1. Values vs. References
Lecture 13

2. Office Hours Feedback
After an appointment ends you'll be asked to complete a 3 question feedback form.
The team does not see feedback immediately and any feedback shared with team members will be anonymized like course evals
Were we welcoming and respectful?
Did we genuinely try to help you learn the concepts?
Overall, were we excellent?
There's an optional field for notes, too. If a TA does a great job, let us know!
Honest feedback here is critical for helping us improve!

3. Office Hours Feedback
After an appointment ends The TAs are asked to complete a 3 question feedback form, too.
This is to help us reach out to students who need extra help and ensure office hours is being used effectively
Did the student come prepared and with a demonstrated effort?
Was the student respectful and engaged in learning?
Overall, do you have any concerns about the student's progress in 110?

4. What's on the horizon... PS3 Part 2 – Due Friday at 11:59pm
WS4 – Out today due Sunday at 11:59pm
MT1 – Next Thursday
PS4 – Emoji - Will be released this week and due after Spring Break.
A simple emoji that earns full credit should take ~2-4 hours
Intricate emoji for fun – as long as you'd like!

5. Warm-up Question #1: What is the output?
int x = 1;
int y = x;
x = 2;
System.out.println(x);
System.out.println(y);

6. Warm-up Question #2: What is the output?
Egg a = new Egg();
Egg b = a;
a.boil();
console.print(a.isRaw());
console.print(b.isRaw());
public class Egg {
private boolean _raw;
public Egg() {
_raw = true; }
public void boil() {
_raw = false;
public boolean isRaw() {
return _raw;
} }

7. Variables that hold simple types
Actually store their values!
When you declare a variable of type int, double, boolean, char you are reserving space in memory to hold a single value of that type
When you assign to a variable, the value is copied into the variable's space in memory
Let's look at an example

8. Code Memory Stack Heap File: Demo0.java int x = 1; int y = x; x = 2;
public static void main(String[] args) {
int x = 1;
int y = x;
x = 2;
System.out.println(x);
System.out.println(y); }
File: Demo0.java

9. Code Memory Stack Heap x File: Demo0.java int x = 1; int y = x; 1
public static void main(String[] args) {
int x = 1;
int y = x;
x = 2;
System.out.println(x);
System.out.println(y); }
File: Demo0.java 1 x
int

10. Code Memory Stack Heap x y File: Demo0.java int x = 1; int y = x; 1
public static void main(String[] args) {
int x = 1;
int y = x;
x = 2;
System.out.println(x);
System.out.println(y); }
File: Demo0.java 1 x
int 1 y
int

11. Code Memory Stack Heap x y File: Demo0.java int x = 1; int y = x; 20
public static void main(String[] args) {
int x = 1;
int y = x;
x = 20;
System.out.println(x);
System.out.println(y); }
File: Demo0.java 20 x
int 1 y
int

12. Variables that "hold" arrays and objects...
A variable cannot actually store arrays or objects
A variable can only store a reference to an array or an object
When you assign to such a variable, you are copying the reference
Thus, different variables can refer to the same thing
"Roy", "Roy Williams", "Coach Williams", "Coach", "me", "I"

13. Code Memory Stack Heap File: Demo1.java Egg a = new Egg(); Egg b = a;
public static void main(String[] args) {
Egg a = new Egg();
Egg b = a;
Egg c = new Egg();
a.boil();
// ... }
File: Demo1.java

14. Code Memory Stack Heap _raw Step 1.
When the new keyword is encountered, space is reserved on the Heap to store every field of an object or every element of an array.
public static void main(String[] args) {
Egg a = new Egg();
Egg b = a;
Egg c = new Egg();
a.boil();
// ... }
File: Demo1.java
Egg
_raw
boolean

15. Then, for object types, the constructor is called.
Code
Memory
Stack
Heap
Step 2.
Then, for object types, the constructor is called.
public static void main(String[] args) {
Egg a = new Egg();
Egg b = a;
Egg c = new Egg();
a.boil();
// ... }
File: Demo1.java
Egg
_raw
boolean

16. Code Memory Stack Heap this _raw Step 3.
Java sets up a new Stack frame and automatically sets up a this variable that points to the object.
Stack
Heap
public Egg() {
_raw = true; }
File: Egg.java
this
Egg
public static void main(String[] args) {
Egg a = new Egg();
Egg b = a;
Egg c = new Egg();
a.boil();
// ... }
File: Demo1.java
Egg
_raw
boolean

17. All field references refer to the fields of the this object.
Code
Memory
All field references refer to the fields of the this object.
Stack
Heap
public Egg() {
_raw = true; }
File: Egg.java
this
Egg
public static void main(String[] args) {
Egg a = new Egg();
Egg b = a;
Egg c = new Egg();
a.boil();
// ... }
File: Demo1.java
Egg
true
_raw
boolean

