B2: Methods and Method Calls

CPSC 210

Learning Goals

Identify the type of an object
Identify a method call and the object on which it is called
Identify a method definition
Draw a call graph starting from a specified method
(Use IntelliJ to navigate a sequence of method calls)

OO Terminology

Class	Cat
Object / Instance	Snowy
Method	meow(3), sleep(), eat()
Attribute / Field	name, age, weight
Constructor	Create a new cat object (e.g. Snowy)

OO Terminology (2)

Class	Dog
Object / Instance	Chuck
Method	bark(2), sleep(), eat()
Attribute / Field	name, age, weight
Constructor	Create a new dog object (e.g. Chuck)

Methods and Objects

Clone the B02-SpaceInvadersBase repo if you have not yet done so:
https://github.students.cs.ubc.ca/CPSC210/B02-SpaceInvadersBase

Class Example: Tank

Class

Methods

Attributes/Fields

Constructor

(View > Tool Windows > Structure)

Tank Code Example

Tank myTank;
myTank = new Tank(400);
myTank.faceLeft();
myTank.move();

Declare new variable myTank of type Tank (just a reference!)

Assign new Tank object to variable

Call faceLeft method on Tank object

Call move method on Tank object

Variables, Objects, & References

Tank myTank;

This does not create an object/instance yet, but only a variable holding a reference to a tank
We need to create an object specifically

myTank = new Tank(400);

Tank object

direction 1

x 400

myTank

A variable is space to hold one specific type
of thing, like a reference to a tank:

Tank myTank = new Tank(400);

Tank object

direction 1

x 400

myTank = new Tank(99);

But we can change what's
in it by assigning to it:

Variables, Objects, & References (2)

myTank

A variable is space to hold one specific type
of thing, like a reference to a tank:

Tank myTank = new Tank(400);

Tank object

direction 1

x 400

myTank

myTank = new Tank(99);

But we can change what's
in it by assigning to it:

Variables, Objects, & References (2)

Tank object

direction 1

x 99

A reference is an address of an object in memory.
That's a number like a building's address.
We draw it as an arrow.

Tank myTank = new Tank(400);

Tank object

direction 1

x 400

myTank

Tank myTank2 = myTank;

But it's really just a number.
When we copy it, we get the
same number: not a new object!

myTank2

Variables, Objects, & References (3)

myTank.faceLeft();

myTank

Tank object

direction 1

x 400

Variables, Objects, & References (4)

Let's call a method with the dot operator:

myTank.faceLeft();

myTank

Tank object

direction -1

x 400

Variables, Objects, & References (4)

Let's call a method with the dot operator:

myTank.move();

x 400

direction -1

Tank object

myTank

Variables, Objects, & References (5)

Again: find the object, execute the method on it:

myTank.move();

x 399

direction -1

Tank object

myTank

Variables, Objects, & References (5)

Again: find the object, execute the method on it:

Call Graph Example

Tank
move

Tank
handleBoundary

Tank.move

GamePanel.drawMissiles

GamePanel.drawMissiles

GamePanel.drawMissiles

GamePanel
drawMissiles

SIGame
getMissiles

GamePanel
drawMissile

Missile
getX

Missile
getY

B2: Methods and Method Calls

CPSC 210

Learning Goals

OO Terminology

OO Terminology (2)

Methods and Objects

Class Example: Tank

Tank Code Example

Variables, Objects, & References

Variables, Objects, & References (2)

Variables, Objects, & References (2)

Variables, Objects, & References (3)

Variables, Objects, & References (4)

Variables, Objects, & References (4)

Variables, Objects, & References (5)

Variables, Objects, & References (5)

Call Graph Example

GamePanel.drawMissiles

Lecture Lab

edX > B2 > Lecture Lab

The End - Thank You!

B2: Methods & Method Calls