Transcript More on Classes

More on Classes
Pepper
With help from
http://docs.oracle.com/javase/tutorial/java/javaOO/classva
rs.html
This
inside the class code that has an instance
 Refers to the instance of the class that is running the
code right now
 Static methods have no “this” because they run the
code from the Class, itself
 http://docs.oracle.com/javase/tutorial/java/javaOO/thi
skey.html
Using Objects (such as in your main
routine)
 First create a variable to hold an instance
 MyClass x ;
 Then fill it with an instance by calling constructor
 x = new MyClass(1);
 Access its public variables with :
 x.myvar1
 Access its methods:
 x.myMethod(‘a’);
Passing Objects
 Just as arrays passed to methods are really just
passing a pointer to the array, objects pass only
pointers.
 When your main method calls mymethod(player1), it
is updating the Player object player1 inside the main
method.
 Note: Strings act like primitives and pass real copies
(not under the covers, but they act as if they do)
Encapsulating
 Why? When other programmers use your class, you must
not change anything public or you will break their code.
 Ex: Fang – if we have a game using advance(), and Fang
upgrades to insist on advance(int y), all our code breaks
 How?
 Public – everyone
 Private – no one
 Protected – extended can
 Nothing - your package (your bluej panel) can
 Method or Variable
http://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html
Class Variable
 Static – just like method static – one per class
 Ex: Class variable total # of bikes
 static int numbBikes = 0; (defaulting to 0)
 each bike instance points to that one variable # of bikes.
 Can change if your program asks it to
 numbBikes = numbBikes + 1;
 Changes the total number for all the bike instances , not just the one you are
accessing.
 Without static – a separate value for each instance
 Ex: speed, color of a bike
 int speed = 0;
 Constants
 Make it unchangeable with the word final after static
 Capitalize by convention
 ex: maximum number of bikes:
 static final int MAXBIKES = 15;
 Cannot later have MAXBIKES = MAXBIKES+1;
 http://docs.oracle.com/javase/tutorial/java/javaOO/classvars.html
Arrays holding objects
 Arrays can hold objects
 If a point has a x and y, an array of 3 points holds 3
point objects, each with their own x and y.
 Point[] p = new Point[2]; // makes 2 player miniboxes
 p[0] = new Point(1,2); // puts a point object into p[0]
 p[1] = new Point(3,4); // puts a point object into p[1]
 p[0].getDistance(p[1]); // asks p[0] for its distance
from p[1]
 See in debug
Exercises
 Do these exercises, but your deck of cards should
only have the face cards so you have less coding.
JQKA
 http://docs.oracle.com/javase/tutorial/java/javaOO/Qa
ndE/creating-questions.html
Exercise 2
 Do question #1 and Exercise 1 & 2. (Skip the garbage
collection questions.)
 http://docs.oracle.com/javase/tutorial/java/javaOO/Qa
ndE/objects-questions.html
More on Arrays – For Each
 Shortcut for loop through an array:
 for (variable to hold value : array name)
 Ex:
int[] arr= {1,2,3,4};
int tot = 0;
for (int x : arr)
{ tot = tot + x;}
System.out.println(tot);