Winter 2006 CISC121 - Prof. McLeod 1

Last Time

• Started Encapsulation (using some rather strange analogies!)

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Stuff

• Note that PrimeNums sample files on web site have been updated (last night). Please download them again if you have already done so.

• Only the files– TestPrimeNumsPrivacy.java– PrimeNumsException.java

have not changed.

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Today

• Continue Encapsulation

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So Far,

• “We” decided that attributes must be declared private. Otherwise the object cannot control the value of the attribute.

• To set the value of the attribute you can use the constructor upon instantiation, or invoke a mutator method.

• Either method could throw an exception if an attempt is made to set the attribute(s) to illegal values.

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PrimeNums Mutator?

• You can decide whether to add a “set” method to your class or not.

• If you do, this means the user can create an empty instance of PrimeNums and then populate it, or you can change the contents of a PrimeNums object any time.

• How does the mutator indicate a problem?

• See PrimeNumsWithMutator.java and TestPrimeNumsWithMutator.java.

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More Questions!

• Why does the constructor and the mutator assign the array nums element by element instead of using nums = array;?

• Why does PrimeNumsWithMutator need an empty (or “default”) constructor?

• Remember the first version of PrimeNums? (Slide 6 – Feb. 7 lecture) It did not have any constructors, but we could still instantiate it. What is going on here?

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Default Constructors

• If you do not have any constructors, the compiler will create an empty, or “default”, constructor for you. It has no parameters and does nothing.

• If you write any constructor yourself, then the compiler no longer supplies the default constructor.– In this case if you want an empty constructor (no

parameters), you will need to write it yourself.

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Accessor Methods

• We need to have some way to get the data back out of the Object. Right? Why?

• This is done using accessor or “get” methods.

• What is the problem with:

public int[] getNums () {

return nums;

}

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A Better Accessor for PrimeNums

public int[] getNums () {

int[] temp = new int[nums.length];for (int i = 0; i < temp.length; i++)

temp[i] = nums[i];

// or use:// int[] temp = nums.clone();

return temp;

} // end getNums accessor

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Aside - the clone() Method• It is the only method that an array has.• It returns a full, but completely independent copy

of the array.• It is not aliased to the original array at all.• (It is called a “deep copy” by some coders.)• Note clone() does not work on 2D arrays – it

produces only a “shallow copy”.

• Nearly all objects in Java have a clone() method, so (as not to be different!) we should put one in our PrimeNums class.

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What Else does PrimeNums Need?

• There are many standard methods that can be added to a class, depending on its purpose. These three are fairly standard to all classes:– toString()– equals()– compareTo()

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toString() Method

• As it is, if we try to print a PrimeNums object, we will just get “gobbldeygook”:

PrimeNums@923e30

• (This String is composed of the object type and its hash code…)

• So, to get a more useful view of the contents of the object, define a toString() method that returns a String.

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toString() Method, Cont.

public String toString () {

String s = "Array contents: ";

for (int i = 0; i < nums.length; i++)

s = s + nums[i] + " ";

return s;

} // end toString method

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equals() Method

• Accepts another object of type PrimeNums and returns true of they are equal, false otherwise.

• You get to define what “equality” means.

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equals() Method, Cont.public boolean equals (CompletePrimeNums

otherPrimeNums) {

if (nums.length != otherPrimeNums.nums.length) return false;

//It would make more sense to sort both arrays first!for (int i = 0; i < nums.length; i++)

if (nums[i] != otherPrimeNums.nums[i])return false;

return true;

} // end equals

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Aside - the equals() Method in Object

• Every object created in Java inherits all the methods in the Object class, including:

public boolean equals (Object o) {….}

• This method only compares memory addresses.

• Not much use!• Advanced topic: How to override this method.

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Aside - the equals() Method in Object, Cont.

• The proper equals method would start out like:

public boolean equals (Object o) {

if (o == null) return false;

if (!(o instanceof CompletePrimeNums)) return false;

CompletePrimeNums otherPrimeNums =(CompletePrimeNums)o;

// rest of method is the same

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Aside - the equals() Method in Object, Cont.

• A couple of new things here:– instanceof – checks to see if an object’s

underlying type matches a certain type.– Casting a generic Object to be a specific type.

(You should never do this unless you have checked the type by using instanceof first!)

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compareTo() Method

• Compares a supplied PrimeNums object with the current one, based on your comparison criteria.

• It returns an int value.• (Like the compareTo() method in the String

class.)

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compareTo() Method, Cont.

public int compareTo (CompletePrimeNums otherPrimeNums) {

// Assume a comparison based on length of array //only

return nums.length - otherPrimeNums.nums.length;

} // end compareTo

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compareTo() Method, Cont.

• Object does not have a compareTo() method, so we don’t have to override one.

• You could still write compareTo() as:

public int compareTo (Object o) {…}

• If instanceof returns false what do you do?

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clone() Method

public CompletePrimeNums clone () {

CompletePrimeNums temp = null;try {

temp = new CompletePrimeNums(getNums());} catch (PrimeNumsException e) {

// do nothing!} // end try catch

return temp;

} // end clone method

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clone() Method, Cont.

• By calling getNums() we are not worried about aliasing, since getNums() already takes care of that.

• Do we ever have to worry about actually catching a PrimeNumsException here?

• (The instantiation still has to be in a try/catch block – as dictated by the merciless compiler!)

• This clone() method makes a proper, “deep” copy of the current object.

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A Complete PrimeNums Class

• Look at “CompletePrimeNums.java”• Methods in CompletePrimeNums:

– CompletePrimeNums ()– CompletePrimeNums (int[])– void setNums (int[])– int[] getNums ()– String toString ()– boolean equals (Object)– int compareTo (CompletePrimeNums)– CompletePrimeNums clone ()

throw PrimeNumsException

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Demonstrating Privacy Violations

• (in a clean way!)

• Look at “CompletePrimeNumsNoPrivacy.java” and “TestPrimeNumsPrivacy.java” to see the effects of privacy leaks!

• (Also contains a more sophisticated toString() method.)