java-overview-interm

Shlomo Hershkop 2007Shlomo Hershkop 2007 11

Intermediate ReviewIntermediate Review


Intermediate ReviewIntermediate Review

ReferencesReferences Basic inheritanceBasic inheritance Time classesTime classes Date ClassesDate Classes File input/output File input/output PackagesPackages


MemoryMemory

One of the good things about java is that One of the good things about java is that it abstracts memory awayit abstracts memory away

You don’t have to worry about how things You don’t have to worry about how things are being put into memoryare being put into memory

But you still need to be aware of how But you still need to be aware of how things are representedthings are represented


Basic ExampleBasic Example

int sum = 100int sum = 100 int arr[];int arr[];

100

X

SUM

arr


NextNext

int sum = 100int sum = 100 int arr[];int arr[]; arr = new int[10];arr = new int[10];

100SUM

arr


QuestionQuestion

What happens on the last line ?What happens on the last line ? int arr[];int arr[]; arr = new int[10];arr = new int[10]; arr[0] = 12;arr[0] = 12; arr = new int[5];arr = new int[5];

100SUM

arr


answeranswer

A new set of ints are allocated and WE A new set of ints are allocated and WE LOSE all data in the old oneLOSE all data in the old one

You need to copy over data (and use a You need to copy over data (and use a temp array)temp array)


ReferencesReferences

So a primitive variable is mapped to a location in memorySo a primitive variable is mapped to a location in memory int x; x= 234; int x; x= 234;

Class Objects are a little more complicated since they have member variables and Class Objects are a little more complicated since they have member variables and methodsmethods

Memory references will point to a location which has been setup with the object you Memory references will point to a location which has been setup with the object you createcreate miniVan mycar;miniVan mycar; mycar = new miniVan(….)mycar = new miniVan(….)

234

ref

x

mycarHonda

Odyssey2000Red


ReferencesReferences Create new primitive variable y, will result in another memory Create new primitive variable y, will result in another memory

location and value copy location and value copy int y = x;int y = x; Create another miniVan instance in the following will simply make it Create another miniVan instance in the following will simply make it

point to same place (unless new is used)point to same place (unless new is used) miniVan oCar = mycar;miniVan oCar = mycar;

234

234

ref

ref

x

mycarHonda

Odyssey2000Red

y

oCar


Who cares ?Who cares ?

So what is the difference ??So what is the difference ??


Difference !Difference !

Messing with x, won’t affect yMessing with x, won’t affect y

Messing with class reference will change Messing with class reference will change both objectsboth objects


Difference IIDifference II

oCar.year = 2005;oCar.year = 2005; Surprise!Surprise!

234

234

ref

ref

x

mycarHonda

Odyssey2005Red

y

oCar


Why?Why?

If its such a bad idea, why have it at all?If its such a bad idea, why have it at all?

Any ideas why we would want to create Any ideas why we would want to create object using references?object using references?


AdvantageAdvantage

If the class is huge If the class is huge

Don’t want to keep copying all the Don’t want to keep copying all the member variables if I plan on only reading member variables if I plan on only reading it it


Back to ArraysBack to Arrays

Can have array of length 0; Can have array of length 0; not the same as null:not the same as null:

int numbers[];int numbers[]; numbers = new int[0];numbers = new int[0]; numbers = null;numbers = null;

What is the difference here ?What is the difference here ?


Two dimensional arraysTwo dimensional arrays

You can create an array of any You can create an array of any object, including arraysobject, including arrays

Person bunch[] = new Person[10];Person bunch[] = new Person[10]; int[][] table = new int[10][20];int[][] table = new int[10][20]; int t = table[i][j];int t = table[i][j];

An array of an array is a two dimensional arrayAn array of an array is a two dimensional array


Before coding…Before coding…

Before we start to code one last thingBefore we start to code one last thing

How to get user input ??How to get user input ??

Most languages give you access to Most languages give you access to something called STANDARD out/insomething called STANDARD out/in


Standard IN/OUTStandard IN/OUT

Assume there is some way to talk to userAssume there is some way to talk to user

And get user inputAnd get user input

Very low levelVery low level Want something a little higher so don’t have to Want something a little higher so don’t have to

worry for example how they enter the worry for example how they enter the informationinformation In theory could be using hieroglyphics In theory could be using hieroglyphics


Reading Input through scannersReading Input through scanners

Construct ScannerConstruct Scanner from input stream (e.g. System.in)from input stream (e.g. System.in)

Scanner in = new Scanner(System.in)Scanner in = new Scanner(System.in) nextInt, nextDouble reads next int or doublenextInt, nextDouble reads next int or double

int n = in.nextInt();int n = in.nextInt();

hasNextInt, hasNextDouble test whether next token is a hasNextInt, hasNextDouble test whether next token is a numbernumber

next reads next string (delimited by whitespace)next reads next string (delimited by whitespace) nextLine reads next linenextLine reads next line


EclipseEclipse

1.1. Start EclipseStart Eclipse2.2. Start a new project (right click in project Start a new project (right click in project

explorer)explorer)3.3. Double click, right click on default Double click, right click on default

package and start a new classpackage and start a new class4.4. Call it InputTesterCall it InputTester

