how can I compare the two lists with/without iterating over each list

Question

How can I compare the above two lists with/without iterating over each list?I would like to compare the people1 list without iterating against people list. when matching occurs it has print the matched object else it has to print non matching object. I have to do this without iterating second list.

import java.util.*;

public class Person implements Comparable<Person> {
    String name;
    int age;
    String mail;

    public String getMail() {
        return mail;
    }

    public void setMail(String mail) {
        this.mail = mail;
    }

    public Person(String name, int age, String mail) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }

    public String toString() {
        return name + " : " + age;
    }

    public int compareTo(Person p) {
        return getMail().compareTo(p.getMail());
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj) {
            return true;
        }

        if (obj == null) {
            return false;
        }

        if (getClass() != obj.getClass()) {
            return false;
        }

        return false;
    }

    public static void main(String[] args) {
        List<Person> people = new ArrayList<Person>();

        people.add(new Person("Homer", 38, "shankar@gmail.com"));
        people.add(new Person("Marge", 35, "shankar6@gmail.com"));
        people.add(new Person("Bart", 15, "ramr@gmail.com"));
        people.add(new Person("Lisa", 13, "ramkumar@gmail.com"));

        System.out.println("\t" + people);

        List<Person> people1 = new ArrayList<Person>();

        people1.add(new Person("jug", 38, "jug@gmail.com"));
        people1.add(new Person("benny", 35, "benny@gmail.com"));
        people1.add(new Person("Bart", 15, "ramr@gmail.com"));
        people1.add(new Person("Lisa", 13, "ramkumar@gmail.com"));

        for (Person people : people) {
            if (people1.contains( people)) { 
                System.out.println("matched");
                System.out.println(people);
            } else {
                System.out.println("not matched");
                System.out.println(people);
            }
        }
    }
}

expected output:

matched
Bart : 15 :ramr@gmail.com

Answer 1

First of all, you need have a correct implementation of equals method. Then you can use List#retainAll which "Retains only the elements in this list that are contained in the specified collection (optional operation). In other words, removes from this list all of its elements that are not contained in the specified collection".

Copy the original list to some list if you want to keep the original list like: copyofPeople then you can use copyofPeople.retainAll(people1) . Now if copyofPeople is blank ( copyofPeople.size()==0 ) then there is no element in common.

If your logical equality depends on the person's email the then equals method should looks like below:

@Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        Person other = (Person) obj;
        if (mail == null) {
            if (other.mail != null)
                return false;
        } else if (!mail.equals(other.mail))
            return false;
        return true;
    }

Answer 2

Collections have:

people.retainAll(people1) // for intersection
people.addAll(people1) // for union

Answer 3

if you are using Java 8 then you can also use stream for this.

    for (Person p : people) {
            if(people1.stream().filter(i -> i.equals(p)).findFirst().isPresent()){
                System.out.println("matched");
                System.out.println(p);
            }
            else{
                System.out.println("not matched");
                System.out.println(p);
            }
     }

Here i am overriding equals method in Person class to return true if all the properties of one object matches other.

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        Person p = (Person) obj;
        if(getName() == p.getName() && getAge() == p.getAge() && getMail() == p.getMail())
            return true;
        else
            return false;
    }