Answers Questioned

Question 1: Why does changing b not affect a, but changing array2 affects array1?

a and b are primitive types. When you assign b = a;, the value 10 is copied. Changing b later does not affect a because they are stored separately.

array1 and array2 are references to the same array in the heap. When array2[0] = 99; runs, both variables point to the same memory. So, changing one changes the other.

Question 2: Describe what’s on the stack vs. the heap for this code

Stack:

a = 10
b = 20
array1 → 0x001
array2 → 0x001

Heap:

0x001 → [99, 2, 3]

The stack stores variables and references. The heap stores the actual array. Both array1 and array2 point to the same array in the heap.

Question 1: After haveBirthday(john) is called, what is John’s age? Why?

After haveBirthday(john) runs, John’s age is 21. The method changes the value of p.age, and since p refers to the same Person object as john, the change affects that object directly. In Java, object references are passed by value, but the value is a reference, so the method can modify the object’s fields.

Question 2: After reassignPerson(john) is called, what is John’s name and age? Why?

After reassignPerson(john), John’s name is still “John” and his age is still 21. Inside the method, p is reassigned to a new Person object, but this does not affect john outside the method. The reassignment only changes what the local variable p points to, not the original john reference.

Question 3: Explain the difference between modifying an object’s contents vs. reassigning a reference.

Modifying an object’s contents means changing the data inside the same object (like updating p.age). Both references to that object will see the change.

Reassigning a reference means making the variable point to a completely new object. This only changes the reference locally and does not affect other variables pointing to the original object.

Homework Hack 1

public class ObjectCreation {
    public static void main(String[] args) {
        Car car1 = new Car("Toyota", 2020);
        Car car2 = new Car("Honda", 2018);

        System.out.println("Car 1");
        car1.fetchInfo();
        System.out.println("");
        System.out.println("Car 2");
        car2.fetchInfo();
    }
}

class Car {
    public String model;
    public int year;
    
    Car(String model, int year) {
        this.model = model;
        this.year = year;
    }

    public void fetchInfo() {
        System.out.println("Model: " + this.model);
        System.out.println("Year: " + this.year);
    }
}

ObjectCreation.main(null);

Car 1
Model: Toyota
Year: 2020

Car 2
Model: Honda
Year: 2018

Homework Hack 2

public class HeapVsStack {
    public static void main(String[] args) {
        int pages = 256;

        int copiedPages = pages;

        Book book1 = new Book("Marvin's Miraculous Mechanical Museum");

        Book book2 = book1;

        pages = 16;

        book1.title = "Carl goes to school";

        System.out.println("Book 1");
        System.out.println("Title: " + book1.title);
        System.out.println("Pages: " + pages);

        System.out.println("");

        System.out.println("Book 2");
        System.out.println("Title: " + book2.title);
        System.out.println("Pages: " + copiedPages);
    }
}

class Book {
    public String title;

    Book(String title) {
        this.title = title;
    }

    public void fetchTitle() {
        System.out.println("Current Book Title: " + this.title);
    }
}

HeapVsStack.main(null);

Book 1
Title: Carl goes to school
Pages: 16

Book 2
Title: Carl goes to school
Pages: 256