Question

我正在阅读Effective Java书籍，并为我将来的参考创建笔记，我遇到了Builder Pattern。

我明白它是什么以及如何使用它。在这个过程中，我创建了两个构建模式的变体。

我需要帮助列出每个人的差异和优势吗？好吧，我当然注意到，Example 1暴露的方法较少，而且限制性较小更通用的，允许它更灵活地使用。

请指出我错过的其他事情？

示例1

package item2;

/**
 * @author Sudhakar Duraiswamy
 *
 */
public  class Vehicle {

    private String type;
    private int wheels;

    interface Builder<T>{
        public  T build();
    }

    public static class CarBuilder implements Builder<Vehicle>{
        private String type;
        private int wheels;     

        CarBuilder createVehicle(){
            this.type= "Car";
            return this;
        }

        CarBuilder addWheels(int wheels){
            this.wheels = wheels;
            return this;
        }

        public Vehicle build(){
            Vehicle v = new Vehicle();
            v.type = type;
            v.wheels = wheels;
            return v;
        }               
    }

    public static class TruckBuilder implements Builder<Vehicle>{       
        private String type;
        private int wheels; 

        TruckBuilder createVehicle(){           
            this.type= "Truck";
            return this;
        }

        TruckBuilder addWheels(int wheels){
            this.wheels = wheels;
            return this;
        }

        public Vehicle build(){
            Vehicle v = new Vehicle();
            v.type = type;
            v.wheels = wheels;
            return v;
        }
    }   

    public Vehicle(){

    }

    public static void main(String[] args) {
        //This builds a car with 4 wheels
        Vehicle car = new Vehicle.CarBuilder().createVehicle().addWheels(4).build();

        //THis builds a Truck with 10 wheels
        Vehicle truck = new Vehicle.TruckBuilder().createVehicle().addWheels(10).build();

    }
}

示例2

package item2;
/**
 * @author Sudhakar Duraiswamy
 *
 */
public  class Vehicle2 {

    private String type;
    private int wheels;

    interface Builder<T>{
        public  T build();      
        public String getType();
        public int getWheels() ;
    }

    public static class CarBuilder implements Builder<Vehicle2>{
        private String type;
        private int wheels;     

        public String getType() {
            return type;
        }
        public int getWheels() {
            return wheels;
        }

        CarBuilder createVehicle(){
            this.type= "Car";
            return this;
        }

        CarBuilder addWheels(int wheels){
            this.wheels = wheels;
            return this;
        }

        public Vehicle2 build(){        
            return new Vehicle2(this);
        }               
    }

    public static class TruckBuilder implements Builder<Vehicle2>{      
        private String type;
        private int wheels; 

        public String getType() {
            return type;
        }

        public int getWheels() {
            return wheels;
        }

        TruckBuilder createVehicle(){           
            this.type= "Truck";
            return this;
        }

        TruckBuilder addWheels(int wheels){
            this.wheels = wheels;
            return this;
        }

        public Vehicle2 build(){
            return new Vehicle2(this);
        }
    }


public Vehicle2(Builder<? extends Vehicle2> builder){
    Vehicle2 v = new Vehicle2();
    v.type = builder.getType();
    v.wheels = builder.getWheels();
}

    public Vehicle2(){
    }

    public static void main(String[] args) {            
        //This builds a car with 4 wheels
        Vehicle2 car = new Vehicle2.CarBuilder().createVehicle().addWheels(4).build();

        //THis builds a Truck with 10 wheels
        Vehicle2 truck = new Vehicle2.TruckBuilder().createVehicle().addWheels(10).build();
    }
}

Answer 1

以上都不是。

第一个不允许构建不可变的Vehicle，这通常是使用Builder模式的原因。

第二个示例是第一个示例的变体，它允许使用其他getter方法从构建器获取信息。但是那些方法不会在任何地方使用，除了在Vehicle构造函数中，它可以直接访问构建器字段。我没有看到添加它们的重点。

我认为有两件更重要的事情需要改进：

两种构建器类型完全相同。不需要两种类型。一个就足够了。
createVehicle()方法应该由构建器构造函数完成。如果你构建一个CarBuilder，显然是要建造一辆汽车，所以一旦构建了构建器，就应该设置车辆的类型。这是我写它的方式：

public final class Vehicle {

    private final String type;
    private final int wheels;

    private Vehicle(Builder builder) {
        this.type = builder.type;
        this.wheels = builder.wheels;
    }

    public static Builder carBuilder() {
        return new Builder("car");
    }

    public static Builder truckBuilder() {
        return new Builder("truck");
    }

    public static class Builder {
        private final String type;
        private int wheels;

        private Builder(String type) {
            this.type = type;
        }

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

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

    public static void main(String[] args) {
        Vehicle car = Vehicle.carBuilder().addWheels(4).build();
        Vehicle truck = Vehicle.truckBuilder().addWheels(10).build();
    }
}

Answer 2

还有第三种变体，代码较少：

构建器也可以改变Vehicle的状态，而不是拥有自己的实例字段。内部类可以编写其外部类的私有成员：

class Vehicle {
  private int wheels;

  private Vehicle() {}

  public static class Builder {
    private boolean building = true;
    private Vehicle vehicle = new Vehicle();

    public Builder buildWheels(int wheels) {
      if(!this.building) throw new IllegalStateException();
      this.vehicle.wheels = wheels;
      return this;
    }

    public Vehicle build() {
      this.building = false;
      return this.vehicle;
    }
  }
}

由于这些字段是私有的，并且您只允许一次构建（building标志），因此构建的Vehicle实例仍然是不可变的对于消费者来说，即使字段不再是final（C＃上不再有realio-trulio immutability, see Eric's blog article，但概念类似）。

您需要更加小心，因为在构造对象期间不需要初始化非最终字段（由编译器强制执行），您必须仔细检查building状态。但是，您可以保存所有实例字段的完整额外副本。一般来说，如果你有一组相当大的实例变量，这些变量是用很少的方法构建的，每个方法一次构建几个字段，这很有用。

我知道这并未指出您的方法的任何优点或缺点。但是，如果您不需要将字段设置为final，则此方法可以节省大量额外代码。

Builder模式：首选哪种变体？

2 个答案: