Foundations of Programming - Java OOP

Dasar-Dasar Pemrograman 2:
Pemrograman Berbasis Objek
Tim DDP 2 Fasilkom UI
Correspondence: Fariz Darari (fariz@cs.ui.ac.id)
Feel free to use, reuse, and share this work:
the more we share, the more we have!

Why?
Try this one: Create cubes, without using classes.
2

Why?
3

Why?
4

Why?
5

Why?
6
It's cumbersome (= ribet)!

Why?
Try this one: Create cubes, now with classes.
7

Why?
8
Much simpler, isn't it?

Why?
9
Much simpler, isn't it?
Here, the class Cube encapsulates
(= membungkus) related data such
as color and length as a single unit

10
Object-oriented thinking is natural!
Here, we have that the class is Car,
and the objects are, well, all those car instances!

11
And these cute cats?
They are objects as well, of type Cat!

12
Ahh, Human..
Beautiful, yet complicated objects :)

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Objects can be abstract,
such as ideas, emotions, and knowledge

Hello, OOP!
An object has:
• states (aka. data fields/properties/attributes)
• behaviors (aka. methods)
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Hello, OOP!
An object has:
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Hello, OOP!
An object has:
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Cube
color: String
length: double
Cube()
Cube(color: String, length: double)
toString(): String
getVolume(): double
UML Class Diagram

Hello, OOP!
An object has:
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Cube
color: String
length: double
Cube()
toString(): String
getVolume(): double
UML Class Diagram
Class name
Data fields
Constructors
and methods

Objects of Cube
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Cube
color: String
length: double
Cube()
toString(): String
getVolume(): double
UML Class Diagram

Objects of Cube
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Cube
color: String
length: double
Cube()
toString(): String
getVolume(): double
UML Class Diagram
cube1: Cube
color = "Blue"
length = 1.0
UML Objects

Objects of Cube
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Cube
color: String
length: double
Cube()
toString(): String
getVolume(): double
UML Class Diagram
cube1: Cube
color = "Blue"
length = 1.0
cube2: Cube
color = "Red"
length = 2.0
UML Objects

Objects of Cube
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Cube
color: String
length: double
Cube()
toString(): String
getVolume(): double
UML Class Diagram
cube1: Cube
color = "Blue"
length = 1.0
cube2: Cube
color = "Red"
length = 2.0
UML Objects
A class is like a factory or a blueprint to create objects

Let's code the Cube class!
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public class Cube {
String color;
double length;
public Cube() {
this.color = "White";
this.length = 1.0;
}
public Cube(String color, double length) {
this.color = color;
this.length = length;
} // code continues..

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public class Cube {
String color;
double length;
public Cube() {
this.length = 1.0;
}
this.color = color;
These two guys are called:
Data fields

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public class Cube {
String color;
double length;
public Cube() {
this.length = 1.0;
}
this.color = color;
And these two guys are:
Constructors
These two guys are called:
Data fields

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public String toString() {
return String.format("Cube with length =
%.2f and color = %s",this.length,this.color);
}
public double getVolume() {
return Math.pow(this.length, 3.0);
}
}

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return String.format("Cube with length =
%.2f and color = %s",this.length,this.color);
}
return Math.pow(this.length, 3.0);
}
}
These are:
Methods

Quiz time: Let's code the Sphere class!
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public class Sphere {
String color;
double radius;
public Sphere() {
this.radius = 1.0;
}
public Sphere(String color, double radius)
{
this.color = color;
this.radius = radius;

Quiz time: Let's code the Sphere class!
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// .. continuation
return String.format("Sphere with radius = %.2f and
color = %s",this.radius,this.color);
}
return 4.0/3 * Math.PI * Math.pow(this.radius, 3.0);
}
}

Quiz time: Is this illegal?
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public class A { }

Quiz time: Is this illegal?
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public class A { }
No, this is not illegal (= not not legal = legal :-).
It's the simplest class definition you can have.
When no constructors are defined,
default constructors are used to
instantiate the objects of the class.

