In contrast, a data structure is a concrete implementation of one or more ADTs, similar to how Java classes implement interfaces. Examples of ADTs include Employee, Vehicle, Array, and List.

Even the most complicated classes in Java can be flattened down to nothing more than the set of primitive data types they represent. But primitive types aren’t objects, and that presents a problem.

An example is String. This is not a primitive type in Java, but rather a reference type: in this case it “points” to a class that can have multiple values and methods.