番石榴的对象类：Equals，HashCode和ToString

如果您有幸使用JDK 7 ，那么新的可用Objects类（至少对我来说）是实现“通用” Java对象方法（例如equals（Object） [with Objects.equals（Object，Object ） ]， hashCode（） [带有Objects.hashCode（Object）或Objects.hash（Object…） ]和toString（） [带有Objects.toString（Object） ]以适当地覆盖默认的Object实现。我写过有关使用Objects类的文章： JDK 7：New Objects类和Java 7 Objects-Powered Compact Equals 。

如果你还没有使用Java 7，你最好的选择可能是Apache的百科全书建设者ToStringBuilder和EqualsBuilder和HashCodeBuilder （如果你之前使用的Java版本，以J2SE 5）或番石榴（如果你使用J2SE 5或后来）。在本文中，我将研究如何使用Guava的Objects类来实现三种常见的方法equals ， hashCode和toString() 。

在没有Guava或其他库的帮助下，本文中讨论的三种常见方法通常会突出显示，如下面的代码清单所示。这些方法是使用NetBeans 7.1 beta生成的。

传统员工

package dustin.examples;import java.util.Calendar;/*** Simple employee class using NetBeans-generated 'common' methods* implementations that are typical of many such implementations created* without Guava or other library.* * @author Dustin*/
public class TraditionalEmployee
{public enum Gender{ FEMALE, MALE };private final String lastName;private final String firstName;private final String employerName;private final Gender gender;/*** Create an instance of me.* * @param newLastName The new last name my instance will have.* @param newFirstName The new first name my instance will have.* @param newEmployerName The employer name my instance will have.* @param newGender The gender of my instance.*/public TraditionalEmployee(final String newLastName, final String newFirstName,final String newEmployerName, final Gender newGender){this.lastName = newLastName;this.firstName = newFirstName;this.employerName = newEmployerName;this.gender = newGender;}public String getEmployerName(){return this.employerName;}public String getFirstName(){return this.firstName;}public Gender getGender(){return this.gender;}public String getLastName(){return this.lastName;}/*** NetBeans-generated method that compares provided object to me for equality.* * @param obj Object to be compared to me for equality.* @return {@code true} if provided object is considered equal to me or*    {@code false} if provided object is not considered equal to me.*/@Overridepublic boolean equals(Object obj){if (obj == null){return false;}if (getClass() != obj.getClass()){return false;}final TraditionalEmployee other = (TraditionalEmployee) obj;if ((this.lastName == null) ? (other.lastName != null) : !this.lastName.equals(other.lastName)){return false;}if ((this.firstName == null) ? (other.firstName != null) : !this.firstName.equals(other.firstName)){return false;}if ((this.employerName == null) ? (other.employerName != null) : !this.employerName.equals(other.employerName)){return false;}if (this.gender != other.gender){return false;}return true;}/*** NetBeans-generated method that provides hash code of this employee instance.* * @return My hash code.*/@Overridepublic int hashCode(){int hash = 3;hash = 19 * hash + (this.lastName != null ? this.lastName.hashCode() : 0);hash = 19 * hash + (this.firstName != null ? this.firstName.hashCode() : 0);hash = 19 * hash + (this.employerName != null ? this.employerName.hashCode() : 0);hash = 19 * hash + (this.gender != null ? this.gender.hashCode() : 0);return hash;}/*** NetBeans-generated method that provides String representation of employee* instance.* * @return My String representation.*/@Overridepublic String toString(){return  'TraditionalEmployee{' + 'lastName=' + lastName + ', firstName=' + firstName+ ', employerName=' + employerName + ', gender=' + gender +  '}';}
}

尽管NetBeans 7.1 Beta在这里完成了繁重的工作，但仍必须维护此代码，并使它们更具可读性。下一个类是相同的类，但是具有Guava支持的通用方法，而不是上面显示的NetBeans生成的“典型”实现。

番石榴员工

package dustin.examples;/*** Simple employee class using Guava-powered 'common' methods implementations.* * I explicitly scope the com.google.common.base.Objects class here to avoid the* inherent name collision with the java.util.Objects class.* * @author Dustin*/
public class GuavaEmployee
{public enum Gender{ FEMALE, MALE };private final String lastName;private final String firstName;private final String employerName;private final TraditionalEmployee.Gender gender;/*** Create an instance of me.* * @param newLastName The new last name my instance will have.* @param newFirstName The new first name my instance will have.* @param newEmployerName The employer name my instance will have.* @param newGender The gender of my instance.*/public GuavaEmployee(final String newLastName, final String newFirstName,final String newEmployerName, final TraditionalEmployee.Gender newGender){this.lastName = newLastName;this.firstName = newFirstName;this.employerName = newEmployerName;this.gender = newGender;}public String getEmployerName(){return this.employerName;}public String getFirstName(){return this.firstName;}public TraditionalEmployee.Gender getGender(){return this.gender;}public String getLastName(){return this.lastName;}/*** Using Guava to compare provided object to me for equality.* * @param obj Object to be compared to me for equality.* @return {@code true} if provided object is considered equal to me or*    {@code false} if provided object is not considered equal to me.*/@Overridepublic boolean equals(Object obj){if (obj == null){return false;}if (getClass() != obj.getClass()){return false;}final GuavaEmployee other = (GuavaEmployee) obj;return   com.google.common.base.Objects.equal(this.lastName, other.lastName)&& com.google.common.base.Objects.equal(this.firstName, other.firstName)&& com.google.common.base.Objects.equal(this.employerName, other.employerName)&& com.google.common.base.Objects.equal(this.gender, other.gender);}/*** Uses Guava to assist in providing hash code of this employee instance.* * @return My hash code.*/@Overridepublic int hashCode(){return com.google.common.base.Objects.hashCode(this.lastName, this.firstName, this.employerName, this.gender);}/*** Method using Guava to provide String representation of this employee* instance.* * @return My String representation.*/@Overridepublic String toString(){return com.google.common.base.Objects.toStringHelper(this).addValue(this.lastName).addValue(this.firstName).addValue(this.employerName).addValue(this.gender).toString();}
}

如上面的代码所示，Guava的使用提高了三种常用方法的实现的可读性。唯一不好的是需要在代码中显式定义Guava的Objects类，以避免与Java SE 7的Objects类发生命名冲突。当然，如果不是使用Java 7，那么这不是问题，如果使用Java 7，则无论如何都应该使用标准版本。

结论

Guava通过其Objects类提供了一种构建更安全，更易读的通用方法的好方法。尽管我将对JDK 7项目使用新的java.util.Objects类，但是Guava的com.google.common.base.Objects类为在JDK 7之前的Java版本中工作提供了一个不错的选择。

参考： Guava的Objects类：来自JCG合作伙伴 Dustin Marx的Equals，HashCode和ToString，来自Inspired by Actual Events博客。