Java 8 Interview Programs

 Find the first non-repeating character in a String using Java 8

javabestinterviewquestion.blogspot.com

import java.util.LinkedHashMap;

import java.util.Map;

import java.util.stream.Collectors;

public class Main {

    public static void main(String[] args) {

        String input = "programming";

        Character result = input.chars()

                .mapToObj(c -> (char) c)

                .collect(Collectors.groupingBy(c -> c, LinkedHashMap::new, Collectors.counting()))

                .entrySet().stream()

                .filter(e -> e.getValue() == 1)

                .map(Map.Entry::getKey)

                .findFirst()

                .orElse(null);

        System.out.println(result);

    }

}

Find duplicate elements in a list using Stream

import java.util.*;

import java.util.stream.Collectors;

public class Main {

    public static void main(String[] args) {

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

        Set<Integer> duplicates = numbers.stream()

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

                .entrySet().stream()

                .filter(e -> e.getValue() > 1)

                .map(Map.Entry::getKey)

                .collect(Collectors.toSet());

        System.out.println(duplicates);

    }

}

3. Sort a list of employees by salary (ascending and descending)

class Employee {

    String name;

    int salary;

    // constructor, getters

}

List<Employee> employees = // initialize list

// Ascending

employees.stream()

    .sorted(Comparator.comparing(Employee::getSalary))

    .forEach(System.out::println);

// Descending

employees.stream()

    .sorted(Comparator.comparing(Employee::getSalary).reversed())

    .forEach(System.out::println);

4. Group employees by department

Map<String, List<Employee>> groupedByDept = employees.stream()

    .collect(Collectors.groupingBy(Employee::getDepartment));

5. Find max salary employee

Employee highestPaid = employees.stream()

    .max(Comparator.comparing(Employee::getSalary))

    .orElse(null);

6. Convert List<String> to List<Integer> (e.g., parsing numbers)

List<String> strNums = Arrays.asList("1", "2", "3");

List<Integer> nums = strNums.stream()

    .map(Integer::parseInt)

    .collect(Collectors.toList());

7. Count frequency of each character in a String

String input = "banana";

Map<Character, Long> freq = input.chars()

    .mapToObj(c -> (char) c)

    .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));

8. Use Optional to avoid null pointer

Optional<String> name = Optional.ofNullable(getName());

System.out.println(name.orElse("Default"));

9. Sum of all even numbers in a list

int sum = numbers.stream()

    .filter(n -> n % 2 == 0)

    .mapToInt(Integer::intValue)

    .sum();

10. Flatten a List of List using flatMap()

List<List<String>> nestedList = Arrays.asList(

    Arrays.asList("A", "B"),

    Arrays.asList("C", "D")

);

List<String> flatList = nestedList.stream()

    .flatMap(Collection::stream)

    .collect(Collectors.toList());

1. Reverse Each Word in a Sentence Input: "Java 8 Stream API" Output: "avaJ 8 maertS IPA" String sentence = "Java 8 Stream API"; String result = Arrays.stream(sentence.split(" ")) .map(word -> new StringBuilder(word).reverse().toString()) .collect(Collectors.joining(" ")); System.out.println(result); 2. Find the Most Repeated Word in a Sentence Input: "apple banana apple orange banana apple" Output: apple String input = "apple banana apple orange banana apple"; String mostRepeated = Arrays.stream(input.split(" ")) .collect(Collectors.groupingBy(Function.identity(), Collectors.counting())) .entrySet().stream() .max(Map.Entry.comparingByValue()) .map(Map.Entry::getKey) .orElse(null); System.out.println(mostRepeated); 3. Find Numbers Starting with '1' Input: [10, 15, 25, 30, 11] Output: [10, 15, 11] List<Integer> numbers = Arrays.asList(10, 15, 25, 30, 11); List<Integer> result = numbers.stream() .map(String::valueOf) .filter(s -> s.startsWith("1")) .map(Integer::valueOf) .collect(Collectors.toList()); System.out.println(result); 4. Sort Map by Value in Descending Order Map<String, Integer> map = Map.of("apple", 3, "banana", 5, "orange", 2); map.entrySet().stream() .sorted(Map.Entry.<String, Integer>comparingByValue().reversed()) .forEach(e -> System.out.println(e.getKey() + ": " + e.getValue())); 5. Remove Duplicate Characters from a String Input: "banana" Output: "ban" String input = "banana"; String result = input.chars() .mapToObj(c -> String.valueOf((char) c)) .distinct() .collect(Collectors.joining()); System.out.println(result); 6. Group Words by Their Length List<String> words = List.of("apple", "bat", "banana", "ant", "cherry"); Map<Integer, List<String>> grouped = words.stream() .collect(Collectors.groupingBy(String::length)); System.out.println(grouped); 7. Sum of Digits of All Numbers in a List Input: [12, 34, 56] Output: 1+2+3+4+5+6 = 21 List<Integer> list = Arrays.asList(12, 34, 56); int sum = list.stream() .map(String::valueOf) .flatMapToInt(str -> str.chars().map(Character::getNumericValue)) .sum(); System.out.println(sum); 8. Convert List of Employees to Map (id -> name) Map<Integer, String> empMap = employees.stream() .collect(Collectors.toMap(Employee::getId, Employee::getName)); 9. Partition Even and Odd Numbers Map<Boolean, List<Integer>> partitioned = numbers.stream() .collect(Collectors.partitioningBy(n -> n % 2 == 0)); System.out.println("Even: " + partitioned.get(true)); System.out.println("Odd: " + partitioned.get(false)); 10. Chained Filter + Map + Collect Input: List of strings, filter non-empty, convert to uppercase, collect** List<String> words = Arrays.asList("java", "", "stream", "api", ""); List<String> result = words.stream() .filter(s -> !s.isEmpty()) .map(String::toUpperCase) .collect(Collectors.toList()); System.out.println(result); // [JAVA, STREAM, API]

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