Data types ,variables,array

Data Types, Variables, Arrays
and Operators
Theandroid-mania.com

What are Variables?
 Variables are the nouns of a programming language-that
is, they are the entities (values and data) that act or are
acted upon.
 A variable declaration always contains two components:
the type of the variable and its name. The location of the
variable declaration, that is, where the declaration
appears in relation to other code elements, determines its
scope.

What are Data-Types
 All variables in the Java language 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, the declaration
int count
declares that count is an integer (int). Integers can contain
only integral values (both positive and negative), and you
can use the standard arithmetic operators (+, -, *, and/) on
integers to perform the standard arithmetic operations
(addition, subtraction, multiplication, and division,
respectively).

Data-Types
 There are two major categories of data types in the Java
language: primitive and reference.
The following table lists, by keyword, all of the primitive
data types supported by Java, their sizes and formats, and
a brief description of each.

Variables
 Instance Variables (Non-Static Fields) Technically speaking, objects store
their individual states in "non-static fields", that is, fields declared without
the static keyword. Non-static fields are also known as instance
variables because their values are unique to eachinstance of a class (to each
object, in other words); the currentSpeed of one bicycle is independent from
the currentSpeed of another.
 Class Variables (Static Fields) A class variable is any field declared with
the static modifier; this tells the compiler that there is exactly one copy of
this variable in existence, regardless of how many times the class has been
instantiated. A field defining the number of gears for a particular kind of
bicycle could be marked as static since conceptually the same number of
gears will apply to all instances. The code static int numGears = 6; would
create such a static field. Additionally, the keyword final could be added to
indicate that the number of gears will never change.

 Local Variables Similar to how an object stores its state in fields, a method
will often store its temporary state in local variables. The syntax for declaring
a local variable is similar to declaring a field (for example, int count = 0;).
There is no special keyword designating a variable as local; that
determination comes entirely from the location in which the variable is
declared — which is between the opening and closing braces of a method.
As such, local variables are only visible to the methods in which they are
declared; they are not accessible from the rest of the class.
 Parameters You've already seen examples of parameters, both in
the Bicycle class and in the main method of the "Hello World!" application.
Recall that the signature for the main method is public static void
main(String[] args). Here, the args variable is the parameter to this method.
The important thing to remember is that parameters are always classified as
"variables" not "fields". This applies to other parameter-accepting constructs
as well (such as constructors and exception handlers) that you'll learn about
later in the tutorial.

Naming
 The rules and conventions for naming your variables can be
summarized as follows:
Variable names are case-sensitive. A variable's name can be any legal
identifier — an unlimited-length sequence of Unicode letters and digits,
beginning with a letter, the dollar sign "$", or the underscore character "_".
The convention, however, is to always begin your variable names with a
letter, not "$" or "_". Additionally, the dollar sign character, by convention, is
never used at all. You may find some situations where auto-generated
names will contain the dollar sign, but your variable names should always
avoid using it. A similar convention exists for the underscore character; while
it's technically legal to begin your variable's name with "_", this practice is
discouraged. White space is not permitted.

 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 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.
 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.

 Subsequent characters may be letters, digits, dollar signs, or underscore
characters. Conventions (and common sense) apply to this rule as well.
When choosing a name for your variables, use full words instead of cryptic
abbreviations. Doing so will make your code easier to read and understand.
In many cases it will also make your code self-documenting; fields
named cadence, speed, and gear, for example, are much more intuitive than
abbreviated versions, such as s, c, and g. Also keep in mind that the name
you choose must not be a keyword or reserved word.
 If the name you choose consists of only one word, spell that word in all
lowercase letters. If it consists of more than one word, capitalize the first
letter of each subsequent word. The names gearRatio and currentGear are
prime examples of this convention. If your variable stores a constant value,
such as static final int NUM_GEARS = 6, the convention changes slightly,
capitalizing every letter and separating subsequent words with the
underscore character. By convention, the underscore character is never used
elsewhere.

 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
the Floating-Point Types, Formats, and Values section 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.

 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 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 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.
 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).

Arrays
 An array is a container object that holds a fixed number of values of a single
type. The length of an array is established when the array is created. After
creation, its length is fixed. You've seen an example of arrays already, in
the main method of the "Hello World!" application. This section discusses
arrays in greater detail.
Each item in an array is called an element, and each element is
accessed by its numerical index
for ex:
declares an array of integers
int[] anArray;
allocates memory for 10 integers
anArray = new int[10];

initialize elements
anArray[index] = value;
Similarly, you can declare arrays of other types:
byte[] anArrayOfBytes;
short[] anArrayOfShorts;
long[] anArrayOfLongs;
float[] anArrayOfFloats;
double[] anArrayOfDoubles;
boolean[] anArrayOfBooleans;
char[] anArrayOfChars;
String[] anArrayOfStrings;

Data types ,variables,array

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