Learning Java, 4th Edition

Types

The type system of a programming language describes how its data elements (variables and constants) are associated with storage in memory and how they are related to one another. In a statically typed language, such as C or C++, the type of a data element is a simple, unchanging attribute that often corresponds directly to some underlying hardware phenomenon, such as a register or a pointer value. In a more dynamic language such as Smalltalk or Lisp, variables can be assigned arbitrary elements and can effectively change their type throughout their lifetime. A considerable amount of overhead goes into validating what happens in these languages at runtime. Scripting languages such as Perl achieve ease of use by providing drastically simplified type systems in which only certain data elements can be stored in variables, and values are unified into a common representation, such as strings.

Java combines many of the best features of both statically and dynamically typed languages. As in a statically typed language, every variable and programming element in Java has a type that is known at compile time, so the runtime system doesn’t normally have to check the validity of assignments between types while the code is executing. Unlike traditional C or C++, Java also maintains runtime information about objects and uses this to allow truly dynamic behavior. Java code may load new types at runtime and use them in fully object-oriented ways, allowing casting and full polymorphism (extending ...