hashCode() and equals() Method in Java

HashCode() and equals() method in Java | Previously we discussed hashCode() method and equals() method of java.lang.Object class in detail. Now let us see the relation between hashCode() and the equals() method in Java. What are the differences between the hashCode() and equals() methods in Java?

What is the contract between the hashcode and the equals method?

There is a contract between the equals() and hashCode() method in Java, if two objects are equal by the equals() method then their hashCode() must be equal i.e. two equivalent objects should have the same hashcode.

class Student {
   int id;
   Student(int id) {
      this.id = id;
   }
}

Assume s1 and s2 are two different objects of the Student class. If s1.equals(s2) is true then s1.hashCode() == s2.hashCode() is always true.

java.lang.Object class equals() and hashCode() method follows above contract. Hence whenever we are overriding the equals() method then compulsory we should override the hashCode() method to satisfy the above contract i.e. two equivalent objects should have the same hashCode. If we don’t follow this contract then we won’t get any compile-time error, or runtime exception but it is not a good programming practice.

In java.lang.String class equals() method is overridden for content comparison and hence hashCode() method is also overridden to generate hashcode value based on the content.

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

      String s1 = new String("KnowProgram");
      String s2 = new String("KnowProgram");
      System.out.println(s1.equals(s2)); 

      System.out.println(s1.hashCode() == s2.hashCode());
      System.out.println(s1.hashCode());
      System.out.println(s2.hashCode());
   }
}

Output:-

true
true
2058267993
2058267993

In StringBuffer and StringBuilder classes, the equals() method is not overridden for content comparison, therefore the hashCode() method is also not overridden.

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

      StringBuilder s1 = new StringBuilder("KnowProgram");
      StringBuilder s2 = new StringBuilder("KnowProgram");
      System.out.println(s1.equals(s2)); 

      System.out.println(s1.hashCode() == s2.hashCode());
      System.out.println(s1.hashCode());
      System.out.println(s2.hashCode());
   }
}

Output:-

false
false
225534817
1878246837

Relation between hashCode() and equals() Method in Java

1) If two objects are not equal by the equals() method then their hashcode value may or may not be the same.

s1.equals(s2)	s1.hashCode() == s2.hashCode()
true	true
false	true/false

2) If the hashcode of two objects is equal then the equals() method return true or false.
3) If the hashcode of two objects is not equal then these objects are always not equal by the equals() method.

s1.hashCode() == s2.hashCode()	s1.equals(s2)
true	true/false
false	false

Overriding hashCode() and equals() Method in Java

Based on which parameter we override the equals() method, it is highly recommended to use the same parameter while overriding hashCode() also.

In all collections classes, wrapper classes, and string classes the equals() method is overridden for content comparison. Hence it is recommended to override the equals() method in our class for content comparison.

In the equals() method we have already discussed how to override equals() and the steps to be followed. Now, we will use that knowledge to develop the Student class.

class Student {
   private int id;

   Student(int id) {
      this.id = id;
   }

   @Override
   public boolean equals(Object obj) {
      if(this == obj) return true;
      if(obj instanceof Student) {
         Student s = (Student) obj;
         return this.id == s.id;
      }
      return false;
   }

   @Override 
   public int hashCode() {
      return id;
   }
}

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

     Student s1 = new Student(1001);
     Student s2 = new Student(1002);
     Student s3 = new Student(1001);
      
     // invoke hashCode() method
     System.out.println("hashCode():: " +
                    s1.hashCode() + " " +
                    s2.hashCode() + " " +
                    s3.hashCode() );

     // invoke equals() method
     System.out.println("equals():: " + 
                  s1.equals(s2) + " " +
                  s1.equals(s3) + " " +
                  s2.equals(s3) );
  }
}

Output:-

hashCode():: 1001 1002 1001
equals():: false true false

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