Learning Java

Types

The type system of a programming language describes how its data elements (variables and constants) are associated with actual storage. In a statically typed language, like C or C++, the type of a data element is a simple, unchanging attribute that often corresponds directly to some underlying hardware phenomenon, like a register value or a pointer indirection. In a more dynamic language like 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 like Perl and Tcl 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 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 type validity of assignments while the code is executing. Unlike C or C++, though, Java also maintains runtime information about objects and uses this to allow truly safe runtime polymorphism and casting (using an object as a type other than its declared type).

Java data types fall into two categories. Primitive types represent simple values that have built-in ...