Java8流map()函数中的附加括号

问题描述

我正在Java8中工作/测试流，并且遇到了非常令人沮丧的问题.我的代码可以很好地编译:

I'm working/testing streams in Java8 and come across very frustrating issue.I've got the code which compiles well:

        List<String> words = Arrays.asList("Oracle", "Java", "Magazine");
    List<String> wordLengths = words.stream().map((x) -> x.toUpperCase())
        .collect(Collectors.toList());

第二个(几乎相同)发出警告:

And second one (nearly the same) which throw a warnings:

        List<String> words = Arrays.asList("Oracle", "Java", "Magazine");
    List<String> wordLengths = words.stream().map((x) -> {
        x.toUpperCase();
    }).collect(Collectors.toList());

警告:

The method map(Function<? super String,? extends R>) in the type Stream<String> is not applicable for the arguments ((<no type> x) -> {})

这个额外的括号有什么变化?

What does this additional brackets have changed?

推荐答案

您的lambda表达式返回一个值.如果使用方括号，则需要在lambda函数中添加return语句:

Your lambda expression returns a value. If you use brackets you need to add a return statement to your lambda function:

List<String> words = Arrays.asList("Oracle", "Java", "Magazine");
List<String> wordLengths = words.stream().map((x) -> {
    return x.toUpperCase();
}).collect(Collectors.toList());

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