Interview Coding Questions | Convert List into Map

May 17, 2026

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Interview Coding Questions | Convert List into Map

package org.practice.basicprgms;

import java.util.Arrays;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.TreeMap;
import java.util.function.Function;
import java.util.stream.Collector;
import java.util.stream.Collectors;
import java.util.stream.IntStream;

public class ConvertListIntoMapDemo {

	public static void main(String[] args) {
		List<String> names = Arrays.asList(
		        "Java", "Spring", "Docker", "Kubernetes", "AWS");

		List<Integer> numbers = Arrays.asList(10, 20, 30, 40, 50);
		// 1. Using collect method for String
		Map<String, Integer> map1 =
		names.stream()
			.collect(Collectors.toMap(name -> name, 
									  name -> name.length()));
		System.out.println("1. Using collect method for String : "+ map1);
		
		Map<Integer, Integer> map2 =
			numbers.stream()
					.collect(Collectors.toMap(n -> n, 
											n -> n * n));
		System.out.println("1. Using collect method for Integer : "+ map2);
		
		// 2. Using Function.identity() method
		Map<String, Integer> map3 =
			names.stream()
				.collect(Collectors.toMap(Function.identity(), 
											String::length));
		System.out.println("2. Using Function.identity() method : "+ map3);
		
		Map<Integer, Integer> map4 =
			numbers.stream()
					.collect(Collectors.toMap(Function.identity(), n -> n + n));
		System.out.println("2. Using Function.identity() method : "+ map4);		
		
		// 3. Using IntStream.range and collect() method
		Map<Integer, String> map5 =
			IntStream.rangeClosed(0, names.size()-1)
					.boxed()
					.collect(Collectors.toMap(i -> i, i -> names.get(i)));
		System.out.println("3. Using IntStream.range and collect() method : "+ map5);			
			
		Map<Integer, String> map6 =
				IntStream.range(0, numbers.size())
						.boxed()
						.collect(Collectors.toMap(n -> n, 
												  n -> n % 2 == 0 ? "Even":"Odd"	
												));
		System.out.println("3. Using IntStream.range and collect() method : "+ map6);			
				
		// 4. Using collect and handle old and new Key
		Map<Integer, String> map7 =
			names.stream()
				.collect(Collectors.toMap(String :: length,
										  Function.identity(),
										  (oldvalue, newvalue) -> oldvalue));
		System.out.println("4. Using collect and handle old and new Key : "+ map7);	
				
		// 5. Using LinkedHashMap Insertion order Preservation
		Map<String, Integer> map8 =
			names.stream()
				.collect(Collectors.toMap(Function.identity(), 
											String::length,
											(a,b) -> a,
											LinkedHashMap::new
						));
		System.out.println("5. Using LinkedHashMap Insertion order Preservation: "+ map8);	
				
		// 6. Using TreeMap used for Sorting		
		Map<String, Integer> map9 =
			names.stream()
				.collect(Collectors.toMap(Function.identity(), 
											String::length,
											(a,b) -> a,
											TreeMap ::new
						));
		System.out.println("6. Using TreeMap used for Sorting: "+ map9);	
		
	}

}

output:
Using collect method for String : {Java=4, Docker=6, Spring=6, AWS=3, Kubernetes=10}
Using collect method for Integer : {50=2500, 20=400, 40=1600, 10=100, 30=900}
Using Function.identity() method : {Java=4, Docker=6, Spring=6, AWS=3, Kubernetes=10}
Using Function.identity() method : {50=100, 20=40, 40=80, 10=20, 30=60}
Using IntStream.range and collect() method : {0=Java, 1=Spring, 2=Docker, 3=Kubernetes, 4=AWS}
Using IntStream.range and collect() method : {0=Even, 1=Odd, 2=Even, 3=Odd, 4=Even}
Using collect and handle old and new Key : {3=AWS, 4=Java, 6=Spring, 10=Kubernetes}
Using LinkedHashMap Insertion order Preservation: {Java=4, Spring=6, Docker=6, Kubernetes=10, AWS=3}
Using TreeMap used for Sorting: {AWS=3, Docker=6, Java=4, Kubernetes=10, Spring=6}