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Starting Java with BlueJ and the BeetleA 2 day course for real beginners – leading to CS12230
Lynda [email protected]
Introducing ourselvesWe are …Introduce yourself to your neighbours
Where are you from?Why you chose the degree scheme you’re
enrolled on?How much (if any) programming have you
done before?What interests you about computing?
Which of the following have you done before? (Choose all that apply)
Used Word or … Made a web site Built a database application Written program in a programming
language None of the above
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What should you be able to do at the end of next week (with some help)?
Use the BlueJ IDE (Integrated Development Environment). This is a way beginners can write Java programs
Have a basic understanding of Iteration: lots of times do somethingSelection: if something then do one thing
if not do something else
Mainly by using a ‘Beetle’ that responds to commands
Write a simple Java program (Java is a programming language – you write programs in it to make things happen)
Why are we starting this way?We want you to get going quickly with
programmingSeeing how things work and Building things
are what computing is all aboutIt is fun (if a bit frustrating at times)It gives you a sense of accomplishment, and
It takes PRACTICE!!!
Where do we start? (let’s do this now)Logging onOpen a browserAccessing the department’s web sitehttp://www.aber.ac.uk/compsciThen pick Intranet, Then TeachingThe pick Module InformationNotes are at either:
Blackboard, or Dept’s Web site
Materials for just these 2 days are on the department’s web site under CS12230, under zz-introductory-2Days
PROGRAMMING
What is programming?
Telling the computer to ‘do’ somethingYou write instructions in a language that you
understandThe instructions are then translated into
something the ‘computer’ understandsThat is then run
The writing is - programming (writing code)The translating is - compiling (sometimes interpreting)The running is - executing (running)
Some examplesWeb page
Written in htmlEvery time a browser looks at it, it is translatedThe equivalent of running would be the
displayingThe program that UCAS uses to process
admissionsWritten in a programming languageTranslated into something that runsRun to accept input from AU and produce
notificationsWhat we are doing today
Written in JavaTranslated into something that runs in BlueJRun to make a little Beetle move around the
screen
BASIC BLUEJBlueJ is the environment in which we will be
writing, translating and running our Java programs
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This is what BlueJ looks like.
We are going to be creating BJBeetle-y kinds of things – you can think of them as little robots you can control that draw as they move
Ignore this
Here we have used the Java programming language to create 2 Beetles and got them to draw some shapes
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Exercise 0 (let’s do this now)Download Beetle.zip from the web page (right click)http://www.aber.ac.uk/~dcswww/Dept/Teaching/CourseNotes/2011-2012/CS12230/zz-introductory-
2Days/
Unzip it (right click and pick extract) and put it in a new directory on your m: drive
Now run BlueJ you will find it under the start button All Programs/ Courseware/ Computer Science
Open the Beetle projectRight click on BJBeetle and pick new BJBeetle()
that will give you a new BJBeetle object(by right clicking on that object you can get it to do
things)
Java works by creating Classes and objectsThe classes describe things in the ‘world’They are like ‘patterns’For instance every employee has a an employee record
with their information in it.
We would have a class called EmployeeAnd then build objects that were instances of that class
each with the same kind of information but different values
In this course we will start with a class called BJBeetle, we can use that class as a ‘pattern’ to make objects with names like bJBeetle1 which you can think of as little robots you can control that draw as they move.
Ignore this
A class called BJBeetle
BJBeetle is a class and we can make objects of that class by right click, selecting new BJBeetle()
The objects then show up on the ‘object bench’You can have more than one of them – do that
Select this
It also brings up the actual Beetle on the pink canvas –
but that is because we have programmed it
to do so
and nothing to do with BlueJ as such
One thing some people get wrong is The class is a pattern for making objects
It is not a collection of objects
So all objects that you make are examples (instances) of the class.
They have the same structure but different values – for instance the colour, the position,…
So, actually here we have created 2 BJBeetle objects and got them to draw some shapes
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The object bench is a picture of what the memory of the computer looks like
We can make objects do things by selecting their ‘methods’ again with a right click
Find out more about methods by double clicking BJBeetle and selecting ‘documentation’ on top right
Arguments / Parameters
The values that you type in are called arguments. When you moveForward(100) the 100 is the argument.
So we ‘pass in’ the argument to the methodBut you need a way to talk about the value that is moved
forward in a general way. The Beetle moves forward by a ‘distance’. In the context of the method moveBackward, distance is called a parameter
The arguments then, get copied into the parametersSome methods have no parameters, some one, some many
Exercise 1 (let’s do this now)Make a BJBeetle object by right clicking on the
BJBeetle class and selecting new BJBeetle() - you can call it anything but the default will be bJBeetle1
Right click on that object and see the list of things it can do - the methods
Pick several of the methods in turn and see what happens as you move the Beetle around
Most of the methods will ask you to enter arguments(colours are typed in as java.awt.Color.red etc.)
