equals() Method in Java Object Class

equals() Method in Java Object Class | We can use the equals() method in Java to check the equality of two objects. The equals() method is given to compare two objects of a class for their equality based on their reference (default implementation) or based on data (after overriding).

The equals() method is defined in java.lang.Object class and compare two objects based on their reference. If both have the same reference then it returns true else it returns false.

The implementation of the equals() method in java.lang.Object class is:-

public boolean equals(Object obj) {
   return (this == obj);
}

Java equals() Method Example

Examples to demonstrate Object class equals() method in Java,

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

      Test t1 = new Test();
      Test t2 = new Test();
      Test t3 = t1;

      System.out.println(t1.equals(t2));
      System.out.println(t1.equals(t3));
      System.out.println(t2.equals(t3));
   }
}

Output:-

false
true
false

The t1 and t2 are two different objects of the Test class, t3 is pointing to the reference of object t1. Hence, t1 and t2 have different references but t3 and t1 are having the same reference.

The equals() method is called on these objects. Since the equals() method is not available in the Test class so, the compiler searches for it in the superclass. Indirectly or directly every valid Java class is the subclass of java.lang.Object class. Hence equals() method will be executed from the Object class which contains logic to compare two objects based on their reference.

Example of Employee class to demonstrate the Object class equals() method in Java,

class Employee {

   int id;
   String name;

   Employee(int id, String name) {
      this.id = id;
      this.name = name;
   }
}

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

      // create objects
      Employee e1 = new Employee(101, "Emma");
      Employee e2 = new Employee(102, "Oliver");
      Employee e3 = e1;
      Employee e4 = new Employee(101, "Emma");

      // display details
      System.out.println(e1.id+" "+ e1.name);
      System.out.println(e2.id+" "+ e2.name);
      System.out.println(e3.id+" "+ e3.name);
      System.out.println(e4.id+" "+ e4.name);

      // compare objects
      System.out.println(e1.equals(e2)); // false
      System.out.println(e1.equals(e3)); // true
      System.out.println(e1.equals(e4)); // false
   }
}

Output:-

101 Emma
102 Oliver
101 Emma
101 Emma
false
true
false

If two references are pointing to the object then only the equals() method of java.lang.Object class returns true, otherwise, it returns false.

In the above example, e1 and e4 contain exactly the same data. Since they have different references and the Object class equals() method compares objects based on reference, therefore, it gives the result as false.

Override equals() Method in Java

If we want to compare two objects with state or value then we must override the equals() method in the subclass. Many times objects are created and passed as method arguments at runtime. So, to know whether the given object are same or not, we must compare objects using their state.

Java Program to compare two objects by overriding equals() method

class Employee {

   int id;
   String name;

   Employee(int id, String name) {
      this.id = id;
      this.name = name;
   }

   @Override
   public boolean equals(Object obj) {
      if(this == obj){
         return true;
      }
      if(obj instanceof Employee) {
         Employee e = (Employee) obj;
         return (this.id == e.id) &&
                 this.name.equals(e.name);
      }
      return false;
   }
}

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

      // create objects
      Employee e1 = new Employee(101, "Emma");
      Employee e2 = new Employee(102, "Oliver");
      Employee e3 = e1;
      Employee e4 = new Employee(101, "Emma");

      // compare objects
      System.out.println(e1.equals(e2)); // false
      System.out.println(e1.equals(e3)); // true
      System.out.println(e1.equals(e4)); // true
   }
}

Output:-

false
true
true

How to Override equals() Method in Java

Step-1) While overriding first check the passed object and current object are pointing to the same reference. If both are pointing to the same reference then both objects are always the same and they will contain the same data.

For this, write below condition in equals method as,

// compare passed object and current 
// object reference
if(this == obj){
   return true;
}

For example- e1 and e3 are two different objects but they are pointing to the same reference, then it will return true.

Employee e1 = new Employee(101, "Emma");
Employee e3 = e1;
System.out.println(e1.equals(e3)); // true

Step-2) Before comparing the values of the object, compare whether both objects are compatible or not. For this, we can use instanceof operator.

if(obj instanceof Employee) {
   // logic to compare data of objects
} else {
   return false;
}

It will handle situations like comparing with null, or incompatible type objects. Examples:-

Employee e1 = new Employee(101, "Emma");
System.out.println(e1.equals(null)); // false
Test t1 = new Test();
System.out.println(e1.equals(t1)); // false

If we try to compare Employee type objects with Student, Test, or some other type object then it must return false because they are not compatible with each other.

Step-3) Convert object type to current class type.

The parameter of the equals() method is an Object type. To compare it with another object, it should be converted to the current class type.

Employee e = (Employee) obj;

Step-4) Finally compare all values of the object.

Here, to compare two primitive values use the == operator, and to compare objects use the equals() method. For example, their id and name are two values in the Employee class. Since id is a primitive variable so we will use the == operator and the name is of String type hence equals() method will be used.

// get current object values
int id1 = this.id;
String name1 = this.name;

// get passed object values
int id2 = e.id;
String name2 = e.name;

// compare values
if((id1 == id2) && (name1.equals(name2)) {
   return true;
}

The current object can be fetched directly by its name or using the “this” keyword. We can reduce the above lines as,

return (this.id == e.id) && this.name.equals(e.name);

Or,

return (id == e.id) && name.equals(e.name);

Uses of the “this” keyword improve the readability of the code therefore it is recommended to use the “this” keyword while comparing the data of the objects.

