Java类设计过程中，如果类的构造器或者静态工厂中具有多个参数，并且其中有大量的可选参数时，我们应该怎么办？
1.Telescoping Constructor模式(重叠构造器)
我们首先想到的方法肯定是传统的构造器
/**
* 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;
}
}
但是设计以及使用过程中我们发现以下几点问题：
（1）属性参数逐渐变多时
属性参数逐渐变多时，由于要对之前的代码做兼容，所以不可以直接在现有构造器后面追加属性，只能不断新增构造器
/**
* 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）属性参数中有大量的可选参数
/**
* 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;
}
}
显而易见，这样写的类构造器虽然无可厚非，但是当有许多参数的时候，客户端代码会很难编写，并且难以阅读。如果读者想知道那些值是什么意思，必须很仔细的数着这些参数来探个究竟。
2.JavaBeans模式
在这种模式下，调用一个无参构造器来创建对象，然后调用setter方法来设置每个必要的参数，以及每个相关的可选参数。
/**
* 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;
}
}
这种模式弥补重叠构造器模式的不足。说的明白一点，就是创建实例很容易，这样产生的代码读起来也很容易:
Person person = new Person();
person.setCity("重庆");
person.setYear(12);
person.setSex("男");
person.setName("itbird");
遗憾的是，JavaBeans模式自身有着很严重的缺点。因为构造过程被分到几个调用中，在构造过程中JavaBean可能处于非一致的状态。JavaBeans模式阻止了把类做成不可变的可能，这就需要程序员付出额外的努力来确保他的线程安全。
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 {
// 必要参数
private String name;
// 可选参数
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;
}
}
}
调用的实例:
Person person = new Person.PersonBuilder("itbird")
.setCity("重庆").setYear(15).build();
显然，使用Builder模式解决了上诉的难题，达到了“以不变(Builder)应万变(参数)”的目的。
总结：
Java类设计过程中，如果类的构造器或者静态工厂中具有多个参数，并且其中有大量的可选参数时，我们应该考虑使用构建器，与传统的重叠构造器模式相比，使用Builder模式的代码更易于阅读和编写，有更好的可扩展性，同时构建器也比JavaBeans更加安全。
从最后的实例演练中我们知道，在实际开发过程中，我们往往是需要几种模式混合使用，这样才能在保证代码健壮性、可读性的同时，去保证代码的可扩展性以及线程安全性等，这才是代码设计开发过程中真正的“以不变应万变”之道。