18. Code Memory Stack Heap this a _raw
When the end of the constructor is reached, a reference to the constructed object is returned.
Stack
Heap
public Egg() {
_raw = true; }
File: Egg.java
this
Egg
public static void main(String[] args) {
Egg a = new Egg();
Egg b = a;
Egg c = new Egg();
a.boil();
// ... }
File: Demo1.java a
Egg
Egg
true
_raw
boolean

19. Code Memory Stack Heap a _raw
Finally, the code jumps back to the main method and the constructor's stack frame is erased.
public static void main(String[] args) {
Egg a = new Egg();
Egg b = a;
Egg c = new Egg();
a.boil();
// ... }
File: Demo1.java a
Egg
Egg
true
_raw
boolean

20. Code Memory Stack Heap a _raw b
Notice we are not constructing a new Egg, we are taking a's value, which is a reference to an egg, and assigning it to b.
public static void main(String[] args) {
Egg a = new Egg();
Egg b = a;
Egg c = new Egg();
a.boil();
// ... }
File: Demo1.java a
Egg
Egg
true
_raw
boolean b
Egg

21. Code Memory Stack Heap c _raw a _raw b
All of the same steps we moved through to construct a, we would also do for c...
Egg
true
_raw
boolean c
Egg
public static void main(String[] args) {
Egg a = new Egg();
Egg b = a;
Egg c = new Egg();
a.boil();
// ... }
File: Demo1.java a
Egg
Egg
true
_raw
boolean b
Egg

22. Notice we now have two Egg objects in the Heap.
Code
Memory
Stack
Heap
Notice we now have two Egg objects in the Heap.
Egg
true
_raw
boolean c
Egg
public static void main(String[] args) {
Egg a = new Egg();
Egg b = a;
Egg c = new Egg();
a.boil();
// ... }
File: Demo1.java a
Egg
Egg
true
_raw
boolean b
Egg

23. Then we reach a method call.
Code
Memory
Stack
Heap
Then we reach a method call.
"Egg a, boil!"
Egg
true
_raw
boolean c
Egg
public static void main(String[] args) {
Egg a = new Egg();
Egg b = a;
Egg c = new Egg();
a.boil();
// ... }
File: Demo1.java a
Egg
Egg
true
_raw
boolean b
Egg

24. Code Memory Stack Heap this c _raw a _raw b
Java sets up a new Stack frame and automatically sets up a this variable that points to the object the method was called on.
Code
Memory
Stack
Heap
public void boil() {
_raw = false; }
File: Egg.java
this
Egg
Egg
true
_raw
boolean c
Egg
public static void main(String[] args) {
Egg a = new Egg();
Egg b = a;
Egg c = new Egg();
a.boil();
// ... }
File: Demo1.java a
Egg
Egg
true
_raw
boolean b
Egg

25. All field references refer to the fields of the this object.
Code
Memory
Stack
Heap
public void boil() {
_raw = false; }
File: Egg.java
this
Egg
Egg
true
_raw
boolean c
Egg
public static void main(String[] args) {
Egg a = new Egg();
Egg b = a;
Egg c = new Egg();
a.boil();
// ... }
File: Demo1.java a
Egg
Egg
false
_raw
boolean b
Egg

26. Code Memory Stack Heap this c _raw a _raw b
End of a void method reached, nothing to return, so we'll just delete the stack frame and return back to our book mark.
Code
Memory
Stack
Heap
public void boil() {
_raw = false; }
File: Egg.java
this
Egg
Egg
true
_raw
boolean c
Egg
public static void main(String[] args) {
Egg a = new Egg();
Egg b = a;
Egg c = new Egg();
a.boil();
// ... }
File: Demo1.java a
Egg
Egg
false
_raw
boolean b
Egg

27. Code Memory Stack Heap c _raw a _raw b Notice the state of memory...
two objects in the heap and
three local variables in main's
stack frame.
Egg
true
_raw
boolean c
Egg
public static void main(String[] args) {
Egg a = new Egg();
Egg b = a;
Egg c = new Egg();
a.boil();
// ... }
File: Demo1.java a
Egg
Egg
false
_raw
boolean b
Egg

28. The "Stack"
All local variables (and thus parameters!) are stored on the Stack
Each method/constructor call has its own "frame" of the Stack
Variables declared inside of either are stored in the frame
When a method/constructor is called, a frame is pushed onto the top of the stack
When a method/constructor completes, that frame is popped off the top of the stack

29. The "Heap" All objects and arrays are stored on the heap.
Whenever your code encounters the new keyword, this is your clue that new space is being reserved on the Heap to hold an object or array.
All variables on the stack which "hold" an object or an array hold references, also called pointers, to the actual values on the heap.
When two variables refer to the same object or array, of course changes you make to one variable will impact the other.

