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Primitive Data Types
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How numbers and other data, such as characters, are represented in memory is of great practical importance. Ideally a single memory representation, or type, could represent all data including numbers, characters and boolean values. Computer memory and transfer rates, however, are not infinite and designers must strike a compromise between the widest possible range of values and conserving memory and maximizing speed. Also, the data representations must use base 2 as dictated by the underlying binary hardware. Thus floating point numbers cannot represent all decimal numbers exactly. For example, converting to base 2 from base 10 means 0.1 will not produce 1.0 if we sum it in a loop 10 times. So it is clear that one universal numerical data type will not be efficient or sufficient for all situations. 8 Primitive Data Types For any language there is the need to provide different storage representations for integers and floating point numbers. In the case of integers, a variety of sizes allows for the most efficient use of memory for a given task. For example, the byte type (8-bit) is often useful for I/O tasks while long types (64-bit) are needed for representing very large values. In between are the 16-bit short and the 32-bit int types. The int type is the default for most integer operations in Java. The integer types use two's complement signed representations. For floating point there are two widths available. The IEEE 754 floating point standard is used for the 32-bit float and 64-bit double types. See also Chapter 2: Tech : More about Floating Point. Historically, characters were typically represented with 7-bit ASCII codes, which allowed for 128 characters. A large number of 8-bit encodings exist to provide both standard ASCII and characters needed for particular languages. However, full internationalization with a single code requires many more characters than is available with a single byte. So a two byte character type- char - is used to hold the Unicode representation. (An even larger 4-byte system is under development but not implemented in Java.) The char type can also represent 16-bit unsigned integers. A boolean (true/false) type is needed for the many logic operations carried out in almost any program. Java comes with 8 primitive data types in all. These include 4 types of integers, two floating points, a character type and a boolean. Below is a table of the Java primitive types with their specifications. We will discuss classes and objects later, but note here that primitive types in Java are not instances of classes. The decision to use simple data types broke the symmetry and elegance of the language to some degree but vastly reduced overhead and execution times compared to the case where all data types are objects.
More comments about primitives:
More About the Primitive Types See these other sections for additional information concernting the primitive data types:
References & Web Resources Latest update: Jan.11, 2006
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