Make sure you make more than one object – just so you can see that one class can describe many similar objects
Have you completed exercise 1?
Created a BJBeetle objectMade it do a couple of
methodsSeen the documentationCan find a method with 4
parametersPlease help me!
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We have to talk about variable names
Variables are names we give to pieces of computer storage/memory so we can manipulate them.
We have seen variables already:
On the object bench we created object variables bJBeetle1 was a variable name
AND ….
Parameters are also variable names
parameters - these are ones that accept the arguments when we call a method bJBeetle1.turn(45); public void turn(int degrees) {
} degree is a parameter variable name - needed so
that in the code for the turn method you can use that instead of ‘45’ or ‘90’.
You need something generic to talk about
The idea that we use names to stand in for values is essential to programming
You can guess that when we tell bJBeetle1 to turn(45) what happens is that the little Beetle ‘s picture is turned by that number of degrees.
When we tell it to moveForward(100) it moves forward by that distance
So we can express what happens generically as an ‘algorithm’ or set of instructions using ‘degrees’ and ‘distance’ to describe the general instructions
THE CODE PAD
As a first step towards programming let’s use the code pad instead of selecting methods
We are now writing (short bits of) Java codeNotice how you write method calls:
bJBeetle1.turn(45);objectName.methodName(argument);
You can define the variables on the code pad instead of the object bench BJBeetle fred=new BJBeetle();
This is how you say fred is a ‘BJBeetle-y thing’fred.turn(45);
Notice how fussy it is!
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Exercise 2 (let’s do this now)Use the code pad to define a new BJBeetle to draw
the letters A and U (nice blocky ones – like the T). If that is too boring do your own initials, Don’t do
‘T’ and you probably should pick letters that are ‘straight’! We’ll do curves later.
You’ll need to use position() in order to move from initial to initial
To start again completely RIGHT CLICK on ‘reset virtual machine’
oops() allows you to back up one stepWhen you are done, hit printscreen button and
paste into a word document for posterity!
Have you completed exercise 2?
Drawn one letterDrawn 2 lettersPlease help me!
<<end of first session?>>
What’s the catch?Come back this afternoon it is all gone!
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What’s the catch?
Come back tomorrow it is all gone!
A temporary solution:double click on the UseThis classwrite your code in there (the ‘main code’)click compilethen click on the void main of the UseThis class
Let’s do it! BJBeetle bob= new BJBeetle();bob.moveForward(100);
Here I did this with the code in draw-T.txt
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A bit more with codingNot all variables represent objects, some are ‘primitive’:int - means a whole numberdouble is a number with a decimal point boolean is true or false
String is not primitive but it is very simple
These kinds of variables are created without saying ‘new’. You just say String name="fred"; int age=19;
Let’s do this now :Create a new class and call it TestOutputDouble click to see its code and replace all its code with the code from UseThis – except replace the word ‘UseThis’ with ‘TestOutput’. Compile.Then replace the ‘main code’ with (or copy output.txt)compile and run (right click the class and select void main)int age=19;
double hourlyWage=4.5;
boolean penUp=false;
String name="fred";
age++;
hourlyWage=hourlyWage*2;
System.out.println(name+" is "+age+” and makes “+ hourlyWage);
System.out.println() prints stuff in the terminal window
So, up pops the Terminal window and the values of the variables are printed (along with explaining text that you determine)
The + comes between things you want to print, to link them
Notice how ++ adds 1 to an int, and *2 multiplies by 2
Notice also how the quotes and ()s need to match
(to clear the window pick Options-clear)
What is happening?
Declares these variables, says what kinds of things they are
allowed to hold, and puts some initial values in
Changes values
Prints values
int age=19;
double hourlyWage=4.5;
boolean penUp=false;
String name="fred";
age++;
hourlyWage=hourlyWage*2;
System.out.println(name+" is "+age+” and makes “+ hourlyWage);
LOOPS
OK, now the first of our ‘control structures’ At the moment our code just runs one
line, then the next etc. Often we want to do the same thing
over and over (for instance printing an invoice of items, a list of students etc)
Called ‘iteration’ There are several different ways of
doing iteration – the loops. We will look at one
The ‘counted for loop’ Does something a certain number of times Looks like this example:
for (int counter=0;counter<5;counter++) {System.out.println(“hi ”+counter);
}Try it now in the UseThis class or a copy
What happened when you tried it?
