Polymorphism

Polymorphism: one method call that runs different code depending on the object behind it. It rests on a single prediction worth getting right: given a variable of one type holding an object of another, which body of code runs. It covers the rule that picks the body, two surface-different examples, what the compiler checks against what the JVM chooses, fields that hide rather than override, casts that throw, and when a chain of instanceof is the better shape. It leaves out generic methods, which belong to Java Collections and Streams.