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By operating on String objects, the CaseInsensitiveString.equals() method violates the second contract requirement (symmetry). Because of the asymmetry, given a String object s and a CaseInsensitiveString object cis that differ only in case, cis.equals(s)) returns true while s.equals(cis) returns false.

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In the noncompliant code example, p1 and p2 compare equal and p2 and p3 compare equal but p1 and p3 compare unequal; this violates the transitivity requirement. The problem is that the Card class has no knowledge of the XCard class and consequently cannot determine that p2 and p3 have different values for the field type.

Compliant Solution

Unfortunately, it is impossible to extend an instantiable class (as opposed to an {{abstract}} class) by adding a value or field in the

Compliant Solution

It is impossible to extend an instantiable class  subclass while preserving the {{equals()}} contract;. It is therefore recommended to use composition rather than inheritance to achieve the desired effect \[[Bloch 2008|AA. Bibliography#Bloch 08]\]. This compliant solution adopts this approach by adding a private {{card}} field to the {{XCard}} class and providing a {{public}} {{viewCard()}} method. 

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