30. Today: Egg Russian Roulette

31. Code Walk

32. Let's Run our Program Open EggRoulette.java Enter 2 Players
We get a NullPointerException. What does that mean?

33. Follow-Along #1 – Let's Run our Program
Open EggRoulette.java
Under todo #1:
_eggs = new Carton();

34. What happens when we construct a Carton?
public Carton() {
_eggs = new Egg[12]; }
What is stored in each of _eggs' elements, though?

35. At this point notice all of the elements are empty
Code
Memory
Stack
Heap
At this point notice all of the elements are empty
public Carton() {
_eggs = new Egg[12];
_eggs[0] = new Egg(); }
File: Carton.java
Carton
_eggs:
Egg[]
this
Carton …
EggRoulette
_carton
Carton
_carton = new Carton();
File: EggRoulette.java
this
EggRoulette

36. Let's imagine assigning index 0 a new Egg…
Code
Memory
Stack
Heap
Let's imagine assigning index 0 a new Egg…
public Carton() {
_eggs = new Egg[12];
_eggs[0] = new Egg(); }
File: Carton.java
Carton
_eggs:
Egg[]
this
Carton …
EggRoulette
_carton
Carton
_carton = new Carton();
File: EggRoulette.java
this
EggRoulette

37. Code Memory Stack Heap _raw _eggs: this _carton this
We've seen that the constructor of will return a reference to the new object on the Heap
Egg
true
_raw
boolean
public Carton() {
_eggs = new Egg[12];
_eggs[0] = new Egg(); }
File: Carton.java
Carton
_eggs:
Egg[]
this
Carton …
EggRoulette
_carton
Carton
_carton = new Carton();
File: EggRoulette.java
this
EggRoulette

38. This reference is assigned to the 0 index of the array…
Code
Memory
Stack
Heap
This reference is assigned to the 0 index of the array…
Egg
true
_raw
boolean
public Carton() {
_eggs = new Egg[12];
_eggs[0] = new Egg(); }
File: Carton.java
Carton
_eggs:
Egg[]
this
Carton …
EggRoulette
_carton
Carton
_carton = new Carton();
File: EggRoulette.java
this
EggRoulette

39. Code Memory Stack Heap _raw _eggs: _carton this
After the Carton construction is complete only one Egg element is initialized… how do we do the rest?
Egg
true
_raw
boolean
Carton
_eggs:
Egg[] …
EggRoulette
_carton
Carton
_carton = new Carton();
File: EggRoulette.java
this
EggRoulette

40. Hands-on #1: Initialize all Eggs in the Carton
Inside of Carton.java
After constructing the _eggs array
Write a for loop that constructs a new Egg for every element of the array…
_eggs[ <index> ] = new Egg();
Check-in on PollEv.com/comp110 when complete

41. for (int i = 0; i < _eggs.length; i++) {
_eggs[i] = new Egg(); }

42. What happens if we don't initialize our _carton or _eggs to new object references?
Arrays that hold references to objects,
as well as fields that hold references to objects…
Are null by default.
This means each element or field refers to nothing.

43. NullPointerException
When you try to call a method on a null value, you will get a NullPointerException
For example, before we initialized _carton, when our code reached:
_carton.size()
It caused a NullPointerException because _carton currently refers to nothing it is null.
To avoid NullPointerExceptions always initialize array elements and fields!

44. How do we shuffle our Eggs in the Carton?
We need to make use of our old friend… the swap
We will also make use of a new class of object we've never seen before but is really handy for making Games…
Introducing: java.util.Random
The following code will print a random number between 0 and x, not inclusive of x:
Random random = new Random();
int x = 12;
int num = random.nextInt(x);
System.out.println(num);

45. Hands-on #2: Randomize the Eggs in the Carton
Inside of the shuffleEggs method's for loop find the todo
Notice 2 indices of the carton are being chosen at Random
Your job is to swap the Eggs at indices a and b
To do so you will need a temp variable
What type should this variable be?
To remove and put back an Egg from the Carton, you will need to refer to Carton's takeEgg and setEgg methods
Check-in on PollEverywhere when you think you have it

46. Egg temp = _carton.takeEgg(a);
_carton.setEgg(a, _carton.takeEgg(b));
_carton.setEgg(b, temp);

47. How do we make it look like 2 rows of Eggs?
The getShapes method of Carton is where our game's graphics are coming from…
Let's see if we can be clever about how we position our X and Y of each egg…

48. double x = (i % 6) * (2 * shape.getRadius() + 10.0);
double y = 0.0;
if (i > 5) {
y = 2 * (shape.getRadius() + 10); }

49. Let's play…