java 对字段的 Lambda 引用
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Lambda reference to a field
提问by Daneel S. Yaitskov
I'd like to know how to get lambda reference to a field. I don't want to use a method because my field is public final. I suspect this is impossible but I don't see an obvious statement.
我想知道如何获取对字段的 lambda 引用。我不想使用方法，因为我的领域是公共决赛。我怀疑这是不可能的，但我没有看到明显的陈述。
class A {
public final String id;
...
}
Map<String, A> f(List<A> l) {
return l.stream().collect(Collectors.toMap(A::id, Function.identity()));
}
采纳答案by Eran
You can always use a lambda expression:
您始终可以使用 lambda 表达式：
return l.stream().collect(Collectors.toMap(a -> a.id, Function.identity()));
I think that "method references" are called this way for a reason, and therefore apply only for methods.
我认为“方法引用”被称为这种方式是有原因的，因此仅适用于方法。
回答by Brian Goetz
It sounds like you're hoping that Java has a corresponding feature for field referencesas it does for method references. But this is not the case. Method references are shorthand for a certain category of lambda expressions, but there is no corresponding syntax for fields. Field literals were explored during the JSR-335 Expert Group deliberation (there is some reference to it here http://mail.openjdk.java.net/pipermail/lambda-dev/2011-November/004235.html) but were not included in Java SE 8.
听起来您希望 Java 对字段引用具有相应的功能，就像对方法引用一样。但这种情况并非如此。方法引用是某一类 lambda 表达式的简写，但没有对应的字段语法。在 JSR-335 专家组审议期间探索了字段文字（此处有一些参考http://mail.openjdk.java.net/pipermail/lambda-dev/2011-November/004235.html）但未包括在内在 Java SE 8 中。
