如果 Map 中的所有 List 值为空/非空，则使用 Streams 返回布尔值 [英] Use Streams to return Boolean if all the List values in a Map are empty/not-empty

问题描述

Given a Map mapping a String to a List, is there a way to use Java Streams to return a Boolean where TRUE means one or more list had elements? If all lists in the map were empty, return FALSE.

Map< String , List<String> > map = …

Can use of Streams replace this conventional code?

// See if any diffs were found. Loop through the Map, look at each List of diffs to see if non-empty.
boolean anyElementsInAnyList = false;
for (List<String> list : map.values () ) {
    if (!list.isEmpty()) {
        anyElementsInAnyList = true;
        break;
    }
}

Note that we can break out of the examination after the first finding. No need to examine all the Map values (all the Lists). Would be nice if, for efficiency, the Stream could do the same stop-work-on-first-finding (a "short-circuiting" operation).

解决方案

Stream::allMatch

In Java 8 you can check that not all lists are empty.

 boolean anyNonEmpty = !map.values().stream().allMatch(List::isEmpty);

Notice that Stream::allMatch is a short-circuiting terminal operation. So the stream is efficient, not running any longer than need be.

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