Find Missing Number in Java (5 Best Methods) | Java 8 Stream Interview Question

February 22, 2026

✔ Sum Formula Method ✔ IntStream.rangeClosed() ✔ HashSet Optimization ✔ Reduce Method ✔ Frequency Map Technique

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package org.practice.basicprgms;

import java.util.Arrays;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.function.Function;
import java.util.stream.Collectors;
import java.util.stream.IntStream;

public class FindMissingNumber {

	public static void main(String[] args) {
		List<Integer> list = Arrays.asList(1,2,3,4,6,7,8);
		int n = list.size() + 1;
		
		// Solution 1: Using Sum Formula:
		int expectedSum = n * (n + 1)/2;
		int actualSum = list.stream()
							.mapToInt(Integer::intValue)
							.sum();
		
		System.out.println("Missing Number : "+ (expectedSum - actualSum));
		
		// Solution 2: Using List and IntStream Class
		int missingNum = IntStream.rangeClosed(1, n)
									.filter(num -> !list.contains(num))
									.findFirst()
									.orElse(-1);
		
		System.out.println("Missing Numer using List: "+ missingNum);
		
		// Solution 3 Using HashSet
		Set<Integer> set = new HashSet<>(list);
		missingNum = IntStream.rangeClosed(1, n)
							.filter(num -> !set.contains(num))
							.findFirst()
							.orElse(-1);
		
		System.out.println("Missing Numer using Set: "+ missingNum);
		
		// Solution 4 Using Reduce MEthod
		expectedSum = IntStream.rangeClosed(1, n).sum();
		
		actualSum = list.stream()
						.reduce(0, Integer::sum);
		
		System.out.println("Missing Number using reduce method: "+ (expectedSum - actualSum));
			
		// Solution 5 : Using HashMap
		Map<Integer, Long> map = 
			list.stream()
				.collect(Collectors.groupingBy(Function.identity(),
												Collectors.counting()));
		
		missingNum = IntStream.rangeClosed(1, n)
								.filter(num -> !map.containsKey(num))
								.findFirst()
								.orElse(-1);
		
		System.out.println("Missing Numer using HashMap: "+ missingNum);
		
	}

}