使用Builder模式创建对象

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创建对象的方法很多, 常见的有构造函数, 静态工厂方法, 工厂方法等.

以上三种方法不适合创建需要提供多个参数的对象, 而其中某些参数是可选的场景.

考虑一个Person类:

public class Person {
// 必须的参数
private int id;
private String name;
private int age;

// 可选参数
private String city;
private String hoby;
private String job;
private String ***;
}

public class Person { // 必须的参数 private int id; private String name; private int age; // 可选参数 private String city; private String hoby; private String job; private String ***;}

如果给这个Person类提供合适的构造函数, 应该怎么做呢?

也许我们可以提供多个构造函数, 每个构造函数都包含必须的参数, 以及0个或多个可选参数. 例如:

public Person(int pid, String name, int age) {
this.id = pid;
this.name = name;
this.age = age;
}

public Person(int id, String name, int age, String city) {
this.id = id;
this.name = name;
this.age = age;
this.city = city;
}

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

public Person(int id, String name, int age, String city, String hoby, String job) {
this.id = id;
this.name = name;
this.age = age;
this.city = city;
this.hoby = hoby;
this.job = job;
}

public Person(int id, String name, int age, String city, String hoby, String job, String ***) {
this.id = id;
this.name = name;
this.age = age;
this.city = city;
this.hoby = hoby;
this.job = job;
this.*** = ***;
}

public Person(int pid, String name, int age) { this.id = pid; this.name = name; this.age = age;} public Person(int id, String name, int age, String city) { this.id = id; this.name = name; this.age = age; this.city = city;} public Person(int id, String name, int age, String city, String hoby) { super(); this.id = id; this.name = name; this.age = age; this.city = city; this.hoby = hoby;} public Person(int id, String name, int age, String city, String hoby, String job) { this.id = id; this.name = name; this.age = age; this.city = city; this.hoby = hoby; this.job = job;}public Person(int id, String name, int age, String city, String hoby, String job, String ***) { this.id = id; this.name = name; this.age = age; this.city = city; this.hoby = hoby; this.job = job; this.*** = ***;}

这种方法至少存在这样的问题:

1. 如果需要创建一个只指定city和***的Person对象, 我们就必须调用public Person(int id, String name, int age, String city, String hoby, String job, String ***)方法, 然后将hoby, job都赋值为空字符串. 这是让人疑惑的, 也是丑陋的.

2. 这样的代码难以阅读, 而且臃肿不堪, 不具备良好的可伸缩性.

当然我们可以同时提供属性的getter和setter方法, 然后只保留Person(int pid, String name, int age)一个构造函数.

如此的话, 我们先通过Person(int pid, String name, int age)方法创建Person对象, 然后调用需要的setter方法给可选参数赋值.

这是不错的解决方法, 但是创建对象被分成了几个步骤, 这会带来问题: 如果一不小心可能会让将尚未完全创建的Person对象泄露出去--结果是严重的.

Builder设计模式就非常适合这样的场景:

public class Person {
// 必须的参数
private int id;
private String name;
private int age;

// 可选参数
private String city;
private String hoby;
private String job;
private String ***;

// 私有构造函数
private Person(Builder builder) {
this.id = builder.id;
this.name = builder.name;
this.age = builder.age;
this.city = builder.city;
this.hoby = builder.hoby;
this.job = builder.job;
this.*** = builder.***;
}

public static class Builder {
// 必须的参数
private int id;
private String name;
private int age;

// 可选参数
private String city;
private String hoby;
private String job;
private String ***;

// 设置必须的参数
public Builder(int id, String name, int age) {
this.id = id;
this.name = name;
this.age = age;
}

// 设置可选参数, 返回Builder对象可以让调用者使用级联
public Builder setCity(String city) {
this.city = city;
return this;
}

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

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

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

public Person build() {
return new Person(this);
}
}
}

public class Person { // 必须的参数 private int id; private String name; private int age; // 可选参数 private String city; private String hoby; private String job; private String ***; // 私有构造函数 private Person(Builder builder) { this.id = builder.id; this.name = builder.name; this.age = builder.age; this.city = builder.city; this.hoby = builder.hoby; this.job = builder.job; this.*** = builder.***; } public static class Builder { // 必须的参数 private int id; private String name; private int age; // 可选参数 private String city; private String hoby; private String job; private String ***; // 设置必须的参数 public Builder(int id, String name, int age) { this.id = id; this.name = name; this.age = age; } // 设置可选参数, 返回Builder对象可以让调用者使用级联 public Builder setCity(String city) { this.city = city; return this; } public Builder setHoby(String hoby) { this.hoby = hoby; return this; } public Builder setJob (String job) { this.job = job; return this; } public Builder set***(String ***) { this.*** = ***; return this; } public Person build() { return new Person(this); } }}

如果需要一个指定city和***的Person对象, 就可以这样创建:

Person.Builder builder = new Person.Builder(1, "xing", 25);
Person person = builder.setCity("beijing").set***().build();

Person.Builder builder = new Person.Builder(1, "xing", 25);Person person = builder.setCity("beijing").set***().build();

在调用build方法完成之前, Person对象都没有被创建, 所以无需担心提前泄露的问题, 而且使用级联的方式使得代码简洁优雅.

总结: Builder模式只适合创建对象时需要指定多个可选参数的场景.