Primitive Data Types
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 listed below, the first 3 is the most common ones and
they are probably the ones that you’ll ever need.
int: The int
data type is a 32-bit signed 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, use
long instead.
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 and 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.
byte: The byte data 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 byte data type can be useful for saving memory in large arrays, where the
memory savings actually matters. They can also be used in place of int 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 with byte, the same guidelines apply: you can use a
short to save memory in large arrays, in situations where the memory savings
actually matters.
long: The long data type is a 64-bit signed 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 by int.
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 for byte and short, use a float (instead of 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
BigDecimal and other useful classes provided by the Java platform.
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).
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. You'll learn more about the String
class in Simple Data Objects
Default 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
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, so I am not going to go into
them.
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:
boolean result = true;
int i = 100000;
Character and String Literals
Literals of types char and String 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' for char literals and "double quotes" for String
literals. Unicode escape sequences may be used elsewhere in a program (such
as in field names, for example), not just in char or String literals.
The Java programming language also supports a few special
escape sequences for char and String literals: \b (backspace), \t (tab), \n (line feed), \f (form feed), \r (carriage
return), \" (double quote), \' (single quote), and \\ (backslash).
Using Underscore Characters in Numeric Literals
In Java SE 7 and later, any number of underscore characters
(_) can appear anywhere between digits in a numerical literal. This feature
enables you, for example. to separate groups of digits in numeric literals,
which can improve the readability of your code.
For instance, if your code contains numbers with many
digits, you can use an underscore character to separate digits in groups of
three, similar to how you would use a punctuation mark like a comma, or a
space, as a separator.
long creditCardNumber = 1234_5678_9012_3456L;
long socialSecurityNumber = 999_99_9999L;
You can place underscores only between digits; you cannot
place underscores in the following places:
At the beginning
or end of a number
Adjacent to a
decimal point in a floating point literal
Prior to an F or L
suffix
In positions where
a string of digits is expected
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