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:

1. Instance Variable

2. 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);