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Objects that serve as keys in ordered sets and maps should be immutable. When some fields must be mutable, the {{equals()}}, {{hashCode()}} and {{compareTo()}} methods must consider only immutable state when comparing objects. Violations of this rule can produce inconsistent orderings in collections. The documentation of {{java.util.Interface Set<E>}} and {{java.util.Interface Map<K,V>}} warns against this. For example, the documentation for the Interface Map states \[[API 2006|AA. Bibliography#API 06]\]: |
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Failure to ensure that the keys used in a comparison operation are immutable can lead to non-deterministic behavior.
Guideline Recommendation | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
OBJ15MET19-J | low | probable | high | P2 | L3 |
Automated Detection
The Coverity Prevent Version 5.0 MUTABLE_COMPARISON checker can detect the instances where compareTo method is reading from a non constant field. If the non-constant field is modified, the value of compareTo might change, which may break program invariants.
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