Recall the Cube class, let's instantiate it!
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Cube cube1 = new Cube();
Cube cube2 = new Cube("Blue", 4.0);
Objects are created using the
new operator, which calls a
relevant constructor

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code inside Cube.java

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code inside Cube.java

Now, let's access the data fields, and
call the methods.
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System.out.println(cube1.color);
System.out.println(cube1.getVolume());
System.out.println(cube2.length);

Now, let's access the data fields, and
call the methods.
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System.out.println(cube2.length);
Output:
White
1.0
4.0
64.0

Special method: toString()
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System.out.println(cube1);
public String toString() { // inside Cube.java
return String.format("Cube with length = %.2f and color = %s",
this.length,this.color);
}

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}
Printing objects calls the toString() method!

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}
Output:
Cube with length = 1.00 and color = White
Cube with length = 4.00 and color = Blue
Printing objects calls the toString() method!

Variables store references of objects
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Cube cube;
cube = new Cube("Red", 2.0);
Cube cubeCopy = cube;
cubeCopy.color = "Blue";
System.out.println(cube.color);
System.out.println(cubeCopy.color);

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Cube cube;
cube = new Cube("Red", 2.0);
Cube cubeCopy = cube;
cubeCopy.color = "Blue";
System.out.println(cube.color);
System.out.println(cubeCopy.color);
Output:
Blue
Blue

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Cube cube1 = new Cube("Red", 2.0);
Cube cube2 = new Cube("Red", 2.0);
cube2.color = "Blue";
Output:
Red
Blue

null
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It's a special value, meaning "no object".
You can print it, but..
don't you ever try accessing an attribute or invoke a method of null!
String str = null;
System.out.println(str.length());

null
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It's a special value, meaning "no object".
You can print it, but..
don't you ever try accessing an attribute or invoke a method of null!
String str = null;
System.out.println(str.length());
Exception in thread "main" java.lang.NullPointerException

Real-world classes: Set
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import java.util.HashSet;
HashSet<Integer> s1 = new HashSet<Integer>();
s1.add(3); s1.add(3); s1.add(1); s1.add(2);
s1.retainAll(s2);
System.out.println(s1);

Real-world classes: Set
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import java.util.HashSet;
s1.retainAll(s2);
System.out.println(s1); Output:
[2, 3]

Real-world classes: String
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String csui = "Fasilkom UI";
System.out.println(csui.charAt(7));
System.out.println(csui.endsWith("UI"));
System.out.println(csui.indexOf("kom"));
System.out.println(csui.replaceAll("UI", "UB"));
System.out.println(csui.toUpperCase());

Real-world classes: String
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String csui = "Fasilkom UI";
System.out.println(csui.charAt(7));
System.out.println(csui.endsWith("UI"));
System.out.println(csui.indexOf("kom"));
System.out.println(csui.replaceAll("UI", "UB"));
System.out.println(csui.toUpperCase());
Output:
m
true
5
Fasilkom UB
FASILKOM UI

Real-world classes: LocalDate
50
import java.time.LocalDate; // introduced in java 8
LocalDate dateNow = LocalDate.now();
System.out.println(dateNow);
System.out.println(dateNow.getDayOfWeek());
System.out.println(dateNow.plusDays(1));

Real-world classes: LocalDate
51
import java.time.LocalDate; // introduced in java 8
LocalDate dateNow = LocalDate.now();
System.out.println(dateNow);
System.out.println(dateNow.getDayOfWeek());
System.out.println(dateNow.plusDays(1)); // besok
System.out.println(dateNow.plusDays(2)); // lusa
System.out.println(dateNow.plusDays(3)); // tulat
System.out.println(dateNow.plusDays(4)); // tubin

Real-world classes: Random
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import java.util.Random;
public class GuessStarter {
public static void main(String[] args) {
// pick a random number from 1 to 100
Random random = new Random();
int number = random.nextInt(100) + 1;
System.out.println(number);
}
}

Quiz time: Create a method to initialize an ArrayList of
Integers with n random Integers from 11 to 20!
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public static void initRandom(ArrayList<Integer> lstInt, int n) {
Random rnd = new Random();
while(n > 0) {
lstInt.add(rnd.nextInt(10)+11);
n--;
}
}
Quiz time: Create a method to initialize an ArrayList of
Integers with n random Integers from 11 to 20!

