry.getKey())
.findFirst()
.get();
System.out.println(result);
}
}

Output:

9. Given a list of integers, sort all the values present in it using Stream functions?

import java.util.*;
import java.util.stream.*;
import java.util.function.Function;

public class SortValues{

public static void main(String args[]) {

List<Integer> myList = Arrays.asList(10,15,8,49,25,98,98,32,15);
myList.stream()
.sorted()
.forEach(System.out::println);
}
}

Output:

8
10
15
15
25
32
49
98
98

10. Given a list of integers, sort all the values present in it in descending order using Stream functions?

import java.util.*;

import java.util.stream.*;

import java.util.function.Function;

public class SortDescending{

public static void main(String args[]) {

List<Integer> myList = Arrays.asList(10,15,8,49,25,98,98,32,15);

myList.stream()

.sorted(Collections.reverseOrder())

.forEach(System.out::println);

}

Output:

11. Given an integer array nums, return true if any value appears at least twice in the array, and return false if every element is distinct.

public boolean containsDuplicate(int[] nums) {

List<Integer> list = Arrays.stream(nums)
.boxed()
.collect(Collectors.toList());
Set<Integer> set = new HashSet<>(list);
if(set.size() == list.size()) {
return false;
}
return true;
}

Input: nums = [1,2,3,1]
Output: true

Input: nums = [1,2,3,4]
Output: false

12. How will you get the current date and time using Java 8 Date and Time API?

class Java8 {

public static void main(String[] args) {

//Used LocalDate API to get the date
System.out.println("Current Local Date: " + java.time.LocalDate.now());

//Used LocalTime API to get the time
System.out.println("Current Local Time: " + java.time.LocalTime.now());

//Used LocalDateTime API to get both date and time
System.out.println("Current Local Date and Time: " + java.time.LocalDateTime.now());

}
}

13. Write a Java 8 program to concatenate two Streams?

import java.util.Arrays;
import java.util.List;
import java.util.stream.Stream;

public class Java8 {

public static void main(String[] args) {

List<String> list1 = Arrays.asList("Java", "8");
List<String> list2 = Arrays.asList("explained", "through", "programs");

// Concatenated the list1 and list2 by converting them into Stream
Stream<String> concatStream = Stream.concat(list1.stream(), list2.stream());

// Printed the Concatenated Stream
concatStream.forEach(str -> System.out.print(str + " "));

}
}

14. Java 8 program to perform cube on list elements and filter numbers greater than 50.

import java.util.*;

public class Main {

public static void main(String[] args) {

List<Integer> integerList = Arrays.asList(4,5,6,7,1,2,3);
integerList.stream()
.map(i -> i*i*i)
.filter(i -> i>50)
.forEach(System.out::println);
}
}

Output:

64
125
216
343

15. Write a Java 8 program to sort an array and then convert the sorted array into Stream?

import java.util.Arrays;
public class Java8 {
public static void main(String[] args) {

int arr[] = { 99, 55, 203, 99, 4, 91 };

// Sorted the Array using parallelSort()
Arrays.parallelSort(arr);

/* Converted it into Stream and then print using forEach */
Arrays.stream(arr).forEach(n -> System.out.print(n + " "));

}
}

16. How to use map to convert object into Uppercase in Java 8?

public class Java8 {

public static void main(String[] args) {

List<String> names = Arrays.asList("aa", "bb", "cc", "dd");
List<String> nameLst = names.stream()
.map(String::toUpperCase)
.collect(Collectors.toList());
System.out.println(nameLst);
}
}

output:
AA, BB, CC, DD

17. How to convert a List of objects into a Map by considering duplicated keys and store them in sorted order?

public class TestNotes {

public static void main(String[] args) {

List<Notes> noteLst = new ArrayList<>();
noteLst.add(new Notes(1, "note1", 11));
noteLst.add(new Notes(2, "note2", 22));
noteLst.add(new Notes(3, "note3", 33));
noteLst.add(new Notes(4, "note4", 44));
noteLst.add(new Notes(5, "note5", 55));
noteLst.add(new Notes(6, "note4", 66));

Map<String, Long> notesRecords = noteLst.stream()
// sorting is based on TagId 55,44,33,22,11
.sorted(Comparator.comparingLong(Notes::getTagId).reversed())
// consider old value 44 for dupilcate key and it keeps order
.collect(Collectors.toMap(Notes::getTagName, Notes::getTagId,
(oldValue, newValue) -> oldValue,LinkedHashMap::new));

