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Trail: Learning the Java Language
Lesson: Language Basics

Data Types

Every variable must have a data type. A variable's data type determines the values that the variable can contain and the operations that can be performed on it. For example, in the MaxVariablesDemo (in a .java source file) program, the declaration int largestInteger declares that largestInteger has an integer data type (int). Integers can contain only integral values (both positive and negative). You can perform arithmetic operations, such as addition, on integer variables.

The Java programming language has two categories of data types: primitive and reference (in the glossary). A variable of primitive type contains a single value of the appropriate size and format for its type: a number, a character, or a boolean value. For example, an integer value is 32 bits of data in a format known as two's complement, the value of a char is 16 bits of data formatted as a Unicode character, and so on.

A variable of primitive type contains a value of a particular size and format.

The following table lists, by keyword, all of the primitive data types supported by the Java platform, their sizes and formats, and a brief description of each. The MaxVariablesDemo program declares one variable of each primitive type.

Primitive Data Types
Keyword Description Size/Format
byte Byte-length integer 8-bit two's complement
short Short integer 16-bit two's complement
int Integer 32-bit two's complement
long Long integer 64-bit two's complement
(real numbers)
float Single-precision floating point 32-bit IEEE 754
double Double-precision floating point 64-bit IEEE 754
(other types)
char A single character 16-bit Unicode character
boolean A boolean value (true or false) true or false

Purity Tip: In other programming languages, the format and the size of primitive data types can depend on the system on which the program is running. In contrast, the Java programming language specifies the size and the format of its primitive data types. Hence, you donít have to worry about system dependencies.

You can put a literal primitive value directly in your code. For example, if you need to assign the value 4 to an integer variable you can write this:

int anInt = 4;
The digit 4 is a literal integer value. The following table gives some examples of literal values of various primitive types.

Examples of Literal Values and Their Data Types
Literal Data Type
178 int
8864L long
37.266 double
37.266D double
87.363F float
26.77e3 double
' c ' char
true boolean
false boolean

Generally speaking, a series of digits with no decimal point is typed as an integer. You can specify a long integer by putting an 'L' or 'l' after the number. 'L' is preferred as it cannot be confused with the digit '1'. A series of digits with a decimal point is of type double. You can specify a float by putting an 'f' or 'F' after the number. A literal character value is any single Unicode character between single quote marks. The two boolean literals are simply true and false.

Arrays, classes, and interfaces are reference (in the glossary) types. The value of a reference type variable, in contrast to that of a primitive type, is a reference to (an address of) the value or set of values represented by the variable (see the following figure). A reference is called a pointer, or a memory address in other languages. The Java programming language does not support the explicit use of addresses like other languages do. You use the variable's name instead.

A variable of reference type contains a reference to (an address of) an object or an array.

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