Java類設(shè)計(jì)過(guò)程中，如果類的構(gòu)造器或者靜態(tài)工廠中具有多個(gè)參數(shù)灸撰，并且其中有大量的可選參數(shù)時(shí)葵礼，我們應(yīng)該怎么辦烧栋？
1.Telescoping Constructor模式(重疊構(gòu)造器)
我們首先想到的方法肯定是傳統(tǒng)的構(gòu)造器
/**
* Created by itbird on 2017/3/23
*/
public class Person {
private String name;
private String sex;
private int year;
public Person(String name, String sex, int year) {
this.name = name;
this.sex = sex;
this.year = year;
}
}
但是設(shè)計(jì)以及使用過(guò)程中我們發(fā)現(xiàn)以下幾點(diǎn)問(wèn)題：
（1）屬性參數(shù)逐漸變多時(shí)
屬性參數(shù)逐漸變多時(shí)，由于要對(duì)之前的代碼做兼容苇倡，所以不可以直接在現(xiàn)有構(gòu)造器后面追加屬性富纸，只能不斷新增構(gòu)造器
/**
* Created by itbird on 2017/3/23
*/
public class Person {
private String name;
private String sex;
private int year;
private String city;
public Person(String name, String sex, int year) {
this.name = name;
this.sex = sex;
this.year = year;
}
public Person(String name, String sex, int year, String city) {
this.name = name;
this.sex = sex;
this.year = year;
this.city = city;
}
}
（2）屬性參數(shù)中有大量的可選參數(shù)
/**
* Created by itbird on 2017/3/23
*/
public class Person {
private String name;
private String sex;
private int year;
private String city;
private String state;
private boolean isFemale;
private boolean isEmployed;
private boolean isHomewOwner;
public Person(String name, String sex, int year) {
this.name = name;
this.sex = sex;
this.year = year;
}
public Person(String name, String sex, int year, String city) {
this.name = name;
this.sex = sex;
this.year = year;
this.city = city;
}
public Person(String name, String sex, int year, String city, String newState,
boolean newIsFemale, boolean newIsEmployed, boolean newIsHomeOwner) {
this.name = name;
this.sex = sex;
this.year = year;
this.city = city;
this.state = newState;
this.isFemale = newIsFemale;
this.isEmployed = newIsEmployed;
this.isHomewOwner = newIsHomeOwner;
}
}
顯而易見(jiàn)，這樣寫的類構(gòu)造器雖然無(wú)可厚非旨椒，但是當(dāng)有許多參數(shù)的時(shí)候晓褪，客戶端代碼會(huì)很難編寫，并且難以閱讀综慎。如果讀者想知道那些值是什么意思涣仿，必須很仔細(xì)的數(shù)著這些參數(shù)來(lái)探個(gè)究竟。
2.JavaBeans模式
在這種模式下寥粹，調(diào)用一個(gè)無(wú)參構(gòu)造器來(lái)創(chuàng)建對(duì)象变过，然后調(diào)用setter方法來(lái)設(shè)置每個(gè)必要的參數(shù)埃元，以及每個(gè)相關(guān)的可選參數(shù)。
/**
* Created by itbird on 2017/3/23
*/
public class Person {
private String name;
private String sex;
private int year;
private String city;
private String state;
private boolean isFemale;
private boolean isEmployed;
private boolean isHomewOwner;
public Person() {
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getSex() {
return sex;
}
public void setSex(String sex) {
this.sex = sex;
}
public int getYear() {
return year;
}
public void setYear(int year) {
this.year = year;
}
public String getCity() {
return city;
}
public void setCity(String city) {
this.city = city;
}
public String getState() {
return state;
}
public void setState(String state) {
