Effective Java 学习笔记（二）

【第2条：遇到多个构造器参数时要考虑用构建器】

遇到参数较多时，静态工厂方法不容易扩展。那么一般来说重叠构造器可能是个好的做法，但是遇到参数多的情况下，需要重叠很多构造器。

代码如下：

public class Person {
// 必须
private String name;
// 必须
private String sex;
// 可选
private int age;
// 可选
private String tel_phone;
// 可选
private String mobile_phone;
// 可选
private String address;

public Person(String name, String sex) {
super();
this.name = name;
this.sex = sex;
}

public Person(String name, String sex, int age) {
super();
this.name = name;
this.sex = sex;
this.age = age;
}

public Person(String name, String sex, int age, String tel_phone) {
super();
this.name = name;
this.sex = sex;
this.age = age;
this.tel_phone = tel_phone;
}

public Person(String name, String sex, int age, String tel_phone, String mobile_phone) {
super();
this.name = name;
this.sex = sex;
this.age = age;
this.tel_phone = tel_phone;
this.mobile_phone = mobile_phone;
}

public Person(String name, String sex, int age, String tel_phone, String mobile_phone, String address) {
super();
this.name = name;
this.sex = sex;
this.age = age;
this.tel_phone = tel_phone;
this.mobile_phone = mobile_phone;
this.address = address;
}
}可以看到需要重叠多个构造器，那么如果参数增加到20多个呢。显然继续重叠不是一个好办法。
那么我们看看第二种setter的方式。

public class Person {
// 必须
private String name;
// 必须
private String sex;
// 可选
private int age;
// 可选
private String tel_phone;
// 可选
private String mobile_phone;
// 可选
private String address;

public Person(String name, String sex) {
super();
this.name = name;
this.sex = sex;
}

public Person(String name, String sex, int age) {
super();
this.name = name;
this.sex = sex;
this.age = age;
}

public Person(String name, String sex, int age, String tel_phone) {
super();
this.name = name;
this.sex = sex;
this.age = age;
this.tel_phone = tel_phone;
}

public Person(String name, String sex, int age, String tel_phone, String mobile_phone) {
super();
this.name = name;
this.sex = sex;
this.age = age;
this.tel_phone = tel_phone;
this.mobile_phone = mobile_phone;
}

public Person(String name, String sex, int age, String tel_phone, String mobile_phone, String address) {
super();
this.name = name;
this.sex = sex;
this.age = age;
this.tel_phone = tel_phone;
this.mobile_phone = mobile_phone;
this.address = address;
}
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 getAge() {
return age;
}

public void setAge(int age) {
this.age = age;
}

public String getTel_phone() {
return tel_phone;
}

public void setTel_phone(String tel_phone) {
this.tel_phone = tel_phone;
}

public String getMobile_phone() {
return mobile_phone;
}

public void setMobile_phone(String mobile_phone) {
this.mobile_phone = mobile_phone;
}

public String getAddress() {
return address;
}

public void setAddress(String address) {
this.address = address;
}

}实例化的时候，如下代码
Person person = new Person();
person.setName("upxiaofeng");
person.setAge(27);
person.setSex("man");
person.setTel_phone("010-12345678");
person.setMobile_phone("17012345678");
person.setAddress("未知");在《Effective Java》中指出的了可能出现的错误：因为在构造过程中，被分配到了几个调用中，有可能会导致JavaBean可能处于不一致的状态。
也就是说在使用settter的时候，参数多的时候，有可能会出现构造了不同的不是我们需要的对象。

至此，《Effective Java》中提出了Builder模式（构建模式）。代码如下：

public class Person {
// 必须
private String name;
// 必须
private String sex;
// 可选
private int age;
// 可选
private String tel_phone;
// 可选
private String mobile_phone;
// 可选
private String address;

// 静态成员类
public static class Builder {
private String name;
private String sex;
private int age = 0;
private String tel_phone = "默认";
private String mobile_phone = "默认";
private String address = "默认";

public Builder(String name, String sex) {
super();
this.name = name;
this.sex = sex;
}

public Builder age(int age) {
this.age = age;
return this;
}

public Builder tel_phone(String tel_phone) {
this.tel_phone = tel_phone;
return this;
}

public Builder mobile_phone(String mobile_phone) {
this.mobile_phone = mobile_phone;
return this;
}

public Builder address(String address) {
this.address = address;
return this;
}
public Person builder(){
return new Person(this);
}
}

private Person(Builder builder) {
name = builder.name;
sex = builder.sex;
age = builder.age;
tel_phone = builder.tel_phone;
mobile_phone = builder.mobile_phone;
address = builder.address;
}

}实例化的时候，代码如下：
Person person = new Person.Builder("upxiaofeng", "man").age(27).address("未知").builder();这样看来代码更容易编写，且builder对参数加了约束，是否在当前的builder对象域当中。
但是builder模式无疑使增加了代码量，创建对象之前，必须先创建构建器。虽然创建构建起的开销在实践中可能不那么明显

但是在一些对性能要求很高的情况下，就可能成为问题。

用书中的一句话总结：

如果类的构造器或者静态工厂中具有多个参数，设计这种类中，Builder模式就是中不错的选中，特别是在大多数参数都是可选的时候。