CS 125 Exam Review Winter 2008 Some Exam
- Slides: 33
CS 125 Exam Review Winter 2008
Some Exam Info • Tuesday (22 nd) at 4: 00 -6: 30 pm in the PAC • CHECK YOUR SEAT!!! • Read Final Exam Information on website – Practice questions & old exams – Reference sheet • Bring WATCARD, pencil, eraser only
Old Topics • Primitive variables • Declare • Initialize • Operations • Mod, Division • String methods • Board class • Coordinate class • Scanner • next(), next. Line() • Boolean statements • AND, OR, NOT • If statements • Loops • for • while • Tracing
Overloading Two methods in the same class with… • • • Exact same method name Different parameters (by number or type) Ignore return type • Examples: index. Of, put. Peg • Midterm: Why the overloaded methods didn’t work?
Class Diagrams public class Cat { //instance variables private String name; //constructor public Cat(String n) {…} //methods public void meow() {…} } Cat - String name … + Cat(String n) - void meow() …
Information Hiding (1/2) • Designing methods so the user doesn’t need to understand how they work – Examples, put. Peg, remove. Peg (we use them, but don’t know how they work) • Why do this? _______, _______ • Pre- and post- conditions
Information Hiding (2/2) • Visibility modifiers - public or private – Public: accessed from anywhere – Private: accessed within its own class • Make sure you understand this key idea : ) • Applies to methods or variables • ____ variables are always private • So how do we use them?
Encapsulation • Type of information hiding • Separating interface & implementation • Interface – What the user accesses…method signatures, pre- and post- conditions • Implementation – What programmer sees • Where do private methods fall?
How Objects are stored • Recall reference variables (objects) reference Holds the memory location of where to find the object Actual object (instance variables, etc. )
Comparing Objects Blob b 1 = new Blob(2); Blob b 2 = new Blob(2); Blob b 3 = b 1; Blob b 1 = new Blob(2); Blob b 2 = new Blob(2); Blob b 3 = b 1 b 3. triple. Value() b 1==b 2 ? b 1. equals(b 2) ? b 1==b 3 ? b 1. equals(b 2) ? b 1. equals(b 3) ? b 1==b 3 ?
Passing Parameters to methods Type of Parameter What is passed? Change to original? “Passed-byvalue” “Passed-byreference” Primitive Object Copy of primitive Reference to object ___
Example 1 Blah my. Object = new Blah(); int n 1=3, n 2=5; my. Object. make. Equal(n 1, n 2); System. out. println(n 1==n 2); //somewhere else public void make. Equal(int n 1, int n 2) { } n 2=n 1; Output: _____ Output?
Example 2 Square s 1 = new Square(5); Square s 2 = new Square(3); s 1. make. Equal(s 2); System. out. println(s 1. equals(s 2)); Output: _____
Top-Down Design • Step-wise refinement • Helper methods • Stubs
Static Methods • Usually, we call methods on objects – my. Board. put. Peg(…), rect. area() • Sometimes it doesn’t make sense to call a method on an object, so we call it directly from a class – Math. abs(-7)
Static Methods • Static methods can only call ______ methods and variables • Interpreting reference sheet: Math class: + static double abs (double a), returns double Store absolute value of -5. 4 in double num: _____________
Constants • Key word: final • Arbitrary numbers should never appear in code (except 0 or 1) • Usually declared/initialized: public static final int PI = 3. 14 (As a side note, why public? why static? )
Declaring Arrays (1/7) type[] array. Name = new type[size] 1. char[] letters = new char[5]; 2. String[] names; names = new String[30/2*3]; 3. int a=10; My. Object[] mult = new int[a]
Array of Objects (2/7) Person[] class = new Person[5]; Here, references are made for 5 Person objects… but they are all null. (Null. Pointer. Exception, anyone? ) class
Initializing Arrays (3/7) • While primitive variables are usually assigned a default value (0, false, etc. ) it is good practice to initialize them. - Or you may want different initial values • Initialize alpha array to the alphabet (I. e. alpha[0]=‘a’, alpha[1]=‘b’, etc. ) char[] alpha = new char[26]; …
Initialize Array of Objects (4/7) for (int i=0; i<class. length; i++) { class[i] = new Person(); } class Person Person
Using Arrays (5/7) Student[] class = new Student[#]; • Find length: class. length • Index range _______ to _______ • To access individual element – class[index] – Treat ‘class[2]’ like any other single Student object
Using Arrays (6/7) • You should easily know how to: – Search – Iterate (go through) – Manipulate values – Pass an array as a method parameter – Return an array from a method
Array Questions (7/7) 1. Find lowest/highest value in array. 2. Shift all values of an array up (and last value is now first) (Think about how you would do these…)
2 -D Arrays Type[][] my. Array = new Type[3][5]; my. Array[0]. length my. Array. length
2 -D Arrays • 2 D arrays are 1 D arrays where each array is a 1 D array
2 -D Arrays • Same rules as 1 -D array • Examples…
Inheritance Person super class (more general) Student has ‘everything’ Person has and (maybe) more. Person Student extends Person, Student is a Person Student inherits everything public the Person has.
… Say Coffee extends Drink: Drink d = new Coffee(); ok? Coffee c = new Drink(); ok? Wanting any drink and getting coffee is good; wanting coffee and getting a drink may not be.
Overriding Methods • A subclass overrides a method in its super class. – Use the exact same method signature – Used to make the method more tailored to the class – Example, Lab 12 Vehicle • Not the same as overloading. What’s the difference?
Accessing Super Class • Keyword: super • Use super(params) – To access parent’s constructor – Must be first line in child’s constructor • Use super. some. Method(params) – To access parent’s some. Method method
Method Resolution Object Somewhere in Hot. Drink class: • this. method. Name() Drink Hot. Drink Coffee – Looks at itself first – Hot. Drink ->… • super. method. Name() – Starts looking in parent class – Drink -> Object
Examples Person p = new Student(“Josh”, 19); p. set. Age(20); - Will compile if _______ has set. Age method - Will look in the _______ class for set. Age
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