Review for Exam 1

1. Review for Exam 1 As you arrive…please get a handout

2. Today is All Review • Inheritance • General Exam Advice • HashCode/Equals • Comparable

3. Inheritance • When one class “extends” another, it gets all of the functions (and variables) that it’s superclass has class Superclass { public intgetNum() { return 45; } } class Subclass extends Superclass { //empty! } //in some function… Subclass var = new Subclass(); //var has a getNum method, even though it’s //got no code! It inherits the method from //Superclass var.getNum();

4. Because a subclass can do everything a superclass can do, you can use a subclass in any variable that expects a superclass class Car { public String honk() { return “honk”; } } class ToyotaCamry extends Car { } public void doHonk(Car carVar} { System.out.println(carVar.honk()); } Car mikesCar = new ToyotaCamry(); //a method parameter is just another kind of variable doHonk(new ToyotaCamry());

5. But subclasses can add new methods or change the way existing methods work class Car { public String honk() { return “honk”; } } class ToyotaCamry extends Car { // we say that ToyotaCamry “overrides” the method honk in car public String honk() { return “beep”; } } public void doHonk(Car carVar} { System.out.println(carVar.honk()); } Car mikesCar = new ToyotaCamry(); Car regularCar = new Car(); //prints beep doHonk(mikesCar); //prints honk doHonk(regularCar);

6. An abstract class is a class that is missing one or more methods abstract class IntCollection { public abstract add(int new value); } //abstract classes can never be created //this will not compile! IntCollection collection = new IntCollection(); Intcollection.add(7);

7. The point of abstract classes is to have subclasses that override their abstract methods abstract class IntCollection { public abstract add(int new value); } class IntArrayList extends IntCollection { public add(intnewValue) { myList.add(newValue); } } //variables of abstract classes can still store //non-abstract subclasses IntCollection collection = new IntArrayList(); Intcollection.add(7);

8. Interfaces are like abstract classes, except they can’t have any non abstract methods (and you use implements not extends) interface IntCollection { public abstract add(int new value); } class IntArrayList implements IntCollection { public add(intnewValue) { myList.add(newValue); } } IntCollection collection = new IntArrayList(); Intcollection.add(7);

9. Write the class definitions to match this code Ninja[] ninjas = new Ninja[2]; ninjas[0] = new FireNinja(); ninjas[1] = new Ninja(); for(Ninja ninja : ninjas) { ninja.attack(); } //code prints // “You are attacked by a fire ninja” // “You are attacked by a ninja” //Question: // Could Ninja have an abstract method? // Could Ninja be an interface? // Could this code compile – FireNinjafoo = new Ninja();

10. class Ninja { public void attack() { System.out.println(“You are attacked by a ninja”) } } class FireNinja extends Ninja { public void attack() { System.out.println(“You are attacked by a fire ninja”) } } // Could Ninja have an abstract method? NO! // Could Ninja be an interface? NO! Abstract class and interfaces can not be instantiated (i.e. you couldn’t say “new Ninja()”) // Could this code compile – FireNinjafoo = new Ninja(); NO! – Ninja is the superclass, not the other way around

11. Today is All Review • Inheritance • General Exam Advice • HashCode/Equals • Comparable

12. What will be on the exam • 3 Questions about just solving ordinary APish problems, often with structures like maps, sets, and lists • 3 Questions about classes hashcode equals and comparable • 1 Question about Big O • 2 Questions about recursion

13. Advice • Don’t forget you get 1 (2-sided) page of notes • Arrive on time, keep track of your time through the exam • When solving coding problems, try to stay as close to genuine Java as you can • Never leave a question blank – if you can solve 50% of a problem, that is worth partial credit • A correct non-recursive solution to a problem that requires recursion is 1/5 points • Don’t forget your base cases for recursion problems! It’s considered a big miss if it’s not there • Stop immediately when time is called. If you don’t it’s 10% off your exam grade.