Static variables, constants, and methods
Static means shared by all objects of the class.
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Cube
color: String
length: double
numOfCubes: int
Cube()
toString(): String
getVolume(): double
getNumOfCubes(): int
UML Class Diagram
static variable

Static variables, constants, and methods
Static means shared by all objects of the class.
56
Cube
color: String
length: double
numOfCubes: int
Cube()
toString(): String
getVolume(): double
getNumOfCubes(): int
UML Class Diagram
static variable
static method

Cube.java with static var and method
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static int numOfCubes = 0; // add this variable
public Cube() {
this.length = 1.0;
numOfCubes++; // add this line
}
this.color = color;
}
public static int getNumOfCubes() { // add this method
return numOfCubes;
}
Edit the previous Cube.java accordingly

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static int numOfCubes = 0; // add this variable
public Cube() {
this.length = 1.0;
}
this.color = color;
}
public static int getNumOfCubes() { // add this method
return numOfCubes;
}
All variables appearing in a static method, must be static!

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System.out.println(Cube.getNumOfCubes());
System.out.println(cube1.numOfCubes);
System.out.println(Cube.numOfCubes);
Output:

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System.out.println(Cube.numOfCubes);
Output:
0
2
2
2
2

Cube.java with static constant
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static final String CUBE_MATERIAL = "Silver";
System.out.println(cube1.CUBE_MATERIAL);
System.out.println(cube2.CUBE_MATERIAL);
System.out.println(Cube.CUBE_MATERIAL);
Output:
Silver
Silver
Silver

Quiz time: Create a class of Time, storing
hours, minutes, and integers.
62

63
public class Time {
int hour;
int minute;
double second;
}

64
public class Time {
int hour;
int minute;
double second;
}
We declare the data fields

65
public class Time {
int hour;
int minute;
double second;
public Time() {
this.hour = 0;
this.minute = 0;
this.second = 0.0;
}
}
We create a constructor method

66
public class Time {
int hour;
int minute;
double second;
public Time() {
this.hour = 0;
this.minute = 0;
this.second = 0.0;
}
}
this behaves like self in Python, it refers to
the object we are creating
We create a constructor method

67
// ...
public Time(int hour, int minute, double second) {
this.hour = hour;
this.minute = minute;
this.second = second;
}
}
We create another constructor:
this time you can provide
your own values for hour,
minute, and second

Quiz time: Given Time.java, what's the output?
68
Time t1 = new Time();
Time t2 = new Time(11, 30, 10.0);
System.out.println(t1.hour + ":" + t1.minute + ":" + t1.second);

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Time t2 = new Time(11, 30, 10.0);
Output:
0:0:0.0
11:30:10.0

Quiz time: Now, make a toString method for
Time.java!
70

Time.java!
71
// inside Time.java
return String.format("%02d:%02d:%04.1f",
this.hour, this.minute, this.second);
}
}

Time.java!
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// inside Time.java
return String.format("%02d:%02d:%04.1f",
this.hour, this.minute, this.second);
}
} Every object type has a method called toString that returns a string
representation of the object. When you display an object using print or
println, Java invokes the object's toString method.

73
Time t2 = new Time(11, 30, 10.0);
System.out.println(t1);

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Time t2 = new Time(11, 30, 10.0);
Output:
00:00:00.0
11:30:10.0

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Time t3 = new Time(1, 3, 2.1098);

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Output:
01:03:02.1
Time t3 = new Time(1, 3, 2.1098);

equals method
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We have seen two ways to check whether values are equal:
the == operator and the equals method. With objects you can use
either one, but they are not the same.
• The == operator checks whether objects are identical; that is,
whether they are the same object (= same memory location).
• The equals method checks whether they are equivalent; that is,
whether they have the same value.