Check off that you want commentsCheck off that you want comments Check off you want a mainCheck off you want a main


Code then run thisCode then run this

public class InputTesterpublic class InputTester{{ public static void main(String[] args)public static void main(String[] args) {{ Scanner in = new Scanner(System.in);Scanner in = new Scanner(System.in); System.out.print("How old are you?");System.out.print("How old are you?"); int age = in.nextInt();int age = in.nextInt(); age++;age++; System.out.println("Next year, you'll be " + System.out.println("Next year, you'll be " +

age);age); }} }}


codingcoding

Let start to codeLet start to code

Tic tac toe gameTic tac toe game A program to allow two used to play a gameA program to allow two used to play a game

Where to start ??Where to start ?? What objects can you think of ?What objects can you think of ?


Basic parts Basic parts

Board – tic tac toe boardBoard – tic tac toe board What kind of method would you need here ?What kind of method would you need here ? Although it’s a 3 by 3 board, lets keep it generalAlthough it’s a 3 by 3 board, lets keep it general

MoveMove The part which can ask for a move and check if legal and place The part which can ask for a move and check if legal and place

into boardinto board Front endFront end

We would put main in hereWe would put main in here It would start a gameIt would start a game When done ask if you want to play againWhen done ask if you want to play again


Board classBoard class

Would need a constructorWould need a constructor

Reset method to set everything to blankReset method to set everything to blank

Place move to put a move into the boardPlace move to put a move into the board

Print out to see the boardPrint out to see the board


Note pleaseNote please

The users in the game are represented by X’s The users in the game are represented by X’s and O’sand O’s

No reason we can’t use 1,2No reason we can’t use 1,2

If have a member , what does it mean ?If have a member , what does it mean ? public static final int X = 1;public static final int X = 1; public static final int O = 2;public static final int O = 2;


Board.XBoard.X Board.OBoard.O

Don’t need to instantiate to use itDon’t need to instantiate to use it Makes it easier to speak a common Makes it easier to speak a common

language when using the classlanguage when using the class


public class TicTacToe{public class TicTacToe{public static final int EMPTY = 0;public static final int EMPTY = 0;public static final int X = 1;public static final int X = 1;public static final int O = 2;public static final int O = 2;

private final int SIZE = 3; //for 3x3private final int SIZE = 3; //for 3x3

private int[][] board; private int[][] board;

//ok lets add a constructor//ok lets add a constructor


codingcoding

Add constructorAdd constructor

Add reset methodAdd reset method

Add toString methodAdd toString method Use for loopUse for loop Need to translate from 1,2 to X,ONeed to translate from 1,2 to X,O

Don’t hard code values, use your final staticsDon’t hard code values, use your final statics


NextNext

Lets code the move classLets code the move class

Very basicVery basic Need method to get the user’s next moveNeed method to get the user’s next move

Assume move is a move from each userAssume move is a move from each user Need to ask board if game is doneNeed to ask board if game is done

Add another method to the board gameAdd another method to the board game If user enters bad move…what do you want to do ?If user enters bad move…what do you want to do ?


Finally Finally

Now lets code the front end with mainNow lets code the front end with main

Create a class MainGameTTTCreate a class MainGameTTT Have a main in itHave a main in it

What do we need to do next ?What do we need to do next ?


Game logicGame logic

Start a gameStart a game Loop Loop

At the end ask user if they want to play At the end ask user if they want to play another game ?another game ? This is a little tricky to loop….any ideas ?This is a little tricky to loop….any ideas ?


Yay!Yay!

Ok you have a working gameOk you have a working game

Test it out on all your friends Test it out on all your friends


Multiple dimensionsMultiple dimensions

No reason cant create 4,5,6 dimension No reason cant create 4,5,6 dimension arraysarrays

Gets hard to manageGets hard to manage Better idea: Better idea:

Think about another way of representing the Think about another way of representing the datadata

Often creating an object is a better Often creating an object is a better approachapproach


Arrays furtherArrays further

Need to explicitly copy contents of arrays when resizing Need to explicitly copy contents of arrays when resizing arrays with temp onearrays with temp one

Better solution:Better solution: ArrayListArrayList VectorVector

Full object versions of arraysFull object versions of arrays Capacity can grow over timeCapacity can grow over time Useful methods bundles with themUseful methods bundles with them


codecode

Create a new class with a main called VectorTestCreate a new class with a main called VectorTest

Create a vectorCreate a vector

Put in some stuffPut in some stuff Print them outPrint them out Replace something in the middleReplace something in the middle Print it outPrint it out Clear the vectorClear the vector Print it outPrint it out


Default valuesDefault values

Should be aware if you forget to set Should be aware if you forget to set valuesvalues

Might mess up your logicMight mess up your logic Think of multiplying a bunch of numbers and Think of multiplying a bunch of numbers and

not setting one of them…not setting one of them… Compiler/IDE will let you know if you Compiler/IDE will let you know if you

forgot to set values (warning)forgot to set values (warning)


Hope you had fun learning this!Hope you had fun learning this!

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java-overview-interm