Using HashCodeBuilder to implement hashCode()

Similar to overriding equals(), you may also need to override the hashCode() method in your custom class. The equals() contract states that two objects which compare equal must have the same hashCode. Moreover, for efficient storage in HashMap, non-equal objects should return different hashcodes.

Since equal objects should have equal hashcodes, you must compare the same fields as in the equals() method.

With all these concerns in mind, writing an effective hashCode() method might be difficult. Fortunately, the Apache Commons Lang library provides a utility class called HashCodeBuilder which makes it fairly easy to implement an efficient hashCode method.

Here’s how you use this class:

public class Account implements Comparable {
  private long id;
  private String firstName;
  private String lastName;
  private String emailAddress;
  private Date creationDate;

  public int hashCode() {
    return new HashCodeBuilder(11, 21)
        .append(this.id)
        .append(this.firstName)
        .append(this.lastName)
        .append(this.emailAddress)
        .append(this.creationDate)
        .toHashCode();
  }
}

First you need to initialize an object of the class HashCodeBuilder. The constructor takes two integers as arguments. You need to supply two random odd integers (preferable prime) as the arguments. You pass a relevant field of the class (that you want to be included while calculating the hashCode) to the append method. The method returns a HashCodeBuilder instance. So, you can chain the calls to the append methods together (as in the example above). Finally, you call the toHashCode() method which returns the computed hash code to the caller.

Some other useful constructors/methods in the class are:

  • appendSuper(int) – use this method if you want to append the hashCode of the super class object. E.g. o.appendSuper(super.hashCode());
  • HashCodeBuilder(int initialNonZeroOddNumber, int multiplierNonZeroOddNumber): These two arguments are used for calculating the hashCode for the object. Try to use different arguments for different classes.
  • HashCodeBuilder(): This uses two hard coded choices for the constants for calculating the hashCode.
  • hashCode(): returns the computed hash code (same as toHashCode()). Note: It doesn’t return the hashCode of the HashCodeBuilder object.
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2 thoughts on “Using HashCodeBuilder to implement hashCode()

  1. Pingback: Using CompareToBuilder to implement Comparable interface | Param Gyaan

  2. Pingback: Using ToStringBuilder to easily write toString() method | Param Gyaan

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