Can someone explain how this code works?

Question

public class MyClass {
public static void main(String[ ] args) {
Person j;
j = new Person("John");
j.setAge(20);
celebrateBirthday(j);
System.out.println(j.getAge());
}
static void celebrateBirthday(Person p) {
p.setAge(p.getAge() + 1);
}
}
//Outputs "21"

Answer 1

@ Carl
You could just assume the class exists. 😛

@Main question
Objects are basically (Okay, yes @ran technically it's just the value of the reference 😛) passed by reference.
Person is an Object.
So,
celibrateBirthDay(j);
Is passing the Object j as arguments.

static void celibrateBirthDay(Person p){}
This means that p will NOT be a new Person.
It will be the same Object as the one you passed through as arguments.

Example with arrays/

int[] myArray = {1,2,3}
doStuff(myArray);
System.out.println(myArray[0]);

static void doStuff(int[] a){
a[0] = 7;
}

Output: 7

This only works differently for primitive data-types, which are passed by value.

Answer 2

It's a big no no.Java is passed by value!never pass by reference.The object is copy by reference,not pass by reference! Remember this!

Answer 3

Code won't work since there is no class called Person. Therefore you can't create an instance of Person.