What is Java hashCode()

Hash Principle

First of all, before we define Java hashcode, we need to understand what is hashing and what is it for. Hashing is a process of applying a hash function to some data. A hash function is just a mathematical function. Don’t worry about this! “Mathematical” is not always means “complicated”. Here it means only that we have some data and a certain rule that maps the data into a set of characters (code). For example, it could be a hexadecimal cipher. We have some data of any size at the input, and apply a hash function to it. At the output, we get a fixed-size data, say, 32 characters. Usually, that kind of function converts a big piece of data into a small integer value. The result of this function work is called a hash code. Hash functions are widely used in cryptography, and some other areas too. Hash functions can be different, but they all have certain properties:

• A particular object has a particular hash code.
• If two objects are equal, their hash codes are the same. The reverse is not true.
• If the hash codes are different, then the objects are not equal for sure.
• Different objects may have the same hash code. However, it is a very unlikely event. At this point, we have a collision, a situation, where we can lose data.

The "proper" hash function minimizes the probability of collisions.

HashCode in Java

In Java hash function is usually connected to hashCode(). Precisely, the result of applying a hash function to an Object is hashCode. Every Java object has a hash code. In general Hash Code is a number calculated by the hashCode() method of the Object class. Usually, programmers override this method for their objects as well as related to hashCode() the equals() method for more efficient processing of specific data. The hashCode() method returns an int (4 bytes) value, which is a numeric representation of the object. This hash code is used, for example, by collections for more efficient storage of data and, accordingly, faster access to them. By default, the hashCode() function for an object returns the number of the memory cell where the object is stored. Therefore, if no changes are made to the application code, then the function should return the same value. If the code changes slightly, the hashCode value also changes. What hashCode Java uses for? First of all Java Hash codes helps programs run faster. For example, if we compare two objects o1 and o2 of some type, the operation o1.equals(o2) takes about 20 times more time than o1.hashCode() == o2.hashCode().

Java equals()

In the parent class Object, along with the hashCode() method, there is also equals(), the function that is used to check the equality of two objects. The default implementation of this function simply checks the links of two objects for their equivalence. equals() and hashCode() have their contract, so if you override the one, you should override the second for not breaking this contract.

Implementing hashCode()

Example

Let’s create a class Character with one field — name. After that, we create two objects of Character class, character1, and character2 and set them the same name. If we use default hashCode() and equals() of Object class, we’ve definitely got the different, not equal objects.That’s how hashcode in Java works. They will have different hashCodes because they are in different memory cells and equals() operation result will be false.

import java.util.Objects;

public class Character {
    private String Name;

    public Character(String name) {
        Name = name;
    }

    public String getName() {
        return Name;
    }

    public void setName(String name) {
        Name = name;
    } 

    public static void main(String[] args) {
        Character character1 = new Character("Arnold");
        System.out.println(character1.getName());
        System.out.println(character1.hashCode());
        Character character2 = new Character("Arnold");
        System.out.println(character2.getName());
        System.out.println(character2.hashCode());
        System.out.println(character2.equals(character1));
    }
}

The result of running the program:

Arnold
1595428806
Arnold
1072408673
false

Two 10-digit numbers in the console are hashCodes. What if we want to have Objects equivalent if they have the same names? What should we do? The answer: we should override hashCode() and equals() methods of Object class for our Character class. We could do it automatically in IDEA IDE, just press alt + insert on your keyboard and choose Generate -> equals() and hashCode(). In the case of our example we’ve got the next code:

import java.util.Objects;

public class Character {
    private String Name;

    public Character(String name) {
        Name = name;
    }

    public String getName() {
        return Name;
    }

    public void setName(String name) {
        Name = name;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Character)) return false;

        Character character = (Character) o;

        return getName() != null ? getName().equals(character.getName()) : character.getName() == null;
    }

    @Override
    public int hashCode() {
        return getName() != null ? getName().hashCode() : 0;
    }

    public static void main(String[] args) {
        Character character1 = new Character("Arnold");
        System.out.println(character1.getName());
        System.out.println(character1.hashCode());
        Character character2 = new Character("Arnold");
        System.out.println(character2.getName());
        System.out.println(character2.hashCode());
        System.out.println(character2.equals(character1));
    }
}

The result of running this code:

Arnold
1969563338
Arnold
1969563338
true

So now the program identifies our objects as equals and they have the same hash codes.

Java hashcode example:

Your own hashcode and equals

You may also create your own equals() and hashCode() realizations, but be careful remembering of collisions minimization of hashcodes. Here is an example of our own hashCode() and equals() methods in Student class:

import java.util.Date;

public class Student {
   String surname;
   String name;
   String secondName;
   Long birthday; // Long instead of long is used by Gson/Jackson json parsers and various orm databases

   public Student(String surname, String name, String secondName, Date birthday ){
       this.surname = surname;
       this.name = name;
       this.secondName = secondName;
       this.birthday = birthday == null ? 0 : birthday.getTime();
   }
//Java hashcode example
   @Override
   public int hashCode(){
       //TODO: check for nulls
       //return surname.hashCode() ^ name.hashCode() ^ secondName.hashCode() ^ (birthday.hashCode());
       return (surname + name + secondName + birthday).hashCode();
   }
   @Override
   public boolean equals(Object other_) {
       Student other = (Student)other_;
       return (surname == null || surname.equals(other.surname) )
               && (name == null || name.equals(other.name))
               && (secondName == null || secondName.equals(other.secondName))
               && (birthday == null || birthday.equals(other.birthday));
   }
}

And the Main class to demonstrate their work:

import java.util.Date;
import java.util.HashMap;
import java.util.Hashtable;

public class Main {
   static HashMap<Student, Integer> cache = new HashMap<Student, Integer>(); // <person, targetPriority>

   public static void main(String[] args) {
       Student sarah1 = new Student("Sarah","Connor", "Jane", null);
       Student sarah2 = new Student("Sarah","Connor", "Jane", new Date(1970, 01-1, 01));
       Student sarah3 = new Student("Sarah","Connor", "Jane", new Date(1959, 02-1, 28)); // date not exists
       Student john = new Student("John","Connor", "Kyle", new Date(1985, 02-1, 28)); // date not exists
       Student johnny = new Student("John","Connor", "Kyle", new Date(1985, 02-1, 28)); // date not exists
       System.out.println(john.hashCode());
       System.out.println(johnny.hashCode());
       System.out.println(sarah1.hashCode());
       System.out.println();
       cache.put(sarah1, 1);
       cache.put(sarah2, 2);
       cache.put(sarah3, 3);
       System.out.println(new Date(sarah1.birthday));
       System.out.println();
       cache.put(john, 5);
       System.out.println(cache.get(john));
       System.out.println(cache.get(johnny));
       cache.put(johnny, 7);
       System.out.println(cache.get(john));
       System.out.println(cache.get(johnny));
   }
}

What hashCode is used for?

First of all Hash codes helps programs run faster. For example, if we compare two objects o1 and o2 of some type, the operation o1.equals(o2) takes about 20 times more time than o1.hashCode() == o2.hashCode(). In Java hashing principle stands behind some popular collections HashMap, HashSet and HashTable.

Conclusion

Every Java Object has methods hashCode() and equals() inherited from Object class. To get a good working equality mechanism, you’d better override hashcode() and equals() methods for your own Classes. Using hash codes makes programs run faster.