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Types
Types are divided into two categories.
There are 8 primitive types: boolean (true and false); char; byte,
short, int, and long (integers); float and double (floating point). Object
types are created whenever a class is defined. Arrays are objects.
Integer Types
The integer types byte, short, int, and long are stored as two's complement,
signed binary integers.?? The leftmost bit represents the sign. char, which
is technically also an integer type, is stored as an unsigned binary number.
Expressions are computed as ints, so a cast is needed to store in a smaller
type.
| Type |
Bytes |
Range |
Literals |
| byte |
1 |
-128..+127 |
none |
| short |
2 |
-32,768..+32,767 |
none |
| int |
4 |
-2,147,483,648..+2,147,483,647 |
23, 0xAF |
| long |
8 |
-9,223,372,036,854,775,808.. |
23L, 0xAFL |
Operators: arithmetic, comparison, bitwise, assignment.
Floating-point Types
The floating-point types are float and double. Calculations
may produce NaN (Not a Number) or +/- infinity. Calculations are done as doubles,
so a cast is needed to store in a float.
| Type |
Bytes |
Range |
Accuracy |
Literals |
| float |
4 |
-3.4E38..+3.4E38 |
6-7 digits |
3.14F 6.02e23F |
| double |
8 |
-1.7E308..+1.7E308 |
14-15 digits |
3.14 6.02e23 |
Operators: arithmetic, comparison, assignment.
char Type
char type is stored as an unsigned
number (integer type) in two bytes (range 0..65,535).
Literals:
- 'A' (single character)
- Unicode '\uxxxx' where x is a hexadecimal digit. Eg '\u0041'
- Escape combinations: '\n' newline, '\\' backslash, '\''
single quote, '\"' double quote, '\r' carriage return, '\t' tab, '\b' backspace,
'\f' form feed.
boolean Type
boolean is used to store
the values true or false. The storage is unspecified.
Operators: logical, ==, !=, assignment. |
Identifiers (variable, class or method names)
- Start identifiers with an alphabetic character (a-z or A-Z),
and continue with alphabetic, numeric (0-9), or '_' (underscore) characters.
Do not use $.
- Do not use keywords (see below).
Standard Naming:
- Class and Method names should start with an uppercase letter
- Variable names should start with a lowercase letter, start class
attributes with m and class attributes
with an s.
- Constants should be all uppercase with underscores between words.
- Second words in a name should start with an uppercase letter.
Keywords (defining the Java language)
assert boolean break byte case catch char class const continue
default do double else extends false final finally float for goto if implements
import instanceof int interface long native new null package private protected
public return short static strictfp super switch synchronized this throw
throws transient true try void volatile while
Variables
Variables may be local, attributes
(field), or class variables. Parameters are local variables that are
assigned values when the method is called.
|
local in method |
attribute |
class |
| Where declared |
In a method. |
In class, but not in a method. |
In class using static keyword. |
| Initial value |
Must assign a value before using. |
Zero for numbers, null for objects, false for
boolean. Can be initialized in constructor. |
Zero/null/false or initialized with static initializer. |
| Visibility |
Only in the same method. |
All methods in the same package. If it is declared
public, other classes can see it. If it is protected, only child classes, and
for private, only this class. |
Same as fields. |
| Lifetime |
Created when the method is called. Destroyed
when the method returns. |
Created when an instance of the class (object)
is created with new. Destroyed after there
are no more references to the object (garbage collection). |
Program lifetime. |
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