Uses of equals() Method in Java

In all wrapper classes, the Arrays class, and String class equals() method is overridden to compare the data of the objects. But in the StringBuffer and StringBuilder class, the equals() method are not overridden, they will use the Object class equals() method to compare the objects. Example:-

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

    String str1 = new String("Know Program");
    String str2 = new String("Know Program");
    System.out.println(str1.equals(str2));
    
    StringBuilder sb1 = new StringBuilder("Know Program");
    StringBuilder sb2 = new StringBuilder("Know Program");
    System.out.println(sb1.equals(sb2));
  }
}

Output:-

true
false

The Set collection classes allow storing only unique elements, they don’t allow storing duplicate elements. The duplicate elements are found with the help of the equals() method. If the equals() method return true then they are duplicate elements.

Example-2 to Override equals method in Java

Write code to override the equals() method in the given Student class.

class Student {

   private int rollno;
   private String name;
   private String dept;
   private double avg;

   // constructor
   Student(int rollno, String name, 
          String dept, double avg) {
      this.rollno = rollno;
      this.name = name;
      this.dept = dept;
      this.avg = avg;
   }
}

Use == operator to compare primitive values, and use equals() method to compare Objects.

@Override
public boolean equals(Object obj) {

   if(this == obj) return true;

   if(obj instanceof Student) {
      Student s = (Student) obj;

      return 
        ( (this.rollno == s.rollno) &&
          this.name.equals(s.name) &&
          this.dept.equals(s.dept) &&
          (this.avg == s.avg) );
   }

   return false;
}

Example-3 to Override equals method in Java

We have 2 classes Student and Address. Each Student has their id, name, and Address. Each Address contains the house number and city. The main task is to check two students are the same or not?

2 Students are the same if they have exactly the same id, name, and Address. In Adress, they must have the same house number and city. Therefore we have to override the equals() method in both Student and Address classes.

Address class with fields hno (house name), and city,

class Address {
  int hno; // house-number
  String city;
  
  Address(int hno, String city) {
    this.hno = hno;
    this.city = city;
  }
  
  @Override
  public boolean equals(Object obj) {
    if(this == obj) return true;
    if(obj instanceof Address) {
      Address ad = (Address)obj;
      return (this.hno == ad.hno) &&
          this.city.equals(ad.city);
    }
    return false;
  }
}

Student class with fields id, name, and adrs(Address),

class Student {
  int id;
  String name;
  Address adrs;

  public Student(int id, String name, Address adrs) {
    this.id = id;
    this.name = name;
    this.adrs = adrs;
  }
  
  @Override
  public boolean equals(Object obj) {
    if(this == obj) return true;
    if(obj instanceof Student) {
      Student st = (Student) obj;
      return (this.id == st.id) &&
           (this.name.equals(st.name)) &&
           (this.adrs.equals(st.adrs));
    }
    return false;
  }
}

Test class to compare students,

public class Test {
  public static void main(String[] args) {
    Student st1 = 
     new Student(101, "Sophia", new Address(8546, "Los Angeles"));
    Student st2 = 
     new Student(101, "Sophia", new Address(8546, "Los Angeles"));
    Student st3 = 
     new Student(101, "Sophia", new Address(9853, "San Francisco"));
    
    System.out.println(st1.equals(st2)); // true
    System.out.println(st1.equals(st3)); // false
  }
}

Output:-

true
false

If you want to display Student data then you have to override the toString() method. See more:- toString() method in Java Object class.

Object Comparison in Java with equals() Method

We can use the equals method to compare different types of objects. We have already seen the comparison of two different objects.

Comparison using equals()	Valid?	Return
Two objects	Yes	false/true
Two primitive variables	No	compile-time error
Two nulls	No	compile-time error
Null with object	No	compile-time error
Object with null	Yes	false
Null referenced with null	No	NullPointerException
Null referenced with object	No	NullPointerException
Two Null referenced	No	NullPointerException
Object with null referenced	Yes	false
Incompatible objects	Yes	false

We can’t use the equals() method on primitive values because equals() is an instance method and to call equals() method object should be there.

int i1 = 10;
int i2 = 20;
System.out.println(i1.equals(i2)); // error

We can’t compare two nulls, or nulls with objects because a null is also treated as a primitive type value.

System.out.println(null.equals(null)); // error

Test t1 = new Test();
System.out.println(null.equals(t1)); // error

Comparing Object with null always gives “false” as a result.

Test t1 = new Test();
System.out.println(t1.equals(null)); // false

Using the equals() method on null referenced with null, null referenced with an object, and two nulls referenced always raise NullPointerException at runtime.

Test t1 = null;
System.out.println(t1.equals(null)); // Exception
    
Test t2 = new Test();
System.out.println(t1.equals(t2)); // Exception
    
Test t3 = null;
System.out.println(t1.equals(t3)); // Exception

Using the equals() method on Object with null referenced, and Incompatible objects always gives false as result.

Test t1 = new Test();
Test t2 = null;
System.out.println(t1.equals(t2)); // false
    
Student st1 = new Student();
System.out.println(t1.equals(st1)); // false

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