Collection Add to Self

ID

java.collection_add_to_self

Severity

low

Remediation Complexity

trivial

Remediation Risk

low

Remediation Effort

low

Resource

Reliability

Language

Java

Tags

best-practice, reliability

Description

Reports calls where a collection adds itself as an element (list.add(list)) or appends its own contents to itself (list.addAll(list)).

Rationale

Adding a collection to itself is almost always a bug:

  • list.add(list) creates a circular reference that causes StackOverflowError in toString(), hashCode(), or equals().

  • list.addAll(list) throws ConcurrentModificationException because it reads from and writes to the same collection simultaneously.

List<Object> items = new ArrayList<>();
items.add(items);      // Circular reference
items.addAll(items);   // ConcurrentModificationException

Remediation

Use a different collection or create a copy:

List<Object> items = new ArrayList<>();
List<Object> copy = new ArrayList<>(items);
items.addAll(copy);   // Safe: reads from a separate collection

References