CS125 Exam Review

Winter 2008

Some Exam Info

• Tuesday (22nd) at 4:00-6:30pm in the PAC• CHECK YOUR SEAT!!!• Read Final Exam Information on website

– Practice questions & old exams– Reference sheet

• Bring WATCARD, pen, pencil, eraser only

Old Topics

• Primitive variables • Declare• Initialize• Operations

• Mod, Division • String methods• Board class• Coordinate class

• Scanner• next(), nextLine()

• 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: indexOf, putPeg

• Midterm: Why the overloaded methods didn’t work?

Class Diagrams

public class Cat{

//instance variablesprivate String name;//constructorpublic Cat(String n){…}//methodspublic 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, putPeg, removePeg (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 object

Holds the memorylocation of whereto find the object

Actual object (instance variables, etc.)

Comparing Objects

Blob b1 = new Blob(2);

Blob b2 = new Blob(2);

Blob b3 = b1;

b1==b2 ?

b1.equals(b2) ?

b1==b3 ?

Blob b1 = new Blob(2);Blob b2 = new Blob(2);Blob b3 = b1b3.tripleValue()

b1.equals(b2) ?b1.equals(b3) ?b1==b3 ?

Passing Parameters to methods

“Passed-by-value”

“Passed-by-reference”

Type of Parameter Primitive Object

What is passed? Copy of primitive

Reference to object

Change to original? ___ ___

Example 1

Output?

Blah myObject = new Blah();int n1=3, n2=5;myObject.makeEqual(n1,n2);System.out.println(n1==n2);

//somewhere elsepublic void makeEqual(int n1,int n2){

n2=n1;}

Output: _________

Example 2

Square s1 = new Square(5);Square s2 = new Square(3);s1.makeEqual(s2);System.out.println(s1.equals(s2));

Output: _________

Top-Down Design

• Step-wise refinement

• Helper methods

• Stubs

Static Methods

• Usually, we call methods on objects– myBoard.putPeg(…), 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[] arrayName = new type[size]

1. char[] letters = new char[5];

2. String[] names;

names = new String[30/2*3];

3. int a=10;

MyObject[] 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.(NullPointerException, 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 Person 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[][] myArray = new Type[3][5];

myArray.length

myArray[0].length

2-D Arrays

• 2D arrays are 1D arrays where each array is a 1D array

2-D Arrays

• Same rules as 1-D array

• Examples…

Inheritance

Person

StudentPerson

Student

Student extends Person, Student is a PersonStudent inherits everything public the Person has.

super class(more general)

Student has ‘everything’Person has and (maybe) more.

…

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.someMethod(params)– To access parent’s someMethod method

Method Resolution

Somewhere in HotDrink class:• this.methodName()

– Looks at itself first– HotDrink -> Drink ->…

• super.methodName()– Starts looking in parent class– Drink -> Object

Object

Drink

HotDrink

Coffee

Examples

Person p = new Student(“Josh”,19);

p.setAge(20);

- Will compile if _______ has setAge method- Will look in the _______ class for setAge