Could someone explain step by step how to get output?(reference type question)

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);}}
public class Person {
private String name;
private int age;
Person (String n) {
this.name = n; }
public int getAge() {
return age;}
public void setAge(int a) {
this.age = a; }}

Answer 1

You create a Person object, then set it's age attribute to 20.
You pass the Person object to the celebrateBirthday method, where you call the getAge method for the object, add 1 and then set the new value (21) to the age attribute of the Person object.
Then you call getAge to print the current age. Output is 21.