Java 8 Composition，Currying 速记

Question

我有一个 BusinessObject 的流s，我需要给每个对象设置一个值，我想用Stream.map但是map需要 Function<T,R>我有目标对象、鉴别器值和新值。 setNewValueInBusinessObjectExample展示了我想做的事和setNewValueInBusinessObjectWithFun这是我需要帮助的。

同意!我可以使用 setNewValueInBusinessObjectExample在 map 上，但我想看看功能样式如何。谢谢

class BusinessObject {
    String firstField;
    String secondField;
}

class SomeDiscriminator {
    String value;
}

BusinessObject setNewValueInBusinessObjectExample(BusinessObject businessObject, 
     SomeDiscriminator discriminator, String newValue) {

    if(discriminator.value.equals("firstField")) {
        businessObject.firstField = newValue;
    } else {//secondField
        businessObject.secondField = newValue;
    }

    return businessObject;
}

Function<BusinessObject, Function<SomeDiscriminator, Function<String, BusinessObject>>> 
       setNewValueInBusinessObjectWithFun = {
    /* todo: using nested Function<T,R> */
}

Answer 1

如果您对函数式接口(interface)的构造和使用有疑问，我建议您将整个事情扩展到结构变得明显的匿名类。

我还注意到整个流程使用三个参数，与您的 setNewValueInBusinessObjectExample 一样。从而将方法体移动到最里面的匿名类。

Function<BusinessObject, Function<SomeDiscriminator, Function<String, BusinessObject>>> setNewValueInBusinessObjectWithFun =
    new Function<BusinessObject, Function<SomeDiscriminator, Function<String, BusinessObject>>>() {
        @Override
        public Function<SomeDiscriminator, Function<String, BusinessObject>> apply(final BusinessObject businessObject) {
            return new Function<SomeDiscriminator, Function<String, BusinessObject>>() {
                @Override
                public Function<String, BusinessObject> apply(final SomeDiscriminator someDiscriminator) {
                    return new Function<String, BusinessObject>() {
                        @Override
                        public BusinessObject apply(final String newValue) {
                            if (someDiscriminator.value.equals("firstField")) {
                                businessObject.firstField = newValue;
                            } else {//secondField
                                businessObject.secondField = newValue;
                            }
                            return businessObject;
                        }
                    };
                }
            };
        }
    };

现在，将所有内容打包到 lambda 表达式中，看看会发生什么:

Function<BusinessObject, Function<SomeDiscriminator, Function<String, BusinessObject>>> setNewValueInBusinessObjectWithFun = 
    businessObject -> someDiscriminator -> newValue -> {
        if (someDiscriminator.value.equals("firstField")) {
            businessObject.firstField = newValue;
        } else {//secondField
            businessObject.secondField = newValue;
        }
        return businessObject;
    };

为了清楚起见，请正确命名 lambda 表达式中的变量，否则您将无法很好地使用它们。用法相当简单(为了简洁起见，我将 setter 移到了构造函数中:

BusinessObject businessObject = new BusinessObject("oldValue");
setNewValueInBusinessObjectWithFun
    .apply(businessObject)                                  // apply to an object
    .apply(new SomeDiscriminator("firstField"))             // finds its field
    .apply("newValue");                                     // sets a new value

---

但是，我建议您使用更直接的定义来定义自定义 @FunctionalInterface...

@FunctionalInterface
interface MyFunction<T, R, U> {
    T apply(T t, R r, U u);
}

...和用法...

MyFunction<BusinessObject, SomeDiscriminator, String> myFunction = 
    (businessObject, someDiscriminator, newValue) -> {
        if (someDiscriminator.value.equals("firstField")) {
            businessObject.firstField = newValue;
        } else {
            businessObject.secondField = newValue;
        }
        return businessObject;
    };

BusinessObject businessObject = new BusinessObject("oldValue");
myFunction.apply(businessObject, new SomeDiscriminator("firstField"), "newValue");