1. CMPE212 – Stuff… Exercises 4, 5 and 6 are all fair game now.
Winter 2018 CMPE212
11/10/2018
CMPE212 – Stuff…
Exercises 4, 5 and 6 are all fair game now.
Quiz 1 is graded.
Winter 2018
CMPE212 - Prof. McLeod
Prof. Alan McLeod

2. Today
Encapsulation, Cont.
Winter 2018
CMPE212 - Prof. McLeod

3. An Example, Cont.
It would be easier to design a Class that contains (or encapsulates) these four fields and then design another class to collect instances of this Class.
No way the fields could get out of sync since moving a record moves all fields at once.
Collection
record 1
record 2
temp
array
year
record 3
record 4
number
record 5
condition
record 6
....
Winter 2018
CMPE212 - Prof. McLeod

4. Encapsulation, Cont.
Attributes must be declared private, so that the class that owns them can control how they are set.
If attributes are private then how can a user of the class assign the values of the attributes?
Through methods, of course!
Constructor(s)
Mutator(s)
when instantiated only
anytime after instantiation
Winter 2018
CMPE212 - Prof. McLeod

5. Encapsulation, Cont.
Within these methods, you can write code to check the parameters for validity.
What do you do if the values are not legal?
Throw an exception!
We need to start adding Behaviour to our Class.
Winter 2018
CMPE212 - Prof. McLeod

6. Defining Exceptions
You can throw an exception already defined in java, but:
Most of the time you will want to throw your own exception objects.
See the next slide for the definition of an Exception object. (Please ignore indentation and line wrapping…)
Winter 2018
CMPE212 - Prof. McLeod

7. Defining Exceptions, Cont.
public class IllegalHalloweenException extends Exception {
public IllegalHalloweenException() {
super("Illegal parameter value supplied to" + "Halloween object."); }
public IllegalHalloweenException(String message) {
super(message);
} // end IllegalHalloweenException
Winter 2018
CMPE212 - Prof. McLeod

8. Defining Exceptions, Cont.
Inside a method that detects an illegal parameter:
throw new IllegalHalloweenException("Illegal number of kids.");
At the end of the method header that contains the above line of code:
… throws IllegalHalloweenException {
Called a “throws decoration”.
Winter 2018
CMPE212 - Prof. McLeod

9. Defining Exceptions, Cont.
This example contains a few Java keywords that we have not yet discussed:
extends
super
throw
throws
The exact meaning and usage of extends and super will become clear when we discuss object hierarchies.
For now, just follow the pattern.
Winter 2018
CMPE212 - Prof. McLeod

10. Assigning Private Attributes - Constructors
Constructors are special methods that have the same name as the class in which they reside, but have no return type (not even void).
They must be public.
They are only executed once, when the object is being instantiated. You can’t invoke them later.
Constructors are often overloaded. This means you can have many constructors with different parameter lists. Why do that?
Winter 2018
CMPE212 - Prof. McLeod

11. Halloween2 Halloween2.java uses a constructor to set all parameters.
It also throws an exception if illegal values are encountered.
See how this object is used in TestHalloween2.java.
Note that this class is not complete – it only has the one constructor, for now.
Winter 2018
CMPE212 - Prof. McLeod

12. Aside - Information Privacy
Having private attributes is a good start.
But is temperatures (an array of int) really private?
Can I change the contents of the private array from outside the class? (How can I tell…?)
How do I fix this?
Winter 2018
CMPE212 - Prof. McLeod

13. What Does .clone() Do?
Suppose you want to make a completely separate copy of an int array called stuff:
int[] clonedStuff = stuff.clone();
Or:
int[] clonedStuff = new int[stuff.length];
for(int i = 0; i < stuff.length; i++)
clonedStuff[i] = stuff[i];
Does the same thing.
Winter 2018
CMPE212 - Prof. McLeod

14. More on .clone() The only method that an array has.
But many other objects have a clone() method of their own.
Ours should too!
(In fact clone() is a member of Object, so all objects in Java inherit clone(), but this method may not do what you want for all objects.)
Winter 2018
CMPE212 - Prof. McLeod

15. Assigning Private Attributes - Constructors, Cont.
Note that once you have written a constructor with parameters, you can no longer use a constructor without any parameters, called the default constructor.
If you want an empty constructor, you must write one yourself, in addition to the parameterized constructor(s).
Why would you want an empty constructor?
Winter 2018
CMPE212 - Prof. McLeod

16. Aside – Preventing Instantiation
Provide only the default constructor and make it private.
Pretty sneaky!!
Used by the Math class, for example.
There is no need to instantiate the Math class since all its methods and attributes are static.
Winter 2018
CMPE212 - Prof. McLeod

17. Assigning Private Attributes - Constructors, Cont.
Suppose you don’t want to have to force the user to supply all parameters when he instantiates the object? How are you going to do this?
Suppose you want to edit parameters after you have created the object? How are you going to do this, too?
Overload the constructors.
Provide mutator(s).
Winter 2018
CMPE212 - Prof. McLeod

18. Assigning Private Attributes - Mutator Methods
Called “set” methods – in fact, by convention, the word “set” is the first word in the method name.
These methods must also check for legal parameters.
Usually the constructor invokes mutators, when they exist.
Should you write a mutator for each parameter?
Winter 2018
CMPE212 - Prof. McLeod

19. Halloween3 Has overloaded constructors. Has mutators.
Better information privacy.
A Halloween3 object can be created with or without the weather condition, but must have the year, the number of munchkins and the temperatures.
All parameters can be changed later.
How can you still let the constructor use mutators, but not allow the user to change an attribute later?
Winter 2018
CMPE212 - Prof. McLeod

20. Aside - The this Thing
Used by one constructor to invoke the other one.
Can be used to supply a reference to the current object in that object’s code during runtime.
this means “myself”.
Another use – suppose I used the same attribute name as a constructor parameter – year, for example. In the constructor, I would need to say:
this.year = year;
Winter 2018
CMPE212 - Prof. McLeod

21. We Need More!
How do we get data values back out of the Halloween3 object?
What happens when you try to print the object?
How can you compare objects for sorting and searching?
How do you clone the object?
Finally – how do we document the object so other people can use it without looking at the code?
Winter 2018
CMPE212 - Prof. McLeod

22. Accessors Accessor methods return the value of an attribute.
By convention, accessor methods start with the word get.
One accessor per attribute.
For example:
public int getNumMunchkins () {
return numMunchkins;
} // end getNumMunchkins Accessor
Winter 2018
CMPE212 - Prof. McLeod

23. Accessors, Cont. These are pretty simple methods.
Except when you are returning a mutable object.
What is wrong with the following?
public int[] getTemperatures() {
return temperatures;
} // end getTemperatures Accessor
Winter 2018
CMPE212 - Prof. McLeod

24. Accessors, Privacy Leaks, Cont.
Better:
public int[] getTemperatures() {
return temperatures.clone();
} // end getTemperature Accessor
Winter 2018
CMPE212 - Prof. McLeod