In Java, why is object composition typically used to build an adapter instead of inheriting from the class being adapted?

An adapter built with composition holds a reference to the incompatible class as a field and delegates to it inside its own methods, which works even when the class being wrapped is final, has no default constructor, or is supplied to the adapter at runtime rather than known in advance. Java also only allows single inheritance, so if an adapter needed to extend the adapted class, it could never also extend anything else, and it would still need to implement the target interface separately. Composition keeps the adapter flexible -- it can wrap any instance handed to it, including ones created elsewhere -- and keeps the adapted class's internals genuinely private, exposed only through whatever the adapter's own methods choose to forward.