System.out.println("Notes : " + notesRecords);
}
}

18. How to count each element/word from the String ArrayList in Java8?

public class TestNotes {

public static void main(String[] args) {

List<String> names = Arrays.asList("AA", "BB", "AA", "CC");
Map<String,Long> namesCount = names
.stream()
.collect( Collectors.groupingBy( Function.identity() , Collectors.counting() ));

System.out.println(namesCount);
}
}

Output:

{CC=1, BB=1, AA=2}

19. How to find only duplicate elements with its count from the String ArrayList in Java8?

public class TestNotes {

public static void main(String[] args) {

List<String> names = Arrays.asList("AA", "BB", "AA", "CC");
Map<String,Long> namesCount = names
.stream().filter(x->Collections.frequency(names, x)>1)
.collect(Collectors.groupingBy (Function.identity(), Collectors.counting()));
System.out.println(namesCount);
}
}

Output:

{AA=2}

20. How to check if list is empty in Java 8 using Optional, if not null iterate through the list and print the object?

Optional.ofNullable(noteLst)

// creates empty immutable list: [] in case noteLst is null
.orElseGet(Collections::emptyList)

//loop throgh each object and consider non null objects
.stream().filter(Objects::nonNull)

// method reference, consider only tag name
.map(note -> Notes::getTagName)

// it will print tag names

.forEach(System.out::println);

21. Write a Program to find the Maximum element in an array?

public static int findMaxElement(int[] arr) {
return Arrays.stream(arr).max().getAsInt();
}

Input: 12,19,20,88,00,9

output: 88

22. Write a program to print the count of each character in a String?

public static void findCountOfChars(String s) {

Map<String, Long> map = Arrays.stream(s.split(""))
.map(String::toLowerCase)
.collect(Collectors
.groupingBy(str -> str, LinkedHashMap::new, Collectors.counting()));
}

Input: String s = "string data to count each character";

Output: {s=1, t=5, r=3, i=1, n=2, g=1, =5, d=1, a=5, o=2, c=4, u=1, e=2, h=2}

================================================================

23: Find the second largest number in a list of integers using stream?

Ans: -

List nums = Arrays.asList(5, 7, 3, 9, 11, 2,9);

int result=nums.stream().

distinct().

sorted(Comparator.reverseOrder()).

skip(1).

findFirst().get();

System.out.println(result);

Output - 11

24: Remove the duplicate elements of a given list using stream.

Ans:

List<Integer> nums = Arrays.asList(5, 4, 3, 9, 3, 4);

nums.stream().

distinct().

forEach(System.out::println);

Output - 5,4,3,9

25 : Find the sum of all the elements of a list using stream.

Ans:

List<Integer> nums = Arrays.asList(5, 7, 3, 9, 11, 2);

int result=nums.stream().

mapToInt(Integer:: intValue)

.sum();

System.out.println(result);

26: Find the Max/Min of a List using stream.

Ans:

List<Integer> nums = Arrays.asList(5, 7, 3, 9, 11, 2, 6);

int min=nums.stream().

mapToInt(Integer:: intValue).

min().getAsInt();

System.out.println(min);

int max=nums.stream().

mapToInt(Integer:: intValue).

max().getAsInt();

System.out.println(max);

27: Find the average of a given list of floating-point list using stream.

Ans:

List<Double> nums = Arrays.asList(5.1, 7.2 ,4.2, 3.0, 9.9, 14.7, 11.9, 2.6);

double average= nums.stream().

mapToDouble(Double::doubleValue)

.average().getAsDouble();

System.out.println(average);

28: Write a program to check if all the elements are satisfied with a given condition or not?
Example to check if the whole list has even numbers.

Ans:

Example1 - to check if the whole list has all even numbers.

List<Integer> nums = Arrays.asList(2, 4, 8, 3, 14, 16);

boolean allEven=nums.stream().allMatch(n->n%2==0);

System.out.println(allEven);

Example1 - to check if the whole list has any even number.

List<Integer> nums = Arrays.asList(3, 13, 11);

boolean anyEven=nums.stream().anyMatch(n->n%2==0);

System.out.println(anyEven);

29: Find out all the duplicate numbers on a given list and print them using stream.

Ans:

List<Integer> nums = Arrays.asList(5,0,7, 4, 2, 13, 9, 14, 11, 7, 9, 14);

nums.stream().

filter(n->Collections.frequency(nums, n)>1).

collect(Collectors.toSet()).

forEach(System.out::println);

30: Find the square of the first three even numbers in a given list using stream.

Ans:

List<Integer> nums = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);

nums.stream().

filter(n->n%2==0).

limit(3).

map(i->i*i).

forEach(System.out::println);

31: Concatenate all the string of a given list of string using stream.

