In-Loop Collection Building

Rule details

Rule ID	in-loop-collection-building
Severity	WARNING — likely performance problem
Confidence	MEDIUM — likely correct, some context-dependent
Category	Loop amplifiers
Complexity	O(n·m) → O(n+m)

Description

Detects patterns that repeatedly copy or rebuild collections inside loops. Calling addAll(), putAll(), or concat() inside a loop copies elements on every iteration, turning an O(n) loop into O(n·m) due to repeated array/list copying.

Typical

JavaJavaScript

List<LineItem> allLineItems = new ArrayList<>();
for (Order order : orders) {
    allLineItems.addAll(order.getLineItems()); // Copies entire list each iteration
}

let allLineItems = [];
orders.forEach(order => {
    allLineItems = allLineItems.concat(order.lineItems); // Creates new array each iteration
});

After

JavaJavaScript

List<LineItem> allLineItems = orders.stream()
    .flatMap(order -> order.getLineItems().stream())
    .collect(Collectors.toList());

const allLineItems = orders.flatMap(order => order.lineItems);

Suggestion

Accumulate into a single collection, then call addAll() once after the loop

Related Rules

• Nested Lookup — another pattern where loop bodies cause quadratic behavior