this.state = state;
}
public boolean isFemale() {
return isFemale;
}
public void setFemale(boolean female) {
isFemale = female;
}
public boolean isEmployed() {
return isEmployed;
}
public void setEmployed(boolean employed) {
isEmployed = employed;
}
public boolean isHomewOwner() {
return isHomewOwner;
}
public void setHomewOwner(boolean homewOwner) {
isHomewOwner = homewOwner;
}
}
這種模式彌補(bǔ)重疊構(gòu)造器模式的不足媚狰。說(shuō)的明白一點(diǎn)岛杀，就是創(chuàng)建實(shí)例很容易，這樣產(chǎn)生的代碼讀起來(lái)也很容易:
Person person = new Person();
person.setCity("重慶");
person.setYear(12);
person.setSex("男");
person.setName("itbird");
遺憾的是崭孤，JavaBeans模式自身有著很嚴(yán)重的缺點(diǎn)类嗤。因?yàn)闃?gòu)造過(guò)程被分到幾個(gè)調(diào)用中，在構(gòu)造過(guò)程中JavaBean可能處于非一致的狀態(tài)辨宠。JavaBeans模式阻止了把類做成不可變的可能遗锣，這就需要程序員付出額外的努力來(lái)確保他的線程安全。
3.Builder模式
/**
* Created by itbird on 2017/3/23
*/
public class Person {
private String name;
private String sex;
private int year;
private String city;
private String state;
private boolean isFemale;
private boolean isEmployed;
private boolean isHomewOwner;
public Person() {
}
public static class PersonBuilder {
// 必要參數(shù)
private String name;
// 可選參數(shù)
private String sex;
private int year;
private String city;
private String state;
private boolean isFemale;
private boolean isEmployed;
private boolean isHomewOwner;
public PersonBuilder(String name) {
this.name = name;
}
public PersonBuilder setSex(String sex) {
this.sex = sex;
return this;
}
public PersonBuilder setYear(int year) {
this.year = year;
return this;
}
public PersonBuilder setCity(String city) {
this.city = city;
return this;
}
public PersonBuilder setState(String state) {
this.state = state;
return this;
}
public PersonBuilder setFemale(boolean female) {
isFemale = female;
return this;
}
public PersonBuilder setEmployed(boolean employed) {
isEmployed = employed;
return this;
}
public PersonBuilder setHomewOwner(boolean homewOwner) {
isHomewOwner = homewOwner;
return this;
}
public Person build() {
Person person = new Person();
person.name = name;
person.sex = sex;
person.city = city;
person.isEmployed = isEmployed;
person.isFemale = isFemale;
person.isHomewOwner = isHomewOwner;
person.state = state;
person.year = year;
return person;
}
}
}
調(diào)用的實(shí)例:
Person person = new Person.PersonBuilder("itbird")
.setCity("重慶").setYear(15).build();
顯然嗤形，使用Builder模式解決了上訴的難題精偿，達(dá)到了“以不變(Builder)應(yīng)萬(wàn)變(參數(shù))”的目的。
總結(jié)：
Java類設(shè)計(jì)過(guò)程中赋兵，如果類的構(gòu)造器或者靜態(tài)工廠中具有多個(gè)參數(shù)笔咽，并且其中有大量的可選參數(shù)時(shí)，我們應(yīng)該考慮使用構(gòu)建器霹期，與傳統(tǒng)的重疊構(gòu)造器模式相比叶组，使用Builder模式的代碼更易于閱讀和編寫，有更好的可擴(kuò)展性历造，同時(shí)構(gòu)建器也比JavaBeans更加安全甩十。
從最后的實(shí)例演練中我們知道，在實(shí)際開(kāi)發(fā)過(guò)程中吭产，我們往往是需要幾種模式混合使用侣监，這樣才能在保證代碼健壯性、可讀性的同時(shí)臣淤，去保證代碼的可擴(kuò)展性以及線程安全性等达吞，這才是代碼設(shè)計(jì)開(kāi)發(fā)過(guò)程中真正的“以不變應(yīng)萬(wàn)變”之道。
