EqualsBuilder和HashCodeBuilder
/*
* Created on Jan 29, 2015
*
* Copyright 2006 ATPCO Confidential and Proprietary. All Rights Reserved.
*/
package com.atpco.textload.util;
import java.text.DecimalFormat;
import java.util.ArrayList;
import java.util.List;
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;
}
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;
}
PoliticalCandidate pc = (PoliticalCandidate) o;
compare = ( new CompareToBuilder( )
.append(firstName, pc.firstName)
.append(lastName, pc.lastName)).toComparison( );
}
return compare;
}
import org.apache.commons.lang.builder.EqualsBuilder;
import org.apache.commons.lang.builder.HashCodeBuilder;
import org.apache.commons.lang.builder.ToStringBuilder;
import org.apache.commons.lang.builder.ToStringStyle;
import bsh.This;
// get/set方法省略
public void toString( ) {
ReflectionToStringBuilder.toString( this );
}
}
PoliticalCandidate pc = (PoliticalCandidate) o;
compare = ( new CompareToBuilder( )
.append(firstName, pc.firstName)
.append(lastName, pc.lastName)).toComparison( );
}
return compare;
}
// 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 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 );
// Create a Political Candidate
PoliticalCandidate candidate =
new PoliticalCandidate( "Jefferson", "Thomas", dob, moneyRaised, va );
System.out.println( candidate );
public class Test {
}
public static void main(String[] args) throws Exception {
MyClass one = new MyClass("Becker", 35);
MyClass two = new MyClass("Becker", 35);
MyClass three = new MyClass("Agassi", 33);
System.out.println("One>>>" + one);
System.out.println("Two>>>" + two);
System.out.println("Three>>>" + three);
}
System.out.println("one equals two? " + one.equals(two));
System.out.println("one equals three? " + one.equals(three));
System.out.println("One HashCode>>> " + one.hashCode());
System.out.println("Two HashCode>>> " + two.hashCode());
System.out.println("Three HashCode>>> " + three.hashCode());
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);
}
}
}
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);
}
}
}
class MyClass {
private String name = null;
private int age = 0;
public MyClass(String name, int age) {
this.name = name;
this.age = age;
}
public boolean equals(Object obj) {
return EqualsBuilder.reflectionEquals(this, obj);
}
public String toString() {
return ToStringBuilder.reflectionToString(this,
ToStringStyle.MULTI_LINE_STYLE);
}
public int hashCode() {
return HashCodeBuilder.reflectionHashCode(this);
}
}
===============================================
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：
package
com.atpco.textload.util;
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。
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。
CompareToBuilder �C 用于辅助实现Comparable.compareTo(Object)方法；
EqualsBuilder �C 用于辅助实现Object.equals()方法；
HashCodeBuilder �C 用于辅助实现Object.hashCode()方法；
ToStringBuilder �C 用于辅助实现Object.toString()方法；
ReflectionToStringBuilder �C 使用反射机制辅助实现Object.toString()方法；
ToStringStyle �C 辅助ToStringBuilder控制输出格式；
StandardToStringStyle �C 辅助ToStringBuilder控制标准格式。
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;
该bean里面有个toString()方法，假设有以下操作
http://www.blogjava.net/19851985lili/articles/95448.html
// 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]]