Last Time

1. Last Time • Started Encapsulation (using some rather strange analogies!) CISC121 - Prof. McLeod

2. 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. CISC121 - Prof. McLeod

3. Today • Continue Encapsulation CISC121 - Prof. McLeod

4. 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. CISC121 - Prof. McLeod

5. 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. CISC121 - Prof. McLeod

6. 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? CISC121 - Prof. McLeod

7. 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. CISC121 - Prof. McLeod

8. 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; } CISC121 - Prof. McLeod

9. 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 CISC121 - Prof. McLeod

10. 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.) • Noteclone() 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. CISC121 - Prof. McLeod

11. 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() CISC121 - Prof. McLeod

12. 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. CISC121 - Prof. McLeod

13. 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 CISC121 - Prof. McLeod

14. equals() Method • Accepts another object of type PrimeNums and returns true of they are equal, false otherwise. • You get to define what “equality” means. CISC121 - Prof. McLeod

15. 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 CISC121 - Prof. McLeod

16. 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. CISC121 - Prof. McLeod

17. 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 CISC121 - Prof. McLeod

18. 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!) CISC121 - Prof. McLeod

19. 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.) CISC121 - Prof. McLeod

20. 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 CISC121 - Prof. McLeod

21. 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? CISC121 - Prof. McLeod

22. 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 CISC121 - Prof. McLeod

23. 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. CISC121 - Prof. McLeod

24. 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 CISC121 - Prof. McLeod

25. 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.) CISC121 - Prof. McLeod