Careeroza

Reference Types

Primitive vs Reference

	Primitive	Reference
Examples	`int`, `double`, `boolean`	`String`, arrays, `Scanner`, your classes
Stores	the value itself	a reference to an object on the heap
Can be null?	no	yes
Has methods?	no	yes (`name.length()`)
Name style	lowercase	starts with a capital letter
Default value (fields)	0 / false	`null`

Copying Values vs Copying References

Example

Assigning a primitive copies the value. Assigning a reference copies the arrow, so both variables point to the same object.

public class Main {
    public static void main(String[] args) {
        int a = 10;
        int b = a;     // copy of the value
        b = 20;
        System.out.println("a = " + a + ", b = " + b);

        int[] marks1 = {70, 80, 90};
        int[] marks2 = marks1;   // copy of the reference (same array!)
        marks2[0] = 100;
        System.out.println("marks1[0] = " + marks1[0]);

        String s = null;  // references can be null
        System.out.println("s is " + s);
    }
}

Output

a = 10, b = 20
marks1[0] = 100
s is null

Memory Picture

Stack (method variables)          Heap (objects)
 +-----------------+
 | a = 10          |
 | b = 20          |
 | marks1  ------------+-------->  [100, 80, 90]
 | marks2  ------------+
 +-----------------+

Tip: Calling a method on a null reference throws NullPointerException, the most common runtime error in Java.