Variables

When your program carries out computations, you will want to store values so that
you can use them later. In a Java program, you use variables to store values.

Variable Declarations

A variable is a storage location in a computer program.
Each variable has a name and holds a value.

A variable is similar to a parking space in a parking garage. The parking space has an identifier (such as “J 053”), and it can hold a vehicle. A variable has a name (such as cansPerPack), and it can hold a value (such as 6).

Ex: int cansPerPack = 6;

Syntax: 

typeName variableName = value; (or)
typeName variableName;

When you declare a variable, you specify the data type which tells the types of values that the variable holds.

Also, you may want to initialize the value with a value.

Variable Names

When you declare a variable, pick a name that explains its purpose. So, better to use a descriptive name.

• Variable names must start with a letter or underscore character(_). (Technically, the $ symbol is allowed as well, but you should not use it—it is intended for names that are automatically generated by tools.)
• Spaces are not permitted inside names.
• You can use upper case letters to denote word boundaries, as in cansPerPack. This notation is called 'camelCaseNotation'
• Variable names are case sensitive.
• You should not use reserved words for variable names.

Convention:

It is a convention among java programmers that variable names should start with a lower case letter and class names should start with an upper case letter.

Number types

int type - denote a whole number without a fractional part.

         Ex: int cansPerPack = 6;

When a fractional part is required (such as in the number 0.335), we use floating-point numbers. The most commonly used in java is double.

        Ex: double pricePerCan = 1.5;

Assignment Statement

You use the assignment statement to place a new value into a variable.

         Ex: cansPerPack = 8;

The left-hand side of an assignment statement consists of a variable. The right-hand side is an expression that has a value.

• An assignment statement stores a new value in a variable, replacing the previously stored value.
• The assignment operator(=) does not denote mathematical equality.

Constants

You cannot change the value of a variable that is defined as final.

Constants are commonly written using capital letters to distinguish them visually from regular variable.

               Ex: final double PI = 3.14;

Syntax: final typeName variableName = expression;

Comments

As your program gets more complex, you should add comments. Comments add explanations for humans who read your code.

Compiler ignores comments.

Single line comments - //

Multi-line comments - /* ........ */

Java doc comments :  /** ........... */

• This commenting style is used by tools that analyze source files, like javadoc tool which generates documentation of java source files based on this notation.

Some tips:

• Do not use magic numbers in code. A magic number is a numeric constant/literal that appears in code without explanation.
• Floating point numbers are usually rounded. Ex: 1/3 calculated as 0.33 and you multiply 0.33 * 3, it is 0.99 not 1.
• Please check for overflow or underflow errors with numeric types.
• Don't use undeclared and uninitialized variables in your program. Compiler will throw errors for these.

Numeric types in Java

Java has several numeric types:

• byte - single byte consisting of 8 bits : -128 to 127, including 0
• short - two bytes : -32,768 ... 32,767
• int - four bytes 
• long - 8 bytes
• float - 4 bytes
• double - 8 bytes.

Computers use base 2.

double data type is the default for floating point numbers.

Ex: double val = 0.235d;

float data type literal value must be suffixed with 'f'

Ex: float val = 0.235f;

long literal value must be suffixed with 'l'

Ex: long val = 125l;

Note: There are no unsigned integers in Java. All are signed and Big-Endian.