What is happening behind the scenes?
for (int counter=0;counter<5;counter++) {
System.out.println(“hi”);
}
This is the loop headerControls the loop with the
counter variable
This (between the { and the }is the loop body
Gets repeated over and over
The loop header
for (int counter=0; counter<5; counter++)
Says what kind of loop
This starts up a variable called counter with value 0(the variable can have any
name)
Do the loop while the counter is less than 5
Every time you go through the loop add
one to counter – after doing body
The loop body
Starts with a {
Ends with a }
Everything between is the body which gets repeated.
How do you design a for loop?Figure out how many times you want to do
something – that is your loop controlFigure out what you want to do over and over – that
is your loop body.Sometimes the control is used in the body:
for (int count=0;count<30; count++) {System.out.println(“Student “+ count +”: “);
}
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Exercise 3 (let’s do these now)Download the code Star.txt from the web sitePaste it into a class as the ‘main code’ like before and try itModify it to draw a 7 pointed star (the angle you need is
720/number of sides)
Now can you draw 4 stars? What about 10 stars?(hint: What do you need to do between drawing each star?)
Write a loop that prints out the 5 times table up to 5*10(hint: easier if you add 1 to counter each time and then print counter*5 )
Have you completed exercise 3?
Drawn one 5 pointed starDrawn one 7 pointed starDrawn 4 starsDone the times tablePlease help me!
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End of teaching day 1
ReviewUsed BlueJ to play with some Java codeCreated several objects of a class like BJbeetle
BJbeetle bob=new BJBeetle(); Called the methods for those objects this involved
passing arguments into the methods – like bob.turn(90);
Done some output like System.out.println(“hi”+name);Looked at loops like
for (int counter=0;counter<10;counter++) {
}
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Have you completed exercise 3?
Drawn one 5 pointed starDrawn one 7 pointed starDrawn 4 starsDone the times tablePlease help me!
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What should you be able to do at the end of next week (with some help)?Use the BlueJ IDE Have a basic understanding of Iteration
Have a basic understanding of Selection: if something then do one thing
if not do something else
Mainly by using a ‘Beetle’ that responds to commands
Write a simple Java program
IF s(also input)
OK, now the second of our ‘control structures’
At first, our code just ran one line, then the next etc.
But now we have seen iteration (doing something over and over)
Sometimes we want to do one thing in one case and something different in another
This is called ‘selection’ We will look at the ‘if’ statement
The ‘if’ statement Does different things in different situations For instance, BJBeetle objects draw if the pen
is down and don’t otherwise
But that means our code usually needs some input to get a different situation
So, first let’s get some input into the program to create different situations
Input (let’s do this now) Download BeetleWithInput.zip from the web
site Unzip it There is a new class called InputGetter that
grabs a String from the user It has 3 methods
Look at the documentation
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The getInputFromUser() method asks for input
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The getInputFromUser(String message) method asks for input but with a message – note these methods have the same name but different parameters needed
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The getCurrentValue() method ‘returns’ a value that could be manipulated
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Returning values from a method This is sort of the opposite of passing a value
into a method through argument/parameter this method produces a value that can then be
used – in this case a String Look at the documentation for these methods
(double click on InputGetter and select documentation)
So, when you create an InputGetter object It sort of sits there and gets a String and returns it to you as
requested Create one, inputGetter1, give it a String NOW double click it This is how you can ‘inspect’ an object in BlueJ
Most of this is not of interest but look at ‘str’ which holds what I typed
The ‘if’ statement Does different things in different situations Looks like this example:InputGetter reader=new InputGetter();reader.getInputFromUser();String name=reader.getCurrentValue();if (name.equals("fred")) {
System.out.println("found fred");}else { System.out.println("not Fred");}
This code is in UseThis Try it!
What is actually happening?
if (name.equals("fred")) {System.out.println("found fred");
}else { System.out.println("not Fred");}
This is where we test the condition – must be true or false
If true do this
If false do this
This is what happened when I typed in fredTry typing in bill and fred yourself
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What happened when you typed in various things?
FredBillfred389458&^%*&^“fred”
Now look at Nested
Here we have the idea that you can ‘nest’ control structures
LOOP{codecodeIF {
….}
}
public class Nested{
public static void main(String args[]) {
InputGetter reader=new InputGetter();
reader.getInputFromUser("how many students");
int number=Integer.parseInt(reader.getCurrentValue());
for (int i=0;i<number;i++) {
System.out.println("student "+i);
//process a student
//read name
InputGetter nameReader=new InputGetter();
nameReader.getInputFromUser("enter name");
String name=nameReader.getCurrentValue();
//read gender
InputGetter genderReader=new InputGetter();
genderReader.getInputFromUser("what is your gender (male or female)");
String answer=genderReader.getCurrentValue();
//process and print
if (answer.equals("male")) {
System.out.println("Mr. "+name);
}
else {
System.out.println("Ms. "+name);
}
}
}
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Try it!!
