What is Variables in Java...?

Variables

What is Variable?

Variable is the name of memory location which can store the data.

int a = 10;

int = data type

a = variable

10 = literal (data)

Value of variable can be vary – lets understand with example

int a = 10;

a = a + 20;

logic --  a = 10 +20;

                   a = 30;

// 10 values came from previous variable which is declared as int a =10;                                              

Type of Variable

1. Local Variable
2. Instance Variable
3. Static Variable

Note :- Global Variable is not present in Java – it is present in C and C++ -- In Java we use instance variable

1. Local Variables :-

• Declaration :- Inside Method, Constructors or Block
• Scope :- Inside Method, Constructors or Block not outside of the this.
• When Variables get allocated :- When Method, Constructors or Blocs get executed, variable allocates the memory and when Method, Constructors or Block get exist – Variable get deleted from the memory
• Stored Memory :- Local variable get allocated/stored in Stack Memory.
• Default Value :- Doesn’t have any default value – we have to provide the value before use else system throws error
• Access Specifiers :-  cannot use with local variables else system throws error.

2. Instance Variable :-

• Declaration :- Inside the class but outside of the Method, Constructors or Block
• Scope :- Inside  all Method, Constructors or Block within a class (but not inside the static method directly – to use within a static method we have to create an object of the class then of the class then by object reference we have to call the variable).
• When Variables get allocated :- When object is created variable get allocated memory and when object gets deleted variable releases memory.
• Stored Memory :- Instance variable get allocated/stored in Heap Memory.
• Default Value :- They have default values

Int = 0 , boolean = false , object = null.

• Access Specifiers :-  can be use with instance variables.
• How to use :-  1. We can call directly if is a simple method.  2. In case of static method – we have to create the object of the class and then we will call the variable with object reference.

3. Static Variable :-

• Declaration :- Inside the class with static keyword but outside of the Method, Constructors or Block.
• Scope :- Inside  all Method, Constructors or Block within a class including static method, static Constructors or static Block.
• When Variables get allocated :- When we run the program and .class file is loaded static variable get allocated and when .class file get unloaded variable get delated.
• Stored Memory :- static variable get allocated/stored in Non heap Memory or Static Memory.
• Default Value :- They have default values

Int = 0, boolean = false, object = null.

• Access Specifiers :-  can be use with static variables.
• How to use :-  1. Directly 2. Bys using class name (class name . Variable name == ABC.b ----- class = ABC and variable =1) 3. By using object reference name Like instance variable.