问题
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I am trying to determine the correct way of changing all the values in a List using the new lambdas feature in the upcoming release of Java 8 without creating a **new** List.
This pertains to times when a List is passed in by a caller and needs to have a function applied to change all the contents to a new value. For example, the way Collections.sort(list) changes a list in-place.
What is the easiest way given this transforming function, and this starting list, to apply the function to all the values in the original list without making a new list?
String function(String s){
return s.toUppercase(); // or something else more complex
}
List<String> list = Arrays.asList("Bob", "Steve", "Jim", "Arbby");
The usual way of applying a change to all the values in-place was this:
for (int i = 0; i < list.size(); i++) {
list.set(i, function( list.get(i) );
}
Does lambdas and Java 8 offer:
- an easier and more expressive way?
- a way to do this without setting up all the scaffolding of the
for(..)loop?
回答1:
List<String> list = Arrays.asList("Bob", "Steve", "Jim", "Arbby");
list.replaceAll(String::toUpperCase);
回答2:
With a popular library Guava you can create a computing view on the list which doesn't allocate memory for a new array, i.e.:
upperCaseStrings = Lists.transform(strings, String::toUpperCase)
A better solution is proposed by @StuartMarks, however, I am leaving this answer as it allows to also change a generic type of the collection.
Another option is to declare a static method like mutate which takes list and lambda as a parameter, and import it as a static method i.e.:
mutate(strings, String::toUpperCase);
A possible implementation for mutate:
@SuppressWarnings({"unchecked"})
public static List mutate(List list, Function function) {
List objList = list;
for (int i = 0; i
回答3:
You seem to have overlooked forEach, which can be used to mutate the elements of a List. Of course, that doesn't cover your use case entirely—it won't help if your elements are immutable or if you want to replace them completely. [edit: the next sentence relates to streams, not to collections as specified in the question] There is a good reason why you can't do those operations: they aren't parallel-friendly (this actually applies to forEach also) and the Stream API strongly discourages the use of sequential-only operations.
来源：https://stackoverflow.com/questions/19697932/can-a-lambda-be-used-to-change-a-lists-values-in-place-without-creating-a-new