14. Approximately how I grade coding questions • 5 points, fully functional algorithm maybe with the occasional missed semicolon or misnamed library call • 4 points, mostly functional algorithm with an edge case missed • 3 points, basically the right idea for the algorithm but several missed cases or errors • 2 points, part of the algorithm done correctly, some other part wrong • 1 point, some functional code that shows me you understood part of what needed be done, even if you couldn’t do the whole problem

15. Today is All Review • Inheritance • General Exam Advice • HashCode/Equals • Comparable

16. HashCode and Equals • All classes inherit from object • All classes have the methods equals and hashCode • But sometimes you want to change them: myMap = new HashMap<ComplexNum, String>(); myMap.put(new ComplexNum(0,0), “ZERO”); // somewhere far away in a different // function myMap.get(new ComplexNum(0,0));

17. What we want myNum = new ComplexNum(0,0); myNum.equals(new ComplexNum(0,0)) { //this should be true } How can we make that happen?(Hint rhymes with “Boveriding”)

18. The catch • Equals needs some boilerplate (just look it up if you need it) • BUT you also must be consistent with hashCode • THE RULES: • If myVar1.equals(myVar2), myVar1.hashCode() must equal myVar2.hashCode() • To the greatest extent possible, it’s good if myVar1 does not equal myVar2 myVar1.hashCode() does not equal myVar2.hashCode()

19. Today is All Review • Inheritance • General Exam Advice • HashCode/Equals • Comparable

20. Comparable • Oftentimes we want to sort our objects • But how do we compare objects to sort them? • Here’s what we want: CheezBurger a = getBurgerA(); CheezBurger b = getBurgerB(); if(a.isBiggerThan(b)) { //a is bigger than b } • But in Java, we don’t use “isBiggerThan” we use “compareTo”

21. compareTo • Returns an int • If the int is possitive, bigger • If the int is negative, smaller • If the int is 0, equal size CheezBurger a = getBurgerA(); CheezBurger b = getBurgerB(); if(a.compareTo(b) > 0) { //a is bigger than b } Note: there are two objects in this comparison, but only 1 parameter

22. Remember class function’s always have a current instance class CheezBurger { //more functions private String mySauce; public String getSauce() { return mySauce; } } //elsewhere CheezBurger b1 = new CheezBurger(“MAYO”); CheezBurger b2 = new CheezBurger(“Mustard”); System.out.println(b1.getSauce());

23. CheezBurger a = getBurgerA(); CheezBurger b = getBurgerB(); if(a.compareTo(b) > 0) { } //far away…in class CheezBurger intcompareTo(CheezBurger other) { //we have two CheezBurgers here //the CheezBurger we’re IN and //The CheezBurger we’re passed return mySauce.compareTo(other.mySauce); }

24. One final detail • Classes that have the compare to function should always implement the interface Comparable<ClassName>: class CheezBurgerimplements Comparable<CheezeBurger> { intcompareTo(CheezBurger other) { return mySauce.compareTo(other.mySauce); } } //elsewhere ArrayList<CheezBurger> burgers = getAllBurgers(); Collections.sort(burgers); //calls our compareTo

25. Ok…your turn Write a new class UtimateCheezBurger. It has two instance variables, a sauce (like the CheezBurger) and a numberOfBurgers. We want to sort UtimateCheezBurger first by sauce, then by numberOfBurgers (increasing). So Mayo,3 comes before Mayo,6 which comes before Mustard,2 Oh and make sure your class has a constructor so I can initialize the UltimateCheezBurger like this: UltimateCheezBurger b = new UltimateCheezBurger(“Mayo”,4);

26. class UltimateCheezBurger implements Comparable<UltimateCheezBurger> { String mySauce; intmyBurgers; public UltimateCheezBurger(String sauce, int burgers) { mySauce = sauce; myBurgers = burgers; } public intcompareTo(UltimateCheezBurger other) { intsauceCompare = mySauce.compareTo(other.mySauce); if(sauceCompare != 0) return sauceCompare; return myBurgers – other.myBurgers; } }