Many methods offer invariants, which can be any or all of the guarantees made about what the method can do, requirements about the required state of the object when the method is invoked, or guarantees about the state of the object when the method completes. For instanceexample, the % operator, which computes the remainder of a number, provides the invariant that
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Many classes also offer invariants, which are guarantees made about the state of their objects' fields upon the completion of any of their methods. For instanceexample, classes whose member fields may not be modified once they have assumed a value are called immutable classes. An important consequence of immutability is that the invariants of instances of these classes are preserved throughout their lifetimes.
Similarly, classes can rely on invariants in order to properly implement their public interfaces. These invariants might relate to the state of member fields or the implementation of member methods. Generally, classes can rely on encapsulation to help maintain these invariants, for example, by making member fields private. However, encapsulation can be incompatible with extensibility. For example, a class designer might want a method to be publicly accessible, yet rely on the particulars of its implementation when using it in another method within the class. In this case, overriding the method in a subclass can break the internal invariants of the class.
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