In Java, all variables must be declared before they can be used.

Few examples

Instance variables

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 each instance of a class

(to each object, in other words);

the currentSpeed of one bicycle is independent

from the currentSpeed of another.

Instance variables are declared in a class,

but outside a method, constructor or any block.

Instance variables are created when an object is created

and destroyed when the object is destroyed.

Instance variables hold object's state, available to all methods,

constructors or blocks of a class, and accessible by using references or

objects out the class.

Instance variables have default values

● numbers : 0

● Booleans : false

● Object references : null

Values can be assigned during the declaration

or within the constructor.

Identify Instance variables

Class Variables (Static Fields)

A class variable is any field declared with the static modifier;

there is exactly one copy of this variable in existence,

regardless of how many times the class has been instantiated.

Example :

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.

static int numGears = 6

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

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.

Access modifiers cannot be used for local variables.

Local variables are visible only

within the declared method, constructor or block.

There is no default value for local variables so local variables should be

declared and an initial value should be assigned before the first use.

Identify Local variable

How we can fix this error?

Error: variable m might not have been initializedm++;1 error

Parameters

args variable is the parameter to main method.

public static void main ( String[ ] args )

Parameters are always classified as “variables” not “fields”.

This applies to parameter-accepting constructs as well.