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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 2014]:

Note: great care must be exercised [when] mutable objects are used as map keys. The behavior of a map is not specified if the value of an object is changed in a manner that affects equals comparisons while the object is a key in the map. A special case of this prohibition is that it is not permissible for a map to contain itself as a key. While it is permissible for a map to contain itself as a value, extreme caution is advised: the equals and hashCode methods are no longer well defined on such a map.

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Some available static analysis tools can detect instances where the compareTo() method is reading from a nonconstant field.

Bibliography

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