CS2030S AY24/25 S2 Lecture 2

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## CS2030<sub>s</sub>

### Programming Methodology II
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## Lecture 2

### 20 January 2024
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### Announcements:

- Quizzes: Lecture 2, Programming Environment, Vim
- Problem Set 1
- Practical:
	- Setup GitHub account
	- Link to PE
	- Submit your username via the online form
    - Exercise 0
- Labs and Recitations start this week

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### Important Dates

.smaller[
Assessment         | Date                       | Time
:------------------|:-------------------------- |:-------------
Midterm | Mon, 10 Mar | 12 - 2 PM
Practical Exam 1   | Thur, 6 Mar | Time: Between 4 to 10 pm
Practical Exam 2   | Thur, 17 Apr | Time:  Between 4 to 10 pm
Final | Wed, 30 Apr | 1 - 3 PM
]

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### The CS2030S Ticketing System™

[https://mysoc.nus.edu.sg/app/cs2030s](https://mysoc.nus.edu.sg/app/cs2030s)

Your first choice for administrative questions and to contact the teaching team.

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### Lab and Recitation Registration

Academic staff are *not* in charge of this process so appeal using CourseReg. The add / swap period ends on Mon, 20 Jan, 5pm. If you do not secure a slot:

- Please submit an appeal through CourseReg under the Add/Change Tutorial Class category.

- Register for any slot that has availability and does not clash with your timetable.

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### Lab and Recitation Registration

Only file a ticket on the CS2030S Ticketing System if both statements below are true for you:

- Cannot secure a tutorial / lab / recitation slot AFTER the Add / Swap period

- Cannot appeal for any slot with vacancy, as all remaining slots with vacancy have timetable clashes.

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### Topics to cover today:

- Information Hiding, Tell Don't Ask
- Class fields and methods
- Composition
- The Heap and the Stack
- Inheritance
- Overriding

(Unit 5 to Unit 12)

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.small[```Java
class Circle {
	double x;
	double y;
	double r;
  
	double getArea() {
	  return 3.141592653589793 * r * r;
	}
}
```]

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### Information Hiding

New Java syntax: _access modifiers_

- private
- public

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New Java keyword: **this**

```Java
this.x = x;
```

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### Constructor

.small[```Java
public Circle(double x, double y, double r) {
    this.x = x;
    this.y = y;
    this.r = r;
}
```]
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### Accessors and Mutators

.small[```Java
public double getR() {
	return this.r;
}

public void setR(double r) {
	this.r = r;
}
```
]

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### Checking if a point is in a circle

.small[```Java
double cX = c.getX();
double cY = c.getY();
double r = c.getR();
boolean isInCircle = ((x - cX) * (x - cX) + 
                     (y - cY) * (y - cY)) 
                     <= r * r;
```]
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### Tell, Don't Ask

.small[```Java
boolean isInCircle = c.contains(x, y);
```]

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.small[```Java
c.contains(x, y);
```]

`c` is called the target of method invocation `contains`

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### Class Fields and Methods

Java keyword `static`

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### Class Fields

.tiny[```Java
class Math {
	:
	public static final double PI = 3.141592653589793;
	:
	:
}
```]

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### Class Methods

.small[```Java
public static int getNumOfCircles() {
    return Circle.lastId;
}
```
]
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### `main`

.small[```Java
public static void main(String[] args) {
	System.out.println("Hello World!");
}
```
]
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### Composition

### Models the "has-a" relationship

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## Stack and Heap

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### Aliasing

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### Inheritance

### Models the "is-a" relationship

In Java, we use the `extends` keyword.

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### Our first subtyping rule for Reference Types

If `S` inherits from `T`, then `S` $<:$ `T`.

Reminder:  Subtyping is Reflexive and Transitive

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### Subtyping with Reference Types

If `ColoredCircle` $<:$ `Circle`

.small[```Java
ColoredCircle c = new Circle(p, 0); // error
Circle c = new ColoredCircle(p, 0, blue); // OK
```]

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### Compile-Time Type vs. Run-Time Type

.small[```Java
Circle c = new ColoredCircle(p, 0, blue);
c = new Circle(p, 0);
```]

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### Ok, but what is at the top of this class hierarchy?

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### java.lang.Object

### The mother of all classes!

Now a chance to have a look at the [java documentation!](https://docs.oracle.com/en/java/javase/21/docs/api/index.html)

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### java.lang.Object

We will discuss two methods:

`toString()`

`equals(Object obj)`

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### Method Signature

The method name, type of arguments, order of arguments, and number of arguments.

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### Overriding

`@Override`
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### Topics covered today:

- Information Hiding, Tell Don't Ask
- Class fields and methods
- Composition
- The Heap and the Stack
- Inheritance
- Overriding

(Unit 5 to Unit 12)

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### Reminders:

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### Reminders:

Don't just **read** the examples given... have a **play** around with them. Try changing something and see what happens.

You can not learn effectively without trying it yourself!

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.tiny[
Version: v1.0

Last Updated: Sun Jan 19 19:45:00 +08 2025
]