构建器模式的抽象类

Question

我有现有的模型类，它们总是使用这样的构建器模式：

public class Model {
    public static class Builder {
        private boolean isValid;
        private List<String> errorMessagesOrNull;

        public Builder setIsValid(final boolean isValid) {
            this.isValid = isValid;
            return this;
        }

        public Builder setErrorMessages(final List<String> errorMessages) {
            this.errorMessagesOrNull = errorMessages;
            return this;
        }

        public List<String> getErrorMessages() {
            return this.errorMessagesOrNull == null ? new ArrayList<>() : this.errorMessagesOrNull;
        }

        public Model Build() {
            return new Model(this);
        }
    }

    private boolean isValid;
    private List<String> errorMessages;

    private Model(final Builder builder) {
        this.isValid = builder.isValid;
        this.errorMessages = builder.getErrorMessages();
    }

    public boolean getIsValid() {
        return isValid;
    }

    public List<String> getErrorMessages() {
        return errorMessages;
    }
}

如您所见，模型类始终具有isValid和errorMessages。我想编写一个抽象类来尽量减少这些模型类的重复逻辑。

所以我想出了这个抽象类：

public abstract class AbstractModel<T extends AbstractModel<T>> {

    public static abstract class Builder<T> {
        private boolean isValid;
        private List<String> errorMessagesOrNull;

        public Builder<T> setIsValid(final boolean isValid) {
            this.isValid = isValid;
            return this;
        }

        public Builder<T> setErrorMessages(final List<String> errorMessages) {
            this.errorMessagesOrNull = errorMessages;
            return this;
        }

        public List<String> getErrorMessages() {
            return this.errorMessagesOrNull == null ? new ArrayList<>() : this.errorMessagesOrNull;
        }

        public abstract T Build();
    }

    private boolean isValid;
    private List<String> errorMessages;

    private AbstractModel(final Builder<T> builder) {
        this.isValid = builder.isValid;
        this.errorMessages = builder.getErrorMessages();
    }

    public boolean getIsValid() {
        return isValid;
    }

    public List<String> getErrorMessages() {
        return errorMessages;
    }
}

但它并没有真正按照我的预期工作。当我扩展抽象类时：

public class Model extends AbstractModel<Model> {
    // Empty here since all fields are extended
}

我不能做类似的事情：

Model model = new Model.Builder.setIsValid(true).Build();

我希望抽象类有Builder静态类，这样我就不需要Builder每次都编写静态类。

请指教。

Answer 1

您还需要实施Builder.

public class Model extends AbstractModel<Model>{


    private Model(final Builder builder) {
        super(builder);
    }

    public static class Builder2 extends AbstractModel.Builder<Model> {

        @Override
        public Model Build() {
            return new Model(this);
        }
    }
}

然后就可以调用

Model model = new Model.Builder2().Build();

编辑

另外，AbstractBuilder 的构造函数也必须是protected.

  protected AbstractModel(final Builder<? extends Builder<T>> builder) {
        this.isValid = builder.isValid;
        this.errorMessages = builder.getErrorMessages();
    }