How do Java wildcards and PECS guide generic API design?

Applies upper and lower bounded wildcards to producer and consumer APIs while explaining their limits.

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TL;DR

PECS means producer extends, consumer super. If a parameter only produces T values for the method to read, List<? extends T> can accept lists of T or its subtypes. If the method consumes T values by adding them, List<? super T> can accept a list of T or one of its supertypes. If the method must both read and write the exact same type, an invariant List<T> is usually appropriate.

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Use-site variance, safe reads and writes, wildcard return-type judgment, and practical API signatures.

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Overview
PECS is a memory aid for safe operations, not a rule to place wildcards on every generic type.
PECS means producer extends, consumer super. If a parameter only produces T values for the method to read, List<? extends T> can accept lists of T or its subtypes. If the method consumes T values by adding them, List<? super T> can accept a list of T or one of its supertypes. If the method must both read and write the exact same type, an invariant List<T> is usually appropriate.

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Details

Reversing producer and consumer bounds.
Calling an extends-bounded collection strictly immutable.
Expecting a super-bounded collection to return values as T.
Returning wildcard-heavy types without a clear caller benefit.

How do Java wildcards and PECS guide generic API design?

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