Java Program to Compare Two Objects

An object is an instance of a class that has its state and behavior. In java, being object-oriented, it is always dynamically created and automatically destroyed by the garbage collector as the scope of the object is over.

Illustration: An example to illustrate an object of a class:

Furniture chair=new Furniture();

Furniture sofa=new Furniture();

// Here, chair and sofa are two objects of the class Furniture

Approaches: 

There are two standard methods:

1. Using equals()
   • Without overriding
   • With overriding
2. Using hashCode() and equals() method

Example 1: Although equals() method can be used to compare the values of two strings, it is not really useful by default to compare two objects without overriding it.

Java

//  Java Program to compare two objects

// Importing java input output library
import java.io.*;

// Class 1
class Pet {
    // attributes of class1
    String name;
    int age;
    String breed;

    // constructor of class 1
    Pet(String name, int age, String breed)
    {
        // Assignment of current attributes
        /// using this keyword with same
        this.name = name;
        this.age = age;
        this.breed = breed;
    }
}

/* Class 2 : where execution is shown
             for class 1 */
public class GFG {

    // Main driver method
    public static void main(String args[])
    {

        // Objects of class1 (auxiliary class)
        // are assigned value */
        Pet dog1 = new Pet("Snow", 3, "German Shepherd");
        Pet cat = new Pet("Jack", 2, "Tabby");
        Pet dog2 = new Pet("Snow", 3, "German Shepherd");

        // Checking objects are equal and
        // printing output- true/false
        System.out.println(dog1.equals(dog2));
    }
}

Output

false

Example 2: Overriding equals() method

Though the values of dog1 and dog2 are the same, equals() method always checks the reference of the two objects i.e if both the objects passed refer to the same object or not and not their values. Therefore, it is advisable not to use this method in comparing objects without overriding it. Implementing the equals method for the before example:

Java

// Java Program to Compare Two Objects

import java.io.*;

class Pet {
    String name;
    int age;
    String breed;

    Pet(String name, int age, String breed)
    {
        this.name = name;
        this.age = age;
        this.breed = breed;
    }
    @Override public boolean equals(Object obj)
    {

        // checking if the two objects
        // pointing to same object
        if (this == obj)
            return true;

        // checking for two condition:
        // 1) object is pointing to null
        // 2) if the objects belong to
        // same class or not
        if (obj == null
            || this.getClass() != obj.getClass())
            return false;

        Pet p1 = (Pet)obj; // type casting object to the
                           // intended class type

        // checking if the two
        // objects share all the same values
        return this.name.equals(p1.name)
            && this.age == p1.age
            && this.breed.equals(p1.breed);
    }
}

public class GFG {
    public static void main(String args[])
    {

        Pet dog1 = new Pet("Snow", 3, "German Shepherd");
        Pet cat = new Pet("Jack", 2, "Tabby");
        Pet dog2 = new Pet("Snow", 3, "German Shepherd");
        System.out.println(dog1.equals(dog2));
    }
}

Output

true

Example 3: In the above example, the equals() method is checking if all the values match or not. However, it can be overridden in any way possible i.e if one or two values match, etc. For example, if we want to check for any two values to make the equals() method consider the two objects to be the same:

Java

// Java Program to Compare Two Objects

// Importing java input/output libraries
import java.io.*;

// Class 1
class Pet {
    // Attributes of objects
    String name;
    int age;
    String breed;

    // Constructor
    Pet(String name, int age, String breed)
    {
        // Assigning current there it self
        // using this keyword
        this.name = name;
        this.age = age;
        this.breed = breed;
    }

    @Override public boolean equals(Object obj)
    {
        if (this == obj)
            return true;
        if (obj == null
            || this.getClass() != obj.getClass())
            return false;
        Pet p1 = (Pet)obj;

