The Java programming language is strongly-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:
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 toint gear = 1;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: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
- byte: The
byte
data type is an 8-bit signed two's compliment integer. It has a minimum value of -128 and a maximum value of 127 (inclusive). Thebyte
data type can be useful for saving memory in large arrays, where the memory savings actually matters. They can also be used in place ofint
where 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
short
data 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 withbyte
, the same guidelines apply: you can use ashort
to save memory in large arrays, in situations where the memory savings actually matters.
- int: The
int
data type is a 32-bit two's complement integer. It has a minimum value of -2,147,483,648 and a maximum value of 2,147,483,647 (inclusive). For integral values, this data type is generally the default choice unless there is a reason (like the above) to choose something else. This data type will most likely be large enough for the numbers your program will use, but if you need a wider range of values, uselong
instead.
- long: The
long
data type is a 64-bit two's complement integer. It has a minimum value of -9,223,372,036,854,775,808 and a maximum value of 9,223,372,036,854,775,807 (inclusive). Use this data type when you need a range of values wider than those provided byint
.
- float: The
float
data 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 section 4.2.3 of the Java Language Specification. As with the recommendations forbyte
andshort
, use afloat
(instead ofdouble
) 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. Simple Data Objects coversBigDecimal
and other useful classes provided by the Java platform.
- double: The
double
data 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 section 4.2.3 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
boolean
data type has only two possible values:true
andfalse
. 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
char
data 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).String
object; for example,String s = "this is a string";
.String
objects are immutable, which means that once created, their values cannot be changed. TheString
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. You'll learn more about theString
class in Simple Data ObjectsDefault Values
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 resonable default by the compiler. Generally speaking, this default will be zero ornull
, 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) byte 0 short 0 int 0 long 0L float 0.0f double 0.0d char '\u0000' String (or any object) null boolean false Local variables are slightly different; the compiler never assigns a default value to an uninitialized local variable. If you cannot initialize your local variable where it is declared, make sure to assign it a value before you attempt to use it. Accessing an uninitialized local variable will result in a compile-time error.
Literals
You may have noticed that the
new
keyword isn't used when initializing a variable of a primitive type. Primitive types are special data types built into the language; they are not objects created from a class. A literal is the source code representation of a fixed value; literals are represented directly in your code without requiring computation. As shown below, it's possible to assign a literal to a variable of a primitive type:
The integral types (boolean result = true; char capitalC = 'C'; byte b = 100; short s = 10000; int i = 100000;byte
,short
,int
, andlong
) can be expressed using decimal, octal, or hexadecimal number systems. Decimal is the number system you already use every day; it's based on 10 digits, numbered 0 through 9. The octal number system is base 8, consisting of the digits 0 through 7. The hexadecimal system is base 16, whose digits are the numbers 0 through 9 and the letters A through F. For general-purpose programming, the decimal system is likely to be the only number system you'll ever use. However, if you need octal or hexadecimal, the following example shows the correct syntax. The prefix0
indicates octal, whereas0x
indicates hexadecimal.
int decVal = 26; // The number 26, in decimal int octVal = 032; // The number 26, in octal int hexVal = 0x1a; // The number 26, in hexadecimalThe floating point types (
float
anddouble
) can also be expressed using E or e (for scientific notation), F or f (32-bit float literal) and D or d (64-bit double literal; this is the default and by convention is omitted).
double d1 = 123.4; double d2 = 1.234e2; // same value as d1, but in scientific notation float f1 = 123.4f;Literals of types
char
andString
may contain any Unicode (UTF-16) characters. If your editor and file system allow it, you can use such characters directly in your code. If not, you can use a "Unicode escape" such as'\u0108'
(capital C with circumflex), or"S\u00ED se\u00F1or"
(Sí Señor in Spanish). Always use 'single quotes' forchar
literals and "double quotes" forString
literals. Unicode escape sequences may be used anywhere in a program (such as in field names, for example), not just inchar
orString
literals.The Java programming language also supports a few special escape sequences for
char
andString
literals:\b
(backspace),\t
(tab),\n
(line feed),\f
(form feed),\r
(carriage return),\"
(double quote),\'
(single quote), and\\
(backslash).There's also a special
null
literal that can be used as a value for any reference type.null
may be assigned to any variable, except variables of primitive types. There's little you can do with anull
value beyond testing for its presence. Therefore,null
is often used in programs as a marker to indicate that some object is unavailable.Finally, there's also a special kind of literal called a class literal, formed by taking a type name and appending "
.class"
; for example,String.class
. This refers to the object (of typeClass
) that represents the type itself.