Java Streams Interview Questions

By Pradyumna Chippigiri

February 17, 2026


I was preparing for a Java Developer interview, so thought of making this a place to come back and revise when I forget something.

1. Convert list of strings to uppercase, sort them and print them.

JAVA CODE
1import java.util.*;
2 
3List<String> fruits = Arrays.asList("cherry", "mango", "apple", "banana");
4fruits.stream().map(String::toUpperCase).sorted().forEach(f -> System.out.println(f));

2. Get the list of strings where the length of each string is greater than 5.

JAVA CODE
1import java.util.*;
2import java.util.stream.Collectors;
3 
4List<String> fruits = Arrays.asList("cherry", "mango", "apple", "banana", "pineapple");
5List<String> result = fruits.stream()
6              .filter(s -> s.length() > 5)
7              .collect(Collectors.toList());
8 
9System.out.println(result);

3. Remove duplicates from a list of strings.

JAVA CODE
1import java.util.*;
2import java.util.stream.Collectors;
3 
4List<String> fruits = Arrays.asList("cherry", "mango", "mango","apple", "banana", "pineapple", "cherry");
5List<String> result = fruits.stream()
6                          .distinct()
7                          .collect(Collectors.toList());  // remove duplicates
8 
9System.out.println(result);

4. Get the list of strings having a particular word.

JAVA CODE
1import java.util.*;
2import java.util.stream.Collectors;
3 
4List<String> fruits = Arrays.asList("cherry", "mango", "mango","apple", "banana", "pineapple", "cherry");
5List<String> result = fruits.stream()
6                      .filter(s -> s.contains("apple"))
7                      .collect(Collectors.toList());  // get strings containing "apple"
8 
9System.out.println(result);

5. Remove strings starting with 'a'.

JAVA CODE
1import java.util.*;
2import java.util.stream.Collectors;
3 
4List<String> fruits = Arrays.asList("cherry", "mango", "mango","apple", "banana", "pineapple", "cherry");
5List<String> result = fruits.stream()
6                          .filter(s -> !s.startsWith("a"))
7                          .collect(Collectors.toList());
8 
9System.out.println(result);

6.Find first element in the list that starts with 'a'.

findFirst() is a short circuiting terminal operation, it returns an Optional<T> object.

JAVA CODE
1import java.util.*;
2import java.util.stream.Collectors;
3 
4List<String> names = Arrays.asList("pushkar", "praddy", "nachu", "arjit", "mamata");
5Optional<String> res = names.stream()
6                          .filter(s -> s.startsWith("a"))  // find first string starting with "a"
7                          .findFirst();  // find first element
8      
9res.ifPresent(System.out::println);  // prints only if present
10System.out.println(res.orElse("No element found")); // if present, print the element, otherwise print "No element found"

7. Length of each string in the list.

JAVA CODE
1import java.util.*;
2import java.util.stream.Collectors;
3 
4List<String> fruits = Arrays.asList("cherry", "mango", "apple", "banana", "pineapple");
5List<Integer> result = fruits.stream()
6                          .map(String::length)
7                          .collect(Collectors.toList());
8 
9System.out.println(result);

8. Find the longest word in the list

max() is a terminal operation, it returns an Optional<T> object.

JAVA CODE
1import java.util.*;
2import java.util.stream.Collectors;
3 
4List<String> fruits = Arrays.asList("cherry", "mango", "apple", "banana", "pineapple");
5Optional<String> longest = fruits.stream()
6                      .max(Comparator.comparingInt(String::length));
7 
8System.out.println(longest.orElse("No element found"));

9. Sort the list of strings, based on their lengths in ascending

JAVA CODE
1import java.util.*;
2import java.util.stream.Collectors;
3 
4List<String> fruits = Arrays.asList("cherry", "mango", "apple", "banana", "pineapple");
5List<String> result = fruits.stream()
6                          .sorted(Comparator.comparingInt(String::length))
7                          .collect(Collectors.toList());
8 
9System.out.println(result);

10. Sort the list of strings, based on their lengths in descending order.

JAVA CODE
1import java.util.*;
2import java.util.stream.Collectors;
3 
4List<String> fruits = Arrays.asList("cherry", "mango", "apple", "banana", "pineapple");
5List<String> result = fruits.stream()
6                          .sorted(Comparator.comparingInt(String::length).reversed())
7                          .collect(Collectors.toList());
8 
9System.out.println(result);

11. Get the list of strings having vowels.

JAVA CODE
1import java.util.*;
2import java.util.stream.Collectors;
3 
4List<String> fruits = Arrays.asList("cherry", "mango", "apple", "banana", "pineapple");
5List<String> result = fruits.stream()
6                          .filter(s -> s.matches(".*[aeiou].*"))
7                          .collect(Collectors.toList());
8 
9System.out.println(result);

12. Convert a list of strings into a string seperated by commas.

JAVA CODE
1import java.util.*;
2import java.util.stream.Collectors;
3 
4List<String> fruits = Arrays.asList("cherry", "mango", "apple", "banana", "pineapple");
5String result = fruits.stream()
6                      .collect(Collectors.joining(","));
7 
8System.out.println(result);

13. Given a list of strings, group by the first letter of each string.

JAVA CODE
1import java.util.*;
2import java.util.stream.Collectors;
3 
4List<String> names = Arrays.asList("pushkar", "praddy", "nachu", "arjit", "mamata");
5 
6Map<Character, List<String>> result = names.stream()
7                                          .collect(Collectors.groupingBy(s -> s.charAt(0))); 
8 
9System.out.println(result);

14. Remove all strings in the list which are null or empty.

JAVA CODE
1import java.util.*;
2import java.util.stream.Collectors;
3 
4List<String> names = Arrays.asList("pushkar", "praddy", "nachu", "arjit", "mamata");
5List<String> result = names.stream()
6                          .filter(s -> s != null && !s.isEmpty())
7                          .collect(Collectors.toList());
8 
9System.out.println(result);

15. Given a list of strings, group it by the length of the string.

JAVA CODE
1import java.util.*;
2import java.util.stream.Collectors;
3 
4List<String> names = Arrays.asList("pushkar", "praddy", "nachu", "arjit", "mamata");
5Map<Integer, List<String>> result = names.stream()
6                                          .collect(Collectors.groupingBy(String::length));
7 
8System.out.println(result);

16. Given a list of strings, group by the list of strings with vowels and no vowels into 2 lists.

JAVA CODE
1import java.util.*;
2import java.util.function.Predicate;
3import java.util.stream.Collectors;
4 
5Predicate<String> hasVowel = s -> s.toLowerCase().matches(".*[aeiou].*");
6 
7List<String> words = Arrays.asList("sky", "rhythm", "apple", "myth", "banana", "crypt");
8 
9Map<Boolean, List<String>> res = words.stream()
10      .collect(Collectors.partitioningBy(hasVowel));
11 
12List<String> withVowels = res.get(true);
13List<String> noVowels  = res.get(false);
14 
15System.out.println(withVowels);
16System.out.println(noVowels);

17. Given a list of strings, find the average length of the strings.

average() is a terminal operation, it only works on numeric streams.

JAVA CODE
1import java.util.*;
2import java.util.stream.Collectors;
3 
4List<String> names = Arrays.asList("pushkar", "praddy", "nachu", "arjit", "mamata");
5Double average = names.stream()
6                      .mapToInt(String::length)
7                      .average()
8                      .orElse(0.0);
9System.out.println(average);

18.