当遇到构造器有多个参数时，考虑使用构建器

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重叠构造器

静态工厂方法和构造器都有一个共同的局限：它们都不能很好地扩展到大量的可选参数。此时一个不错的方案就是重叠构造器模式，此模式的机制原理是：创建第一个构造方法包含必须参数，第二个构造器方法包含一个可选参数，第三个包含两个可选参数，以此类推最后一个构造器包含所有的可选参数。
但是呢，重叠构造器模式虽然可行，但是但遇到大量可选参数时，客户端代码会很难写，而且比较难以阅读。如果API文档缺乏对各参数的描述，客户端代码会很容易因为参数顺序不对而出现意想不到的结果。
代码展示：

package com.stone.designpattern.builder;/*** @author Stone Wang* 重叠构造器的代码展示**/
public class Person {private String name;private String phone;private short age = 0;private String address = "";private String email = "";public Person(final String name, final String phone) {this(name, phone, (short)0);}public Person(final String name, final String phone, short age) {this(name, phone, age, "");}public Person(final String name, final String phone, short age,final String address) {this(name, phone, age, address, "");}public Person(final String name, final String phone, short age,final String address, final String email) {this.name = name;this.phone = phone;this.age = age;this.address = address;this.email = email;}public String getName() {return name;}public String getPhone() {return phone;}public short getAge() {return age;}public String getAddress() {return address;}public String getEmail() {return email;}
}

JavaBean模式

鉴于重叠构造器造成客户端代码可读性不好的局限，引入Java bean模式。此模式会提供一个带有所有必要参数的构造器和一系列的setter方法，客户端调用最小参数构造器生成类的实例，然后调用setter方法设置可选参数。
代码展示：

package com.stone.designpattern.builder;/*** @author Stone Wang* JavaBean模式的代码展示**/
public class PersonJavaBean {private String name;private String phone;private short age = 0;private String address = "";private String email = "";public PersonJavaBean(final String name, final String phone) {this.name = name;this.phone = phone;}public String getName() {return name;}public String getPhone() {return phone;}public short getAge() {return age;}public String getAddress() {return address;}public String getEmail() {return email;}public void setName(final String name) {this.name = name;}public void setPhone(final String phone) {this.phone = phone;}public void setAge(short age) {this.age = age;}public void setAddress(final String address) {this.address = address;}public void setEmail(final String email) {this.email = email;}
}

此模式在多线程的情况下会导致对象状态不一致的线程安全问题， 而且还破坏了JavaBean对象不可变的特性。

构建器

鉴于重叠构造器和JavaBean的局限，引入构构建器模式。
它不直接生成想要的对象，而是让客户端利用所有必要的参数调用构造器（或者静态工厂方法）， 得到一个builder对象，然后客户端在builder对象上调用类似setter 方法来设置可选参数。

代码展示：

package com.stone.designpattern.builder;/*** @author Stone Wang* 构建器模式的代码展示**/
public class PersonBuilder {private String name;private String phone;private short age = 0;private String address = "";private String email = "";public PersonBuilder(Builder builder) {this.name = builder.name;this.phone = builder.phone;this.age = builder.age;this.address = builder.address;this.email = builder.email;}public String getName() {return name;}public String getPhone() {return phone;}public short getAge() {return age;}public String getAddress() {return address;}public String getEmail() {return email;}public static class Builder{private String name;private String phone;private short age = 0;private String address = "";private String email = "";public Builder(final String name, final String phone) {this.name = name;this.phone = phone;}public Builder setAge(short age) {this.age = age;return this;}public Builder setAddress(final String address) {this.address = address;return this;}public Builder setEmail(final String email) {this.email = email;return this;}public PersonBuilder build() {return new PersonBuilder(this);}}
}

客户端代码

package com.stone.designpattern.builder;/*** @author Stone Wang*客户端代码展示*/
public class Main {public static void main(String[] args) {// 重叠构造器的客户端Person p = new Person("Stone", "1888886688", (short)18);// JavaBean模式的客户端PersonJavaBean personJavaBean = new PersonJavaBean("Stone", "1888886688");personJavaBean.setAge((short)18);//构建器的客户端代码PersonBuilder.Builder builder = new PersonBuilder.Builder("Stone", "1888886688");PersonBuilder pb = builder.setAge((short)18).setAddress("Shanghai").build();}}

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