EqualsBuilder和HashCodeBuilder(重写equal和hashcode)

EqualsBuilder和HashCodeBuilder

自动化hashCode()和equals()
问题产生：当需要自动实现hashCode()和equals()方法
解决方法：使用EqualsBuilder和HashCodeBuilder
使用举例：

import org.apache.commons.lang.builder.HashCodeBuilder;

import org.apache.commons.lang.builder.EqualsBuilder;

public class PoliticalCandidate {

// Member variables - omitted for brevity

// Constructors - omitted for brevity

// get/set methods - omitted for brevity

// A hashCode which creates a hash from the two unique identifiers

public int hashCode( ) {

return new HashCodeBuilder( 17 , 37 )

.append(firstName)

.append(lastName).toHashCode( );

}

// An equals which compares two unique identifiers

public boolean equals(Object o) {

boolean equals = false ;

if ( o != null && PoliticalCandidate. class .isAssignableFrom(o) ) {

PoliticalCandidate pc = (PoliticalCandidate) o;

equals = ( new EqualsBuilder( )

.append(firstName, ps.firstName)

.append(lastName, ps.lastName)).isEquals( );

}

return equals;

}

Discussion：
1.在上述例子中，当有相同的firstname和lastname时,认为两个对象的hashCode相同，从而equals()返回true.
如果hashCode取决于该class的所有filed时需要使用反射机制来产生一个hashCode。

public int hashCode( ) {

return HashCodeBuilder.reflectionHashCode( this );

}

和ToStringBuilder 与 HashCodeBuilder一样EqualsBuilder 也是使用append()方法进行配置， EqualsBuilder的append()方法可以接受基本类型、对象、数组作为参数。EqualsBuilder强大的地方在于可以直接把数组作为参数传入append()方法，EqualsBuilder会依次比较数组中的每个元素。
2.如果两个对象相等当且仅当每个属性值都相等这句话可以由以下代码实现：

public boolean equals(Object o) {

return EqualsBuilder.reflectionEquals( this , o);

}

问题提出：需要快速实现compareTo()方法
解决方法：使用CompareToBuilder提供的compareTo()方法。同样的CompareToBuilder也使用了反射机制。以下代码提供了一个compareTo()方法，用于比较两个对象所有的非static和非transient成员变量。

import org.apache.commons.lang.builder.CompareToBuilder;

// Build a compareTo function from reflection

public int compareTo(Object o) {

return CompareToBuilder.reflectionCompare( this , obj);

}

Discussion： CompareToBuilder.reflectionCompare()提供了两个对象non-static和nontransient成员变量的方法。 reflectionCompare()方法不予理会static和transient变量，因此以下代码中的averageAge和fullName变量是不会进入比较表达式的。

public class PoliticalCandidate {

// Static variable

private static String averageAge;

// Member variables

private String firstName;

private String lastName;

private transient String fullName;

// Constructors

// get/set methods

// Build a compareTo function from reflection

public int compareTo(Object o) {

return CompareToBuilder.reflectionCompare( this , obj);

}

比较对象成员变量的时候应该有一个比较的次序存在，上述代码中默认的应该是先比较lastName,然后是firstName。调用append()方法可以把要比较的变量加入比较表达式中，并且遵循后加入的先比较的次序。
例如：

public int compareTo(Object o) {

int compare = - 1 ; // By default return less-than

if ( o != null && PoliticalCandidate. class .isAssignableFrom( o.getClass( ) ) ) {

PoliticalCandidate pc = (PoliticalCandidate) o;

compare = ( new CompareToBuilder( )

.append(firstName, pc.firstName)

.append(lastName, pc.lastName)).toComparison( );

}

return compare;

}

在比较的时候会先比较lastName，只有在lastName相同的情况下才会比较firstName。
ps：实现compareTo()的时候应保证和equals()规则相同，即当compareTo()返回是0的时候equals()应该返回true。

1.1 ReflectionToStringBuilder

本笔记是在阅读Jakarta Commons Cookbook时所留下的。
1.使用ReflectionToStringBuilder 或者ToStringBuilder 自动产生toString()的内容。
使用举例：假设有一个表征校长候选人信息的javabean－PoliticalCandidate。

public class PoliticalCandidate {

private String lastName;

private String firstName;

private Date dateOfBirth;

private BigDecimal moneyRaised;

private State homeState;

// get/set方法省略

public void toString( ) {

ReflectionToStringBuilder.toString( this );

}

该bean里面有个toString()方法，假设有以下操作：

// Create a State

State va = new State( "VA", "Virginia");

