Before attending a Java interview, it's important to practice Stream API coding questions, as the Stream API is one of the most frequently asked topics in Java 8 interviews.
Below are some commonly asked Java 8 Stream API coding questions and their answers..
1) Write a program to print the list of elements from a list using java 8 stream
import java.util.Arrays;
import java.util.List;
public class Java8Stream {
public static void main(String[] args) {
List<String> list = Arrays.asList(new String[] {"ABC", "BCD", "CDE"});
list.stream().forEach(s -> System.out.println(s));
}
}Output:- ABC
BCD
CDE
2) Write a program to convert lowercase string element of a list into uppercase.
import java.util.Arrays;
import java.util.List;
public class Java8Stream {
public static void main(String[] args) {
List<String> list = Arrays.asList(new String[] {"Abc", "bcd", "cde"});
list.stream().map(s-> s.toUpperCase()).forEach(
s -> System.out.println(s));
}
}
Output:- ABC
BCD
CDE
3) Print a list of strings those start with letter "A" and return updated list.
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
public class Java8Stream {
public static void main(String[] args) {
List<String> list = Arrays.asList(new String[] {"Abc", "bcd", "cde"});
List<String> updatedList = list.stream().filter(s-> s.startsWith("A"))
.collect(Collectors.toList());
updatedList.stream().forEach(s -> System.out.println(s));
}
}
Output:- Abc
4) Write a code to calculate the summation of the integers from the list.
import java.util.Arrays;
import java.util.List;
public class Java8Stream {
public static void main(String[] args) {
List<Integer> list = Arrays.asList(new Integer[] {1,2,3, 4,5});
//Using lambda expression
Integer sum = list.stream().mapToInt(i -> i).sum();
System.out.println(sum);
//Using Integer valueOf method
Integer sumOfIntegers = list.stream().mapToInt(Integer::valueOf).sum();
System.out.println(sumOfIntegers);
//Using Integer intValue method
Integer sumOfIntegerValue = list.stream().mapToInt(Integer::intValue).sum();
System.out.println(sumOfIntegerValue);
}
}
Output:- 15
15
15
5) Create a Map object from List using Stream API
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
public class Java8Stream {
public static void main(String[] args) {
List<String> list = Arrays.asList(new String[] {"Ab", "Bcd", "Ce"});
Map<String, Integer> map = list.stream().collect(Collectors.toMap(
s->s, s->s.length()));
map.forEach((x,v) -> System.out.println("key - "+x+", value - "+v));
}
}
Output:-
key - Ab, value - 2
key - Ce, value - 2
key - Bcd, value - 3
6) Write a program to list the distinct integers from list or remove duplicates from the list.
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
public class Java8Stream {
public static void main(String[] args) {
List<Integer> list = Arrays.asList(new Integer[] {1, 2, 1,4});
List<Integer> listWithDistinct = list.stream().distinct().collect(Collectors.toList());
listWithDistinct.stream().forEach(s-> System.out.println(s));
}
}
Output:-1
2
4
7) Write a program to find the Minimum number from a Stream/list.
import java.util.Arrays;
import java.util.List;
public class Java8Stream {
public static void main(String[] args) {
List<Integer> list = Arrays.asList(new Integer[] {1, 2,3,4});
Integer minNumber = list.stream().mapToInt(s->s).min().getAsInt();
System.out.println(minNumber);
}
}
8) Write a program to find the Maximum number from a Stream/list.
import java.util.Arrays;
import java.util.List;
public class Java8Stream {
public static void main(String[] args) {
List<Integer> list = Arrays.asList(new Integer[] {1, 2,3,4});
Integer maxNumber = list.stream().mapToInt(s->s).max().getAsInt();
System.out.println(maxNumber);
}
}
Output:-4
9) Write a program to sort the given list using java 8 stream.
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
public class Java8Stream {
public static void main(String[] args) {
List<Integer> list = Arrays.asList(new Integer[] {8, 2,3,4});
List<Integer> sortedList = list.stream().sorted().collect(Collectors.toList());
sortedList.stream().forEach(s-> System.out.println(s));
}
}
Output:-2
3
4
8
10) Write a program to sort the given list in descending order using stream.
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.stream.Collectors;
public class Java8Stream {
public static void main(String[] args) {
List<Integer> list = Arrays.asList(new Integer[] {8, 2,3,4});
List<Integer> sortedList = list.stream().sorted(
Collections.reverseOrder()).collect(Collectors.toList());
sortedList.stream().forEach(s-> System.out.println(s));
}
}
Output:-8
4
3
2
11) Write a program to count the total number of integers in the list.
import java.util.Arrays;
import java.util.List;
public class Java8Stream {
public static void main(String[] args) {
List<Integer> list = Arrays.asList(new Integer[] {8, 2,3,4});
long count = list.stream().count();
System.out.println(count);
}
}
Output:-4
12) Convert List<List<String>> into List<String> using flatMap method.
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
public class Java8Stream {
public static void main(String[] args) {
List<List<Integer>> listListOfStrings = new ArrayList<List<Integer>>();
List<Integer> list = Arrays.asList(new Integer[] {8, 2,3,4});
listListOfStrings.add(list);
//use flatMap method
List<Integer> newList = listListOfStrings.stream().flatMap(
s-> s.stream()).collect(Collectors.toList());
newList.stream().forEach(s-> System.out.println(s));
}
}
Output:-8
2
3
4
13) Group by a list element and display the total count of the element using Collectors groupingBy method.
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;
public class Java8Stream {
public static void main(String[] args) {
List<String> letters = Arrays.asList("A", "B", "C","A", "D", "C", "D");
Map<String, Long> mapWithLetterCount = letters.stream().collect(
Collectors.groupingBy(
Function.identity(), Collectors.counting()));
System.out.println(mapWithLetterCount);
}
}
Output:- {A=2, B=1, C=2, D=2}
14) Find the first element from the given list using java 8 stream.
import java.util.Arrays;
import java.util.List;
public class Java8Stream {
public static void main(String[] args) {
List<String> letters = Arrays.asList("A", "B", "C","A", "D", "C", "D");
String firstElement = letters.stream().findFirst().orElse("Default");
System.out.println(firstElement);
}
}
Output:-A
15) anyMatch code example - if any one of the list element matches with given character "A" then return true.
import java.util.Arrays;
import java.util.List;
public class Java8Stream {
public static void main(String[] args) {
List<String> letters = Arrays.asList("A", "B", "C", "D");
boolean foundElement = letters.stream().anyMatch(s-> s.equalsIgnoreCase("A"));
System.out.println(foundElement);
}
}
Output:-true
16) allMatch code example - if all of the elements matches with the given element say "A" then return true else false.
import java.util.Arrays;
import java.util.List;
public class Java8Stream {
public static void main(String[] args) {
List<String> letters = Arrays.asList("A", "B", "C", "D");
boolean foundMatches = letters.stream().allMatch(s-> s.equalsIgnoreCase("A"));
System.out.println(foundMatches);
}
}
Output:-false
17) Write a program to remove the duplicates from the list using Collectors toSet method.
import java.util.Arrays;
import java.util.List;
import java.util.Set;
import java.util.stream.Collectors;
public class Java8Stream {
public static void main(String[] args) {
List<String> letters = Arrays.asList("A", "B", "C", "D", "A", "B");
Set<String> nonDuplicateSet = letters.stream().collect(Collectors.toSet());
nonDuplicateSet.stream().forEach(s-> System.out.println(s));
}
}
Output:-A
B
C
D
Thank you for reading the post.
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