Building equals(), hashCode(), compareTo() and toString() with ease

Implementing equals(), hashCode(), compareTo() and toString() is a common task for java programmers. If you're unfamiliar with this topic, check out Joshua Blochs best practices on how to override hashCode().
Modern IDEs like Eclipse offer commands to auto-generate these methods for particular classes. While this is ok in terms of time saving, the generated code is verbose and therefor hardly maintainable.



Let's take a look at what Eclipse Galileo does with an example class Customer containing a few members:

public class Customer {
private String firstName;
private String lastName;
private Date birthday;
private String street;
private String city;
private String zipcode;

@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((birthday == null) ? 0 : birthday.hashCode());
result = prime * result + ((city == null) ? 0 : city.hashCode());
result = prime * result + ((firstName == null) ? 0 : firstName.hashCode());
result = prime * result + ((lastName == null) ? 0 : lastName.hashCode());
result = prime * result + ((street == null) ? 0 : street.hashCode());
result = prime * result + ((zipcode == null) ? 0 : zipcode.hashCode());
return result;
}

@Override
public boolean equals(Object obj) {
if (this == obj) {
return true;
}
if (obj == null) {
return false;
}
if (getClass() != obj.getClass()) {
return false;
}
Customer other = (Customer) obj;
if (birthday == null) {
if (other.birthday != null) {
return false;
}
}
else if (!birthday.equals(other.birthday)) {
return false;
}
if (city == null) {
if (other.city != null) {
return false;
}
}
else if (!city.equals(other.city)) {
return false;
}
if (firstName == null) {
if (other.firstName != null) {
return false;
}
}
else if (!firstName.equals(other.firstName)) {
return false;
}
if (lastName == null) {
if (other.lastName != null) {
return false;
}
}
else if (!lastName.equals(other.lastName)) {
return false;
}
if (street == null) {
if (other.street != null) {
return false;
}
}
else if (!street.equals(other.street)) {
return false;
}
if (zipcode == null) {
if (other.zipcode != null) {
return false;
}
}
else if (!zipcode.equals(other.zipcode)) {
return false;
}
return true;
}
}

The methods are generated by using the Eclipse Galileo command Menu -> Source -> Generate equals() and hashCode(). As you can see the generated equals() method is damn long. At first glance you can hardly see which members are included in the equals-logic. What happens if you add another member to the class which has to be involved into equals/hashcode? I for myself would delete and re-generate the methods. But I'm no fan of such boiler plate code so let me introduce another approach to implement these methods.

Instead of letting the IDE generate those methods I prefer to use the utility classes from Apache Commons Lang. The library contains convient classes like EqualsBuilder, HashCodeBuilder, CompareToBuilder and ToStringBuilder to build the appropriate methods.
Let's see how the above example would look like using the apache commons:

import java.util.Date;

import org.apache.commons.lang.builder.CompareToBuilder;
import org.apache.commons.lang.builder.EqualsBuilder;
import org.apache.commons.lang.builder.HashCodeBuilder;
import org.apache.commons.lang.builder.ToStringBuilder;

public class Customer implements Comparable<Customer> {
private String firstName;
private String lastName;
private Date birthday;
private String street;
private String city;
private String zipcode;

@Override
public int hashCode() {
return new HashCodeBuilder(17, 37)
.append(firstName)
.append(lastName)
.append(birthday)
.append(street)
.append(city)
.append(zipcode)
.toHashCode();
}

@Override
public boolean equals(Object obj) {
if (obj == null) { 
return false; 
}
if (obj == this) { 
return true;
}
if (obj.getClass() != getClass()) {
return false;
}
Customer rhs = (Customer) obj;
return new EqualsBuilder()
.append(firstName, rhs.firstName)
.append(lastName, rhs.lastName)
.append(birthday, rhs.birthday)
.append(street, rhs.street)
.append(city, rhs.city)
.append(zipcode, rhs.zipcode)
.isEquals();
}

@Override
public String toString() {
return new ToStringBuilder(this)
.append("firstName", firstName)
.append("lastName", lastName)
.append("birthday", birthday)
.append("street", street)
.append("city", city)
.append("zipcode", zipcode)
.toString();
}

public int compareTo(Customer rhs) {
return new CompareToBuilder()
.append(firstName, rhs.firstName)
.append(lastName, rhs.lastName)
.append(birthday, rhs.birthday)
.append(street, rhs.street)
.append(city, rhs.city)
.append(zipcode, rhs.zipcode)
.toComparison();
}
}

The implementation is self-explanatory. At first glance you can see which members are involved into the method. Also you can easily add or remove members from these methods if requirements are changing.

See also:

• Apache Commons Lang
  
• Package org.apache.commons.lang.builder
  
• Implementing hashcode as described in Effectice Java by Joshua Bloch