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

1

2

3

4

(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

Lecture Lab

edX > B2 > Lecture Lab

Note: constructors are just methods in call graphs, e.g.

Person
Person

The End - Thank You!

B2: Methods & Method Calls

B2 Methods and Method Calls

By Steven Wolfman

Source: Felix Grund

B2 Methods and Method Calls

  • 517

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