Java Builder模式

Builder模式的作用是将一个复杂的构建与它的表示分离，使得同样的构建过程可以创建不同的表示。它可以不直接生成想要的对象。然后客户端使用者在builder对象上调用类似于setter的方法，来设置每个相关的可选参数。最后，使用者调用无参的build方法来生成不可变的对象。这个builder是它构建类的静态类。示例：

class Person {
  private int id;
  private String name;
  private int age;
  private double weight;
  private int score;
  private Location loc;
  private Person() {
  }
  
  public static class PersonBuilder {
     private Person p  = new Person();
     public PersonBuilder baseInfo(int id,int age,String name){
            p.id = id;
            p.age = age;
            p.name = name;
            return this;
        }

        public PersonBuilder weight(double weight){
            p.weight = weight;
            return this;
        }

        public PersonBuilder score(int score){
            p.score = score;
            return this;
        }

        public PersonBuilder loc(String street,String roomNo){
            p.loc = new Location(street,roomNo);
            return this;
        }

        public Person build(){
            return p;
        }
  }
}
class Location {
  String street;
  String roomNo;
  public Location(String street,String roomNo) {
    this.street = street;
    this.roomNo = roomNo;
  }
}

class Test {
  public static void main(String[] args) {
    Person p = new Person.PersonBuilder()
                          .baseInfo(10,20,"Tom")
                          .weight(60.0)
                          .loc("南一道","25")
                          .build();
  }
}

注意Person是不可变的，所有的默认参数值都单独放在一个地方。builder的setter方法返回builder本身，以便可以把调用链接起来。如上面代码所示，这样客户端的代码就很容易编写而且易于阅读。