Java泛型，其返回类型具有@NonNull和Collections [英] Java generic with return type that has @NonNull with Collections

问题描述

我想在Java8中实现一个泛型函数，该函数验证集合没有空成员并返回带有@NonNull批注的类型.

I want to implement a generic function in Java8, that verifies that a collection has no null members and returns a type with @NonNull annotation.

输入类型:T扩展Collection，其中T + U可为空.
结果类型:@NonNull T，带有@NonNull U

input type: T extends Collection, where T+U are nullable.
result type: @NonNull T, with @NonNull U

对于数组，它看起来像这样:

For array this would look like this:

public static <T> @NonNull T @NonNull[] arrayHasNoNullMember( @Nullable T @Nullable[] value) {

但是对于集合的情况，我不知道如何定义结果类型与输入类型相同，但是对于集合和元素类型具有@NonNull批注.
这是我想做的，但是它不是有效的语法:

But for the collection case, i don't know how to define that the result type is the same as the input type, but has the @NonNull annotations for the collection and the element type.
This is what i want to do, but it is not valid syntax:

public static <T extends Collection<U>, U> @NonNull T<@NonNull U> collectionHasNoNullMember(T<U> col) {

你能帮忙吗?

推荐答案

不幸的是，Java中无法更改通用类型的通用类型参数，例如T extends Collection<U>中的U.与您的array方法等效的方法是采用并返回Collection实例的方法，如下所示:

Unfortunately there's no way in Java to alter a generic type parameter of a generic type, like the U in T extends Collection<U>. The equivalent of your array method would be a method that takes and returns Collection instances, like so:

public static <E> @NonNull Collection<@NonNull E> collectionHasNoNullMember(@Nullable Collection<? extends @Nullable E> col)

这可以接受Collection的任何子类，但是返回类型不能比Collection更具体.

This accepts any subclass of Collection, but the return type can't be more specific than Collection.

要处理更具体的情况，我建议为通用子类提供更多方法:

To handle more specific cases, I would suggest having a few more methods for common subclasses:

public static <E> @NonNull List<@NonNull E> listHasNoNullMember(@Nullable List<? extends @Nullable E> list)
public static <E> @NonNull Set<@NonNull E> setHasNoNullMember(@Nullable Set<? extends @Nullable E> set)
public static <E> @NonNull Queue<@NonNull E> queueHasNoNullMember(@Nullable Queue<? extends @Nullable E> queue)

或者，只要您返回相同的对象，就可以始终手动将其强制转换回其实际类:

Or, as long as you're returning the same object, you can always manually cast it back to its actual class:

ArrayList<@NonNull E> list2 = (ArrayList<@NonNull E>) collectionHasNoNullMember(arrayList);

这篇关于Java泛型，其返回类型具有@NonNull和Collections的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！