Emptiness tested through length instead of isEmpty
ID |
dart.performance.container_size_empty |
Severity |
info |
Remediation Complexity |
trivial |
Remediation Risk |
low |
Remediation Effort |
low |
Resource |
Idiom |
Language |
Dart |
Description
Reports an emptiness test written as a comparison on length - length == 0, 0 == length, length ⇐ 0 or length < 1 - on a collection, String or any other object exposing length.
Rationale
length is not a stored field on every Iterable. For a lazy sequence - the result of map, where, expand, takeWhile or a synchronous generator - reading it walks and materialises the whole sequence just to compare the count against zero, while isEmpty stops after the first element. On an infinite or very long generator that difference is the whole cost of the expression. Comparing against zero also states the test indirectly: a reader has to work out which side of the comparison means empty, and the four spellings above all mean the same thing while looking different in review.
The following code illustrates the pattern detected by this rule:
class Cart {
final List<String> lines;
final Map<String, int> quantities;
Cart(this.lines, this.quantities);
String summary() {
// FLAGGED: Emptiness tested through length instead of isEmpty
if (lines.length == 0) {
return 'empty cart';
}
// FLAGGED: Emptiness tested through length instead of isEmpty
if (0 == quantities.length) {
return 'nothing to price';
}
return 'ok';
}
}