// Create a Birth Date

Calendar calendar = new GregorianCalendar( );

calendar.set( Calendar.YEAR, 1743 );

calendar.set( Calendar.MONTH, Calendar.APRIL );

calendar.set( Calendar.DAY_OF_MONTH, 13 );

Date dob = calendar.getTime( );

BigDecimal moneyRaised = new BigDecimal( 293829292.93 );

// Create a Political Candidate

PoliticalCandidate candidate =

new PoliticalCandidate( "Jefferson", "Thomas", dob, moneyRaised, va );

System.out.println( candidate );

假设State对象也是一个使用ReflectionToStringBuilder的javabean，上述程序一种可能的输出为com.discursive.jccook.lang.builders.PoliticalCandidate@187aeca
    [lastName=Jefferson,\firstName=Thomas,
     dateOfBirth=Sat Apr 13 22:38:42 CST 1743,
     moneyRaised=\293829292.930000007152557373046875,
     state=\com.discursive.jccook.lang.builders.State@87816d
         [abbreviation=VA,name=Virginia]]

org.apache.commons.lang.builder
　　CompareToBuilder – 用于辅助实现Comparable.compareTo(Object)方法；
　　
　　EqualsBuilder – 用于辅助实现Object.equals()方法；
　　
　　HashCodeBuilder – 用于辅助实现Object.hashCode()方法；
　　
　　ToStringBuilder – 用于辅助实现Object.toString()方法；
　　
　　ReflectionToStringBuilder – 使用反射机制辅助实现Object.toString()方法；
　　
　　ToStringStyle – 辅助ToStringBuilder控制输出格式；
　　
　　StandardToStringStyle – 辅助ToStringBuilder控制标准格式。

自动化hashCode()和equals()
问题产生：当需要自动实现hashCode()和equals()方法
解决方法：使用EqualsBuilder和HashCodeBuilder
使用举例：

import org.apache.commons.lang.builder.HashCodeBuilder;

import org.apache.commons.lang.builder.EqualsBuilder;

public class PoliticalCandidate {

// Member variables - omitted for brevity

// Constructors - omitted for brevity

// get/set methods - omitted for brevity

// A hashCode which creates a hash from the two unique identifiers

public int hashCode( ) {

return new HashCodeBuilder( 17 , 37 )

.append(firstName)

.append(lastName).toHashCode( );

}

// An equals which compares two unique identifiers

public boolean equals(Object o) {

boolean equals = false ;

if ( o != null && PoliticalCandidate. class .isAssignableFrom(o) ) {

PoliticalCandidate pc = (PoliticalCandidate) o;

equals = ( new EqualsBuilder( )

.append(firstName, ps.firstName)

.append(lastName, ps.lastName)).isEquals( );

}

return equals;

}

public int hashCode( ) {

return HashCodeBuilder.reflectionHashCode( this );

}

public boolean equals(Object o) {

return EqualsBuilder.reflectionEquals( this , o);

}

import org.apache.commons.lang.builder.CompareToBuilder;

// Build a compareTo function from reflection

public int compareTo(Object o) {

return CompareToBuilder.reflectionCompare( this , obj);

}

public class PoliticalCandidate {

// Static variable

private static String averageAge;

// Member variables

private String firstName;

private String lastName;

private transient String fullName;

// Constructors

// get/set methods

// Build a compareTo function from reflection

public int compareTo(Object o) {

return CompareToBuilder.reflectionCompare( this , obj);

}

public int compareTo(Object o) {

int compare = - 1 ; // By default return less-than

if ( o != null && PoliticalCandidate. class .isAssignableFrom( o.getClass( ) ) ) {

PoliticalCandidate pc = (PoliticalCandidate) o;

compare = ( new CompareToBuilder( )

.append(firstName, pc.firstName)

.append(lastName, pc.lastName)).toComparison( );

}

return compare;

}

1.1 ReflectionToStringBuilder

public class PoliticalCandidate {

private String lastName;

private String firstName;

private Date dateOfBirth;

private BigDecimal moneyRaised;

private State homeState;

// get/set方法省略

public void toString( ) {

ReflectionToStringBuilder.toString( this );

}

该bean里面有个toString()方法，假设有以下操作：

// Create a State

State va = new State( "VA", "Virginia");

// Create a Birth Date

Calendar calendar = new GregorianCalendar( );

calendar.set( Calendar.YEAR, 1743 );

calendar.set( Calendar.MONTH, Calendar.APRIL );

calendar.set( Calendar.DAY_OF_MONTH, 13 );

Date dob = calendar.getTime( );

BigDecimal moneyRaised = new BigDecimal( 293829292.93 );

// Create a Political Candidate

PoliticalCandidate candidate =

new PoliticalCandidate( "Jefferson", "Thomas", dob, moneyRaised, va );

System.out.println( candidate );