java - 只有一个类的构建器模式

Question

我正在学习构建器模式。我有一个问题，为什么不使用一个类(例如:MyCoffe)而不是 2 个类(例如:MyCoffe、BuilderClass)。感谢您的阅读。

package Creational.Builder;

public class MyCoffe {

        private String name;
        private int id;

        public MyCoffe(){

        }

        public MyCoffe setName(String newName){
            this.name  = newName;
            return this;
        }

        public MyCoffe setId(int newId){
            this.id = newId;
            return this;
        }

        public MyCoffe build(){
            return this;
        }

        public String toString(){
            return name + "/" + id;
        }

        public static void main(String[] args){
            MyCoffe myCoffe = new MyCoffe().setName("HelloWorld").setId(9999);  
            System.out.println(myCoffe);

            System.out.println("Thanks for help!");
        }
}

Answer 1

在某些特定情况下，您当然可以，但构建器模式的目的之一是避免类的无效实例。所以除非它对 MyCoffe 有效没有名称或 ID，您不希望它的实例在没有名称或 ID 的情况下运行，如下所示:

MyCoffe invalid = new MyCoffe();

另请注意，当类(class)像那样自建时，很容易忘记调用最终的 build。方法(事实上，您在 main 中所做的)，因此该实例永远不会得到验证。

因此在构建完整、有效的实例之前使用单独的构建器来保存不完整的信息。

在 Java 中，构建器是静态嵌套类的情况并不少见:

public class MyCoffe {

    public static class Builder {
        private String name;
        private int id;

        public Builder() {
        }

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

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

        public MyCoffe build() {
            if (this.name == null) {
                throw new AppropriateException("'name' is required");
            }
            if (this.id == 0) { // Assumes 0 isn't a valid ID
                throw new AppropriateException("'id' is required");
            }
            return new MyCoffe(name, id);
        }
    }

    private String name;
    private int id;

    public MyCoffe(String name, int id) {
        this.name = name;
        this.id = id;
    }

    @Override
    public String toString(){
        return name + "/" + id;
    }

    public static void main(String[] args){
        MyCoffe myCoffe = new MyCoffe.Builder().setName("HelloWorld").setId(9999).build();
        System.out.println(myCoffe);

        System.out.println("Thanks for help!");
    }
}

或者如果像那样声明属性两次让您感到困扰，MyCoffe可以允许私有(private)不完整的实例，像这样:

public class MyCoffe {

    public static class Builder {
        private MyCoffe instance;

        public Builder() {
            this.instance = new MyCoffe();
        }

        public Builder setName(String name) {
            this.instance.name = name;
            return this;
        }

        public Builder setId(int id) {
            this.instance.id = id;
            return this;
        }

        public MyCoffe build() {
            if (this.instance.name == null) {
                throw new AppropriateException("'name' is required");
            }
            if (this.instance.id == 0) { // Assumes 0 isn't a valid ID
                throw new AppropriateException("'id' is required");
            }
            return this.instance;
        }
    }

    private String name;
    private int id;

    private MyCoffe() {
    }

    public MyCoffe(String name, int id) {
        this.name = name;
        this.id = id;
    }

    @Override
    public String toString(){
        return name + "/" + id;
    }

    public static void main(String[] args){
        MyCoffe myCoffe = new MyCoffe.Builder().setName("HelloWorld").setId(9999).build();
        System.out.println(myCoffe);

        System.out.println("Thanks for help!");
    }
}