Variables

A variable is a memory location used to store data.
Each variable has a data type and stores value accordingly.

java

<data_type> <variable_name> = <value>;

int age = 25;
String name = "Anil";

final Keyword

final is used to make a variable constant
Once assigned → value cannot be changed

java

final int a = 10;
// a = 20;  Error

Types of Variables

🔹1. Class Level Variables

Declared inside class but outside methods
Accessible throughout the class
Stored in Heap Memory
JVM provides default values if not initialized

🔸 Types of Class Level Variables:

Instance Variable
Static Variable

Instance Variable (Non-Static)

Declared inside class without static keyword
Each object has its own copy

Created when object is created
Stored in Heap memory

java

class Student {

    int sid;
    String sname;
    String semail;

}

class Main {
    public static void main(String[] args) {

        Student s1 = new Student();
        s1.sid = 101;
        s1.sname = "Anil";

        Student s2 = new Student();
        s2.sid = 102;
        s2.sname = "Rahul";

        System.out.println(s1.sid + " " + s1.sname);
        System.out.println(s2.sid + " " + s2.sname);
    }
}

Each object stores different data

Static Variable

Declared using static keyword
Shared among all objects

Created once at class loading time
Common for all objects
Stored in method area (JVM memory)

java

class Student {

    int sid;
    String sname;
    static String nationality = "Indian";

}

class Main {
    public static void main(String[] args) {

        Student s1 = new Student();
        Student s2 = new Student();

        System.out.println(s1.nationality);
        System.out.println(s2.nationality);
    }
}

Both objects share the same static variable

2. Local Variables

Declared inside method / block / constructor

Must be initialized before use
Accessible only within their block
Stored in Stack Memory
No default value

java

class Test {

    void m1() {
        int b = 30;  // local variable
        System.out.println(b);
    }
}

Heap vs Stack Memory

JVM Memory

When JVM starts, it divides memory into parts. Important ones:

🔸 Heap Memory

Stores objects and class variables
Shared memory

🔸 Stack Memory

Stores local variables and method calls
Each method has its own stack frame

After method execution → stack frame is removed

java

class Test {

    // Class level variables
    int a = 10;
    byte b;
    short s;
    int i;
    long l;
    float f;
    double d;
    boolean bl;
    char c;

    void m1() {
        int b = 30;  // local variable
        System.out.println(b);
    }
}

public class Main {
    public static void main(String[] args) {
        
        int b = 29;  // local variable
        
        Test t1 = new Test();

        System.out.println(t1.a);
        System.out.println(t1.b);
        System.out.println(t1.s);
        System.out.println(t1.i);
        System.out.println(t1.l);
        System.out.println(t1.f);
        System.out.println(t1.d);
        System.out.println(t1.bl);
        System.out.println(t1.c);

        t1.m1();

        System.out.println("Hello I am Java");
    }
}

🔍 Key Observations:

Class variables → get default values
Local variables → must be initialized
Object (t1) → stored in Heap
Reference → stored in Stack

Ways to Access Static Variable

1. Using Class Name (Best Practice)

java

Student.nationality;

2. Using Reference Variable (even null)

java

Student s = null;
System.out.println(s.nationality);

3. Using Object

java

Student s1 = new Student();
System.out.println(s1.nationality);