Ans:

List<String> fruits = Arrays.asList("Apple", "Banana","Guava","cherry","coconut","Berry");

String concatenatedString=fruits.stream().collect(Collectors.joining("::"));

System.out.println(concatenatedString);

32: Count the number of strings in a given, which starts with a specific character.

Ans:

List<String> fruits = Arrays.asList("Apple", "Banana","Guava","cherry","coconut","Berry");

long count=fruits.stream().filter(e->e.startsWith("B")).count();

System.out.println(count);

32: Write a program to remove duplicate words in a given string using stream

Ans:

String s= "This is java language and java is versatile language";

Arrays.stream(s.split(" ")).distinct().forEach(System.out::println);

33: Write a program to count words frequency in a given string using stream

Ans:

1.Map<String, Long> wordFrequency=Arrays.stream(str.split(" ")).

collect(Collectors.groupingBy(String::toLowerCase, Collectors.counting()));

System.out.println(wordFrequency);

2.String str = "this is demo but because this is my first program";

Arrays.stream(str.split(" "))

.collect(Collectors.groupingBy(a->a,Collectors.counting()))

.forEach((key,value)->{

System.out.println(key+" :: "+value);

});;

34: Given a list of names, group them by their first letter, and then count the number of names in each group.

Ans:

1. String[] names = { "Anand", "Bipin", "Rakesh", "Abhi", "Rohan", "Akash", "Ravi", "Sima" };

Map<Character, Long> result = Arrays.stream(names)

.collect(Collectors.groupingBy(s -> s.charAt(0), Collectors.counting()));

System.out.println(result);

2. List<String> list = Arrays.asList("apple","aakash","arya","yogi","yoga","yogita","hina","himani");

list.stream().collect(Collectors.groupingBy(a->a.charAt(0),Collectors.counting())).forEach((a,b)->{

System.out.print(a+" :: ");

System.out.println(b);

});

35: How do you merge two integer arrays into a single sorted array using stream API

Ans:

1. int[] array1 = { 1, 3, 32, 5, 7 };

int[] array2 = { 2, 4, 6, 62, 8 };

IntStream.concat(Arrays.stream(array1), Arrays.stream(array2)).

sorted().

forEach(System.out::println);

List<Integer> list1 = Arrays.asList(1,2,3,4,5,6);

List<Integer> list2 = Arrays.asList(7,8,9,10,55);

Stream.concat(list1.stream(), list2.stream()))

.forEach(System.out::println);

36: Concatenate two list of strings & remove duplicates using stream API

Ans: 1. List<String> list1 = List.of("apple", "banana", "orange", "pinaaple"); List<String> list2 = List.of("banana", "kiwi", "grape", "guava", "apple"); Stream.concat(list1.stream(), list2.stream()). distinct(). forEach(System.out::println); 2. List<String> list1 = Arrays.asList("apple","hina","banana","didi","didi"); List<String> list2 = Arrays.asList("singh","hina","banana","didi","didi"); List<String> collect = Stream.concat(list1.stream(),list2.stream())

.distinct().collect(Collectors.toList()); System.out.println(collect);

37: Filter all the palindrome words of a list using stream

Ans: String[] str = { "abc", "ada", "kiran", "level", "apple", "narendra", "madam" }; Arrays.stream(str). filter(s -> new StringBuilder(s).reverse().toString().equals(s)) .forEach(e -> System.out.println(e));

38: Partition a list of numbers into two groups: even and odd, using a stream

Ans:

List<Integer> nums = Arrays.asList(1,2,3,4,5,6,7,8,9,10);

Map<Boolean, List<Integer>> result = nums.stream()

.collect(Collectors.partitioningBy(n->n%2==0));

System.out.println(result);

39: Sort a list of custom objects based on a specific attribute for example,
sort a list of employees by employee name using a stream.

Ans:

package com.javahubpoint;

public class Employee {

	Integer empId;
	String empName;
	Long salary;
	Integer deptId;

	public Employee(Integer empId, String empName, Integer deptId) {
		super();
		this.empId = empId;
		this.empName = empName;
		this.deptId = deptId;
	}
	public Employee(Integer empId, String empName) {
		this.empId = empId;
		this.empName =  empName;
	}

	public Employee(Integer empId, String empName, Long salary) {
		super();
		this.empId = empId;
		this.empName = empName;
		this.salary = salary;
	}
	
	public Integer getDeptId() {
		return deptId;
	}

	public void setDeptId(Integer deptId) {
		this.deptId = deptId;
	}

	public Long getSalary() {
		return salary;
	}

	public void setSalary(Long salary) {
		this.salary = salary;
	}

	public Integer getEmpId() {
		return empId;
	}

	public void setEmpId(Integer empId) {
		this.empId = empId;
	}

	public String getEmpName() {
		return empName;
	}

	public void setEmpName(String empName) {
		this.empName = empName;
	}

	@Override
	public String toString() {
		return "Employee [empId=" + empId + ", empName=" + empName + ",
                         salary=" + salary + ", deptId=" + deptId + "]";
	}

