Lecture2.pdf

Lecture 2:
Programming with class

Plan
▪ Basic components of Object Orientated Programming
and of Java
▪ Classes and Objects
▪ Methods and Attributes
▪ Object declaration vs creation
▪ Java standard class

Recall…
▪ Demo from last lecture…
public class L1{
public static void main(String args[]) {
System.out.println("Hello World!");
}
}

Recall…
public class HelloWorld {
public static void main(String[] args)
System.out.println("Hello World");
Everything in Java is written inside a class
This class shares a name with the file
public class HelloWorld  HelloWorld.java
main() – required method for every
Java program
Prints a line of text to the screen.

Classes and Objects
▪ Java is an object-orientated programming language.
▪ Formally, objects are a combination of variables
(data structures) and functions.
▪ Intuitively, objects are a useful programming construct
to represent the real world.

Classes and Objects
▪ Example: A car can be described by a set of
characteristics:
– manufacturer, model, colour, engine capacity, number of seats
etc…
– For objects, these are called the attributes.
▪ A car can also do various things:
– accelerate, brake, change gear, turn left, turn right etc.
– For objects, these are called the methods.
▪ EXERCISE: Describe the attributes and methods of
– A printer
– A bank account

Classes and Objects
▪ Class vs Object
▪ A class is a blueprint (template).
▪ An object is an instance of a class.
– A specific creation with specific values.

Classes and Objects
▪ Intuitively (not strictly accurate):
▪ Type ↔ Variable is similar to Class ↔ Object
▪ Variables are created from a type, objects are created
from classes
▪ Types define variables size, how they are treated etc…
▪ Classes give definition to objects, what information they
hold, how they behave, what they can do etc.
▪ Think cookie cutter vs cookie.

Classes and Objects
Summary so far:
▪ Classes give the blueprints
▪ Objects are instances of classes
▪ We say that objects belong to classes
▪ Classes and objects have properties (variables) – attributes
or data values or data members
▪ Classes and objects can do things (functions) – methods
(sometime called messages…)
▪ Together, the attributes and methods of a class or object are
called its members.

Classes and Objects
▪ I need to create a class for a robot
– Attributes?
– Methods?
▪ What about a class for an Engineering degree?
– Attributes?
– Methods?

Class declaration
▪ Structure of a class declaration
▪ Example: Class with attribute x and methods to display
its value and increment its value.
class MyX {
int x = 0;
void printX() {
System.out.println(x);
}
void incrememtX(int a) {
x += a;
}
}

Class declaration
class MyX {
int x = 0;
void printX() {
}
x += a;
}
}
Class name

Class declaration
class MyX {
int x = 0;
void printX() {
}
x += a;
}
}
Data member / Attribute / Member variables

Class declaration
class MyX {
int x = 0;
void printX() {
}
x += a;
}
}
Method

Class declaration
▪ Attributes work the same as variables.
– Assign values to them, include them in expressions etc.
▪ Methods work the same as functions.
– Call them, received values back from them, pass variables to them.

Class declaration
▪ EXERCISE: Write the class declaration for a car with:
▪ Attributes – Make, Year, CurrentSpeed and
SteeringAngle
▪ Methods - Accelerate(float SpeedInc), TurnLeft() and
TurnRight()

Class declaration
class Car{
String Make;
int Year;
float CurrentSpeed = 0.0f;
int SteeringAngle = 0;
void Accelerate(float SpeedInc) {
CurrentSpeed +=SpeedInc;
}
void TurnLeft() {
SteeringAngle ‐= 5;
}
void TurnRight() {
SteeringAngle += 5;
}
}

Using classes
▪ Now if we have a class and we want to make an object from
that class (instantiate the class)…
▪ Recall: difference between declaration and creation…
▪ Object declaration: designates the name of an object and
the class it belongs to:
▪ Object creation: allocation of space in memory.

Using classes
▪ Now if we have a class and we want to make an object from
that class (instantiate the class)…
▪ Recall: difference between declaration and creation…
▪ Object declaration: designates the name of an object and
the class it belongs to:
<class name> <object name>;
Car c;
▪ Normal identifier naming rules apply
– Start with nondigit
– Can use letters, digits, underscores etc
– Case sensitive

Using classes
▪ Object creation: allocation of space in memory.
<object name> = new <class name>;
c = new Car();
▪ Can do both together:
<class name> <object name> = new <class name>;
Car c = new Car();

Using classes
▪ Declaration: Identifier is created, points to nothing.

Using classes
▪ Creation: Object is created and identifier now points to it.

Using classes
Accessing methods and data members:
▪ This is done using a .
– <object name>.<attribute name>
– <object name>.<method name>
▪ e.g.
– if (c.CurrentSpeed > 120f)
c.Accelerate(‐10f);

Using classes
▪ Back to example:
– Everything in Java happens in a class.
– The main function is just a method inside of a class – should
have the same name as the file
– Now, inside main, create an object from the class car we created.
public class L2 {
public static void main(String[] args) {
<SOME STUFF SHOULD HAPPEN HERE>
}
}

Using classes
public class L2 {
Car c = new Car();
c.Accelerate(10f);
c.TurnLeft();
c.TurnRight();
c.TurnRight();
}
}

Using classes
public class L2 {
Car c = new Car();
c.Accelerate(10f);
c.TurnLeft();
c.TurnRight();
c.TurnRight();
}
}
Object declaration and
creation

Using classes
public class L2 {
Car c = new Car();
c.Accelerate(10f);
c.TurnLeft();
c.TurnRight();
c.TurnRight();
}
}
Calling some object
methods

Using classes
▪ We can create multiple objects from the same class –
same attributes, different values.
public class L2 {
Car c1 = new Car();
Car c2 = new Car();
c1.Accelerate(10f); //c1.CurrentSpeed will be 10.0
c2.Accelerate(20f); //c2.CurrentSpeed will be 20.0
}
}

Using classes
▪ EXERCISE: What will this do?
public class L2 {
Car c;
c = new Car();
c = new Car();
}
}

Using some Java classes
▪ The classes we have considered so far are
programmer-defined classes.
▪ There are also Java standard classes, which are
predefined and come with Java.

▪ Classes are grouped together in packages
▪ Often Java classes are imported from packages.
▪ To use a class from a package, similar to objects and members,
use the . :
<package name>.<class name>
▪ Packages can contain subpackages and are referenced the same
way
<package name>.<package name>...<package name>.<class name>

▪ Usually we use import to avoid this long name
import <package A>.<package B>.*
which means “import everything from package B”.
▪ Using import means we can use
<class C>
instead of using
<package A>.<package B>.<class C>
every time we use the <class C>

▪ Note: java.lang package is automatically included
▪ Can simply use System.out() instead of java.lang.System.out()

▪ Math class – not the thing you’re all looking forward to after
this
▪ Also included automatically in java.lang
▪ Contains functions for many common mathematical functions
▪ Example: Math.abs(x), Math.sin(x), Math.pow(x, y),
Math.random()

▪ Next week: practice some OOP using the Java standard
classes.

Lecture2.pdf

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