Equals Method

The equals method is used to determine whether 2 objects of a class are the same.

Syntax

Objects equality

Let's look at the following example:

class Smartphone
{
    private String model;
    private double price;

Smartphone(String model, double price)
    {
        this.model = model;
        this.price = price;
    }

public boolean equals(Object o) {
        // Casting the passed object to Smartphone:
        Smartphone smartphone = (Smartphone) o;

// two smartphones are considered identical if
        // they are the same model and have the same price:
        return this.getModel().equals(smartphone.getModel()) &&
                this.getPrice() == smartphone.getPrice();
    }

public double getPrice() {
        return this.price;
    }

public String getModel() {
        return this.model;
    }
}

public class Main {

public static void main(String[] args) {
        Smartphone phone_1 = new Smartphone("Iphone 11", 2500);
        Smartphone phone_2 = new Smartphone("Iphone 11", 2600);
        Smartphone phone_3 = new Smartphone("Iphone 11", 2600);
        Smartphone phone_4 = new Smartphone("Samsung Galaxy Fold", 2600);

if(phone_1.equals(phone_2)) {
            System.out.println("Phone 1 and Phone 2 are the same.");
        }

if(phone_2.equals(phone_3)) {
            System.out.println("Phone 2 and Phone 3 are the same.");
        }

if(phone_3.equals(phone_4)) {
            System.out.println("Phone 3 and Phone 4 are the same.");
        }
    }
}

Output:

Explanation:

At lines 12 - 20 the equals method is defined. Two Smartphone objects are considered identical if they have the same model and price.
That's why only phone_2 and phone_3 are considered identical.

Equals And Contains Methods

It can be checked whether an object is contained in List collections via the equals and contains methods.

The rule whether an object is contained in the array is specified in the equals method.

Example:

import java.util.ArrayList;
import java.util.List;

class Smartphone
{
    private String model;
    private double price;

Smartphone(String model, double price)
    {
        this.model = model;
        this.price = price;
    }

public boolean equals(Object o) {
        // Casting the passed object to Smartphone:
        Smartphone smartphone = (Smartphone) o;

// two smartphones are considered identical if
        // they are the same model.
        return this.getModel().equals(smartphone.getModel());
    }

public double getPrice() {
        return this.price;
    }

public String getModel() {
        return this.model;
    }
}

public class Main {

public static void main(String[] args) {
        Smartphone phone_1 = new Smartphone("Iphone 11", 2500);
        Smartphone phone_2 = new Smartphone("Iphone 11", 2600);
        Smartphone phone_3 = new Smartphone("Oneplus 7", 1200);
        Smartphone phone_4 = new Smartphone("Samsung Galaxy Fold", 2600);
        Smartphone phone_5 = new Smartphone("Samsung Galaxy S20", 1700);

List<Smartphone> smartphones = new ArrayList<Smartphone>();
        smartphones.add(phone_1);
        smartphones.add(phone_4);

if(smartphones.contains(phone_1)) {
            System.out.println("phone_1's model is contained in the array");
        }

if(smartphones.contains(phone_2)) {
            System.out.println("phone_2's model is contained in the array");
        }

if(smartphones.contains(phone_3)) {
            System.out.println("phone_3's model is contained in the array");
        }

if(smartphones.contains(phone_4)) {
            System.out.println("phone_4's model is contained in the array");
        }

if(smartphones.contains(phone_5)) {
            System.out.println("phone_5's model is contained in the array");
        }
    }
}

Output:

Explanation:

At line 15 the equals method states that two Smartphone objects are considered identical if their model is the same.
The contains method at lines 46, 50, 54, 58, 62 relies on the equals method.
This means that a Smartphone object is considered to be contained in the list, if the list has an object with the same model.