Exercise 4 (let’s do this now)Write come code with a loop and an if statement
that Asks someone whether they are over 18. If they say “yes” print “iechyd da”, if not print
“have a coke”
(you can use the Nested as a pattern)
<<end of session 3>>
Have you completed exercise 4?
Loop through some peopleAsk if over 18Print iechyd da if they say
yes
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Note you are ‘nesting’ an IF in a LOOPYou can ‘nest’ all control structures! Note ‘comments’
Already nested loops yesterday when we drew 4 starsBJBeetle bob=new BJBeetle();
//four times
for (int counter=0;counter<4;counter++) {
//draw a star this also involves a loop
bob.setPenUp(true); bob.moveForward(100); bob.setPenUp(true);
for (int i=0;i<5;i++) {
bob.turn (720/5);
bob.moveForward(70);
}
}
You can nest control structures to any depthfor (int i=0;i<4;i++) { for (int j=0;j<3;j++){ System.out.println
(i+" "+j);}
}
if (age<3) { System.out.println("play at home");}else if (age<6) { System.out.println("at nursery");}else if (age<13) { System.out.println("play outside!");}else if (age<18) { System.out.println("no - I won't give you
any money");}else { System.out.println("you are on your own
love");}
if (something) {for (…) {}
}
for (…) {if (…) {
//one possibility is break out of the loop}
}
So you can have an if in a loop or a loop in an ifor a loop in an if in a loopThe trick is to figure out ‘in words’ what you want to do and only actually code just a bit at a time:
Suppose you want to ask 10 students their names and genders and print something like:
Mr. John BrownMs. Mary Smith etc.
appropriately
First think about the overall structure:
Loop through 10 studentsprocess a student
for (int i=0; i<10; i++){
//process a student}Now how process just one student? (ie do the body of the loop)
String name=reader1.getCurrentValue();
String gender=reader2.getCurrentValue();
if (gender.equals("male")) {
System.out.println("Mr. "+name);
}
else {
System.out.println("Ms. "+name);
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Or you could do this in the other orderie first how do one student and then stick a loop around it for lots of students!
Exercise 5 (let’s start these now)Modify the code for testif.txt so it recognises
your User ID when you input it and outputs a suitable message.
Modify the code for testif.txt so if you input “circle” a BJBeetle draws a circle.
If not it prints System.out.println(“ok”);you can find the code for a circle in zzCircle.txt this is a loop (the circle) inside an if!
Modify your last code so that also if the user input “star” a BJBeetle draws a star (two loops inside the if!)
Have you completed exercise 5?
Recognises your initialsDrawn the circle if asked forOr draws the star if asked
forPlease help me!
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End of teaching day 2
What should you be able to do at the end of next week (with some help)?Use the BlueJ IDE Have a basic understanding of Iteration - LOOPHave a basic understanding of Selection - IF
Mainly by using a ‘Beetle’ that responds to commands
Write a simple Java program – for that we need to be breaking up our code! Do you notice how complicated it is getting?
Aaaaargh! I am feeling a bit lost!!!!
If this describes you We will be going over all this again in CS122, so don’t panic!I do hope that this brief introduction will have given you some experience that you can bring into the course.If only this much:
BJBeetle bob=new BJBeetle(); //creating an objectbob.moveForward(100); //making if do something
//with an argument
There are some things called loops and if statements
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Aaaaargh! This is really easy!!!!
If this describes you Try
The CS12130 workshop at 11 on Monday in B23 and lecture at 3pm Tuesday in Physics A (I think)
The CS12230 lecture at 5pm Monday in Physics mainYou may decide to change module
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What next?Instead of just using pre-built classes you will be
building your ownInstead of always using the UseThis class you will
create your own proper classes to run proper applications
These classes will be able to be run independent of BlueJ altogether in other ways (actually we already have this – type java UseThis at the command line prompt)
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How do I get marked on this? By Monday Oct 7th:Modify the code you produced in Exercise 5 so that
your ‘program’ asks for input 10 times.If it gets the word “circle” it draws a circleIf it gets the word “star” it draws a starIf it recognises your user id it draws it in block letters
(ie my program would draw LTT)Otherwise it just says “cannot recognise”
That means you will have to have designed code for your UID. (Start by just recognising it.)
Completing this counts as 2 worksheets in CS122 (there are 10 worth 10%)
If you don’t get this done then:Completing Exercise 5 - counts as 1 worksheet in
CS122.
How do I submit it?On Blackboard you should find an assignment for
CS12230 called ‘intro assignment’It is due Monday Oct 7th at 10amTake screenshots of your code working and paste
into a document.Also paste in the code itselfSubmit that document (don’t forget to press the
submit button)
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