ChallengeForInsideFor.java

package demointerviews.collection;

import java.util.*;

/**
 * You must improve the processing time of the "solution" method
 */

public class ChallengeForInsideFor {

    public static final long NUMBERS_OF_ITEMS = 100000;

    public static void main(String[] args) {

        ChallengeForInsideFor self = new ChallengeForInsideFor();
        List<Customer> customersName = self.getCustomersName();
        List<Customer> customersAge = self.getCustomersAge();

        var start = System.currentTimeMillis();
        List<Customer> solution = new Solution().solution(customersName, customersAge);
        System.out.println("Time of solution method -> " + (System.currentTimeMillis() - start));
        if(solution.stream().anyMatch(it -> "ERROR".equals(it.ref()))) {
            throw new RuntimeException("ERROR, elements didn't match");
        }
    }

    private List<Customer> getCustomersName() {
        var list = new ArrayList<Customer>();
        for (int i = 1; i <= NUMBERS_OF_ITEMS; i++) {
            list.add(
                new Customer(
                        (long) i, UUID.randomUUID().toString().substring(0, 4), null, i+""
                )
            );
        }
        return list;
    }

    private List<Customer> getCustomersAge() {
        var list = new ArrayList<Customer>();
        for (int i = 1; i <= NUMBERS_OF_ITEMS; i++) {
            list.add(
                    new Customer(
                            (long) i, null, new Random().nextInt(18, 82), i+""
                    )
            );
        }
        return list;
    }

}

record Customer(Long id, String name, Integer age, String ref) {}

class Solution {

    public List<Customer> solution(List<Customer> listName, List<Customer> listAge) {
        // TODO ==> You must fix only this method, keep solution method signatue
        List<Customer> customersComplete = new ArrayList<Customer>();
        for(var cName : listName) {
            for (var cAge : listAge) {
                if(Objects.equals(cName.id(), cAge.id())) {
                    customersComplete.add(
                            new Customer(
                                    cName.id(),
                                    cName.name(),
                                    cAge.age(),
                                    cName.ref().equals(cAge.ref()) ? cName.ref() : "ERROR"
                            )
                    );
                }
            }
        }
        return customersComplete;
    }
}