}

List<Employee> empList = Arrays.asList(new Employee(101, "Narendra Modi"), new Employee(102, "Kiran Panda"), new Employee(103, "Anand Mishra"), new Employee(104, "Dillip Kumar")); //Sort using empName. Default is ascending order empList.stream().sorted(Comparator.comparing(Employee::getEmpName))

.forEach(e -> System.out.println(e)); //Sort using empName. But in descending order empList.stream().sorted(Comparator.comparing(Employee::getEmpName,

Comparator.reverseOrder())).forEach(e -> System.out.println(e));

40: How to sort Employees by Name and Salary using stream

Ans:

List<Employee> empList = Arrays.asList(new Employee(101, "Narendra Modi", 50000),

new Employee(102, "Kiran Panda", 80000),

new Employee(103, "Anand Mishra", 70000),

new Employee(104, "Kiran Panda", 60000));

//Sort using empName. Default is ascending order

empList.stream().sorted(Comparator.comparing(Employee::getEmpName))

.forEach(e -> System.out.println(e));

System.out.println("==========================================");

// In the above employees are sorted based on name. If we want to order by name and salary then we have to write code like below

empList.stream().sorted(Comparator.comparing(Employee::getEmpName)

.thenComparing(Employee::getSalary))

.forEach(e -> System.out.println(e));

41: How to group Employees by department using stream

Ans:

List<Employee> empList = Arrays.asList(new Employee(101, "Aditya Sahoo", 10),

new Employee(102, "Kiran Panda",20),

new Employee(103, "Anand Mishra", 10),

new Employee(104, "Dillip Kumar",30));

Map<Integer, List<Employee>> result = empList.stream() .collect(Collectors.groupingBy(Employee::getDeptId));

System.out.println(result);

42: Find and print all the employees who belongs to a particular dept using stream

Ans:

List<Employee> empList = Arrays.asList(new Employee(101, "Aditya Sahoo", 10),

new Employee(102, "Kiran Panda",20),

new Employee(103, "Anand Mishra", 10),

new Employee(104, "Dillip Kumar",30));

//suppose dept number is 10

empList.stream().filter(emp -> emp.deptId ==10)

.forEach(System.out::println);

43 : Student Grade Classification - if a student secured 60 and above mark then "PASS" otherwise "FAIL "

Ans:

public class Student {

	String stdName;
	Integer stdMark;
	public String getStdName() {
		return stdName;
	}
	public void setStdName(String stdName) {
		this.stdName = stdName;
	}
	public Integer getStdMark() {
		return stdMark;
	}
	public void setStdMark(Integer stdMark) {
		this.stdMark = stdMark;
	}
	
	@Override
	public String toString() {
		return "Student [stdName=" + stdName + ", 
                        stdMark=" + stdMark + "]";
	}
	public Student(String stdName, Integer stdMark) {
		super();
		this.stdName = stdName;
		this.stdMark = stdMark;
	}
	
}

List<Student> stdList = Arrays.asList(new Student("Aditya Sahoo", 50), 
			    new Student("Kiran Panda",80),
			    new Student("Anand Mishra", 30),
			    new Student("Dillip Kumar",60));
	Map<String, List<Student>> result = stdList.stream()
                .collect(Collectors.groupingBy(std-> 
                            std.stdMark > 60  ? "PASS":"FAIL"));
	
	System.out.println(result);

44: How to sort a Map based on keys or values using stream

Ans:

Map<String, Integer> map = new HashMap<String, Integer>();
	map.put("Dillip", 40);
	map.put("Bikash", 20);
	map.put("Kiran", 30);
	map.put("Aditya", 80);
	
	//sorting a map based on key
	System.out.println("sorting a map based on key");
	map.entrySet().stream()
            .sorted(Map.Entry.comparingByKey())
            .forEach(System.out::println);
	
	
	
	//sorting a map based on Value
	System.out.println("sorting a map based on value");
	map.entrySet().stream()
            .sorted(Map.Entry.comparingByValue())
            .forEach(System.out::println);

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Java 8 - Questions & Answers