从Optional<>到ArrayList<>

问题描述

我有以下情况:

public ArrayList<A> getMethods(){
    return b.c.test();
}

所以，我的问题是b.c.test()返回一个以Optional<A>作为返回类型的值.但是我需要返回一个ArrayList<A>.

So, my problem is that b.c.test() returns a value with Optional<A> as return type. But I need to return an ArrayList<A>.

因此，我尝试将其强制转换并重写为:

So, I tried to cast it and rewrite it to :

public ArrayList<A> getMethods(){
    return (ArrayList<A>)b.c.test();
}

但是Eclipse表示不可能从Optional<A>转换为ArrayList<A>.

But Eclipse says that such a cast from Optional<A> to ArrayList<A> is not possible.

我该如何解决这个问题?

How can I solve this problem?

推荐答案

我假设您的预期语义是如果存在该值，则返回一个包含单个项目的列表，否则返回一个空列表."在这种情况下，我会建议以下内容:

I am presuming your intended semantic is 'if the value is present return a list with a single item, otherwise return an empty list.' In that case I would suggest something like the following:

ArrayList<A> result = new ArrayList<>();
b.c.test().ifPresent(result::add);
return result;

但是，我建议您的返回类型应为List<A>而不是ArrayList<A>，因为这使您可以在不更改调用方的情况下更改列表的类型.如果不存在可选值，它还允许您返回Collections.EMPTY_LIST，这比创建不必要的ArrayList更为有效.

However I would suggest your return type should be List<A> rather than ArrayList<A> as that gives you the opportunity to change the type of list without changing the callers. It would also allow you to return Collections.EMPTY_LIST if the optional value is not present which is more efficient than creating an unnecessary ArrayList.

更新:Java 9现在有一个更简单的选项:

Update: there's now an easier option with Java 9:

b.c.test().stream().collect(Collectors.toList());

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