        // Checking only if attribute- name
        // and age is same and ignoring breed
        return this.name.equals(p1.name)
            && this.age == p1.age;
    }
}

public class GFG {

    // Main driver method
    public static void main(String args[])
    {

        // Assigning values to attributes of object
        // of class 1
        Pet dog1 = new Pet("Snow", 3, "German Shepherd");
        Pet cat1 = new Pet("Jack", 2, "Tabby");
        Pet dog2 = new Pet("Snow", 3, "German Shepherd");
        Pet cat2 = new Pet("Jack", 2, "Persian");

        // Checking if object are equal and
        // printing boolean output
        System.out.println(cat1.equals(cat2));
    }
}

Output

true

Using hashCode() and equals()

This method is more like an add-on to the previous one. Checking the hash values using hashCode() before entering the equals() reduces the time taken to produce the solution drastically. In this way, many comparisons between two objects need not go through the comparison of every value within them.

Example 1: The above implementation along with usage of hashCode():

Java

// Java Program to Compare Two Objects

// Importing java input/output libraries
import java.io.*;

// Class 1
class Pet {
    // Attributes of objects of class
    String name;
    int age;
    String breed;

    // Constructor
    Pet(String name, int age, String breed)
    {
        this.name = name;
        this.age = age;
        this.breed = breed;
    }

    // Overriding using hashCode() method
    @Override public int hashCode()
    {
        /* overriding hashCode() method
        to check the length of the names */
        return this.name.length() % 10;
    }

    // Boolean function to check
    @Override public boolean equals(Object obj)
    {
        if (this == obj)
            return true;
        if (obj == null
            || this.getClass() != obj.getClass())
            return false;
        Pet p1 = (Pet)obj;

        return this.name.equals(p1.name)
            && this.age == p1.age && this.breed == p1.breed;
    }
}
// main class (class2)
public class GFG {

    // Main driver method
    public static void main(String args[])
    {

        // Assigning values to object of class 1(Pet class)
        Pet dog1 = new Pet("Snow", 3, "German Shepherd");
        Pet cat1 = new Pet("Jack", 2, "Tabby");
        Pet dog2 = new Pet("Snow", 3, "German Shepherd");
        Pet cat2 = new Pet("Jack", 2, "Persian");

        /* hashCode() generates true as the lengths of
           the name value of the two objects are same*/

        // Condition check using hashCode() method
        if (dog1.hashCode() == cat1.hashCode())

            /* On entering equals() method, it checks for
               other values and hence, returns false */
            System.out.println(dog1.equals(cat1));
        else
            System.out.println("Not equal");
    }
}

Output

false

Example 2: 

Java

// Java Program to Compare Two Objects

import java.io.*;

class Pet {
    String name;
    int age;
    String breed;

    Pet(String name, int age, String breed)
    {
        this.name = name;
        this.age = age;
        this.breed = breed;
    }
    @Override public int hashCode()
    {
        // overriding hashCode() method to first
        // check the length of the names*/
        return this.name.length() % 10;
    }
    @Override public boolean equals(Object obj)
    {
        if (this == obj)
            return true;
        if (obj == null
            || this.getClass() != obj.getClass())
            return false;
        Pet p1 = (Pet)obj;

        return this.name.equals(p1.name)
            && this.age == p1.age && this.breed == p1.breed;
    }
}

public class GFG {
    public static void main(String args[])
    {

        Pet dog1 = new Pet("Snow", 3, "German Shepherd");
        Pet cat1 = new Pet("Jack", 2, "Tabby");
        Pet dog2 = new Pet("Snow", 3, "German Shepherd");
        Pet cat2 = new Pet("Jack", 2, "Persian");
        Pet dog3 = new Pet("Ray", 1, "Siberian Husky");

        // here, hashCode() generates false and condition
        // reverts to the else statement as soon as it finds out
        // the lengths of the name value of the objects are
        // differenT
        if (dog1.hashCode() == dog3.hashCode())
            System.out.println(dog1.equals(dog3));
        else
            System.out.println("Not equal");
    }
}

Output

Not equal