The Java programming language is statically-typed, which means that all variables must first be declared before they can be used. This involves stating the variable’s type and name, as you’ve already seen:
int gear = 1;
Doing so tells your program that a field named “gear” exists, holds numerical data, and has an initial value of “1”. A variable’s data type determines the values it may contain, plus the operations that may be performed on it. In addition to
int, the Java programming language supports seven other primitive data types. A primitive type is predefined by the language and is named by a reserved keyword. Primitive values do not share state with other primitive values. The eight primitive data types supported by the Java programming language are:
- byte: The
bytedata type is an 8-bit signed two’s complement integer. It has a minimum value of -128 and a maximum value of 127 (inclusive). The
bytedata type can be useful for saving memory in large arrays, where the memory savings actually matters. They can also be used in place of
intwhere their limits help to clarify your code; the fact that a variable’s range is limited can serve as a form of documentation.
- short: The
shortdata type is a 16-bit signed two’s complement integer. It has a minimum value of -32,768 and a maximum value of 32,767 (inclusive). As with
byte, the same guidelines apply: you can use a
shortto save memory in large arrays, in situations where the memory savings actually matters.
- int: By default, the
intdata type is a 32-bit signed two’s complement integer, which has a minimum value of -231 and a maximum value of 231-1. In Java SE 8 and later, you can use the
intdata type to represent an unsigned 32-bit integer, which has a minimum value of 0 and a maximum value of 232-1. Use the Integer class to use
intdata type as an unsigned integer. See the section The Number Classes for more information. Static methods like
divideUnsignedetc have been added to the
Integerclass to support the arithmetic operations for unsigned integers.
- long: The
longdata type is a 64-bit two’s complement integer. The signed long has a minimum value of -263 and a maximum value of 263-1. In Java SE 8 and later, you can use the
longdata type to represent an unsigned 64-bit long, which has a minimum value of 0 and a maximum value of 264-1. Use this data type when you need a range of values wider than those provided by
Longclass also contains methods like
divideUnsignedetc to support arithmetic operations for unsigned long.
- float: The
floatdata type is a single-precision 32-bit IEEE 754 floating point. Its range of values is beyond the scope of this discussion, but is specified in the Floating-Point Types, Formats, and Values section of the Java Language Specification. As with the recommendations for
short, use a
double) if you need to save memory in large arrays of floating point numbers. This data type should never be used for precise values, such as currency. For that, you will need to use the java.math.BigDecimal class instead. Numbers and Strings covers
BigDecimaland other useful classes provided by the Java platform.
- double: The
doubledata type is a double-precision 64-bit IEEE 754 floating point. Its range of values is beyond the scope of this discussion, but is specified in the Floating-Point Types, Formats, and Values section of the Java Language Specification. For decimal values, this data type is generally the default choice. As mentioned above, this data type should never be used for precise values, such as currency.
- boolean: The
booleandata type has only two possible values:
false. Use this data type for simple flags that track true/false conditions. This data type represents one bit of information, but its “size” isn’t something that’s precisely defined.
- char: The
chardata type is a single 16-bit Unicode character. It has a minimum value of
'\u0000'(or 0) and a maximum value of
'\uffff'(or 65,535 inclusive).
In addition to the eight primitive data types listed above, the Java programming language also provides special support for character strings via the java.lang.String class. Enclosing your character string within double quotes will automatically create a new
String object; for example,
String s = "this is a string";.
String objects are immutable, which means that once created, their values cannot be changed. The
String class is not technically a primitive data type, but considering the special support given to it by the language, you’ll probably tend to think of it as such.
It’s not always necessary to assign a value when a field is declared. Fields that are declared but not initialized will be set to a reasonable default by the compiler. Generally speaking, this default will be zero or
null, depending on the data type. Relying on such default values, however, is generally considered bad programming style.
The following chart summarizes the default values for the above data types.
|Data Type||Default Value (for fields)|
|String (or any object)||null|