78
What does it mean by "same" in the same value?
equals method

79
What does it mean by "same" in the same value?
We define the equals method for our objects.
equals method

Now, make the equals method for Time.java!
80
// inside Time.java
public boolean equals(Time that) {
return this.hour == that.hour
&& this.minute == that.minute
&& this.second == that.second;
}
}

Now, make the equals method for Time.java!
81
// inside Time.java
public boolean equals(Time that) {
return this.hour == that.hour
&& this.minute == that.minute
&& this.second == that.second;
}
} this always refers to the object that calls the equals method,
the naming of that, however, is arbitrary.
You can replace that with x, bla, or whatever.

82
Time t1 = new Time(11, 30, 10.0);
Time t2 = new Time(11, 30, 10.0);
Time t3 = new Time(1, 10, 8.1);
System.out.println(t1 == t2);
System.out.println(t1.equals(t2));

83
Time t1 = new Time(11, 30, 10.0);
Time t2 = new Time(11, 30, 10.0);
Time t3 = new Time(1, 10, 8.1);
Output:
false
true
false
false

Another use of this
84
public class Time {
// ...
public Time() {
this(0, 0, 0.0); // this can be used to call another constructor
}
public Time(int hour, int minute, double second) {
this.hour = hour;
}
// ...

Visibility modifiers
85
• Data fields inside a class are too "open".
• Other classes can directly access the values of those data
fields.
• We need information hiding: a way to control what can be
accessed from outside, and what cannot.
public class Time {
int hour;
int minute;
double second;
}

86
• Solution: Add private to the data fields.
public class Time {
private int hour;
private int minute;
private double second;
}

87
public class Time {
private int hour;
private int minute;
}
Can you now access those variables outside the Time class?
Try it out.

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public class Time {
private int hour;
private int minute;
}
We can control what to access by providing:
- Getter methods
- Setter methods

Getters and setters
89
• Recall that the data fields (instance variables) of Time are private. We
can access them from within the Time class, but if we try to access
them from another class, the compiler generates an error.
• For example, here's a new class called TimeClient, trying to access the
private data fields:
public class TimeClient {
public static void main(String[] args) {
Time time = new Time(11, 59, 59.9);
System.out.println(time.hour); // compiler error
}
}

Getters and setters
90
// inside Time.java
public int getHour() {
return this.hour;
}
public int getMinute() {
return this.minute;
}
public double getSecond() {
return this.second;
}
}

Getters and setters
91
// inside Time.java
public void setHour(int hour) {
this.hour = hour;
}
public void setMinute(int minute) {
}
public void setSecond(int second) {
}
}

92
Time t1 = new Time(11, 30, 10.0);
System.out.println(t1.getSecond());
System.out.println(t1.getHour());
t1.setMinute(50);
System.out.println(t1.getMinute());

93
Time t1 = new Time(11, 30, 10.0);
System.out.println(t1.getSecond());
System.out.println(t1.getHour());
t1.setMinute(50);
System.out.println(t1.getMinute());
Output:
10.0
11
50

Recall our Cube.java
94
Cube
color: String
length: double
Cube()
toString(): String
getVolume(): double
UML Class Diagram
This is the original UML,
now suppose we want the data fields to be private,
and all the methods to be public, what would change?

Recall our Cube.java
95
Cube
-color: String
-length: double
+Cube()
+Cube(color: String, length: double)
+toString(): String
+getVolume(): double
UML Class Diagram
- sign indicates private, while
+ sign indicates not private

Quiz time:
96
Can a Java object have a field of another object?

Quiz time:
97
Can a Java object have a field of another object?
Yes, why not. We have seen an example of this
in Cube.java.

Take-away messages
• Defining a class creates a new object type.
• Every object belongs to some object type.
• A class definition is like a template for objects, it specifies:
• what attributes the objects have; and
• what methods can operate on them.
• The new operator creates new instances of a class.
• Think of a class like a blueprint for a house: you can use the
same blueprint to build any number of houses.
98

Inspired by:
Liang. Introduction to Java Programming. Tenth Edition. Pearson 2015.
Chapter 10 & 11 of Think Java book by Allen Downey and Chris Mayfield.

Foundations of Programming - Java OOP

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