Java Building Blocks

Java: Building Blocks Cate Huston @kittenthebad

What We’ll Cover • Java: an Object Oriented Language • The Eclipse IDE • Writing your ﬁrst program • Primitive Types • Strings • Conditions • Loops

Java: an Object Oriented Language

What Does Object Oriented Mean? • If Ikea were made of code, it would totally be written in an Object Oriented language. • Object-Oriented means that we break our code down into components (objects) with properties (ﬁelds), that can be used to make other objects, or interact with each other. • (See? It’s a little bit like Ikea furniture!)

OK, Give Me an Example! • Imagine a bike. If we wanted to “code” a bike, it would be a lot easier if we split it down into its component parts. • wheels (x2) • breaks • seat • frame • peddles...

Another? • How about a ToDo list? • It’s make up of tasks. • Each task should have things associated with it, such as: • It’s name • The date it’s due • The date we actually complete it • An estimate of how long it will take • The date we started it

Try It! • Think of a complex object • Break it down into it’s component parts • What information does each component need to know about itself?

The Eclipse IDE

Eclipse is a powerful, free and open source Java IDE (Integrated Development Environment). It has some very useful features for learning to program.

Let’s start by making a new project. File => New => Java Project

Let’s call it “Hello World”. It’s a programming tradition.

Now we make a new “class”. Java classes are where we represent our “objects”.

Let’s call this “HelloWorld”. Notice how there are no spaces? That’s important.

Our ﬁrst class! It should look like this.

Writing Your First Program

Finally! Write Some Code • For our ﬁrst program, we’re going to write something that prints out “Hello World” in the terminal. • (Sorry, programmer tradition)

Click on “Run” (the green “play button), and see what happens.

What Does It All Mean? • public class HelloWorld { • Declares our class/object with name “HelloWorld”. Everything inside the “{“ is part of the class. Public is to do with it’s visibility (don’t worry about that for now). • public static void main(String[] args) { • This is our “main” method, what’s called when we click “run”. The String[] args means we can pass arguments to it, if we want to. Everything inside the “{“ is part of the main method. • System.out.println("Hello World"); • This means - print out “Hello World” to the terminal. The “;” is important, it denotes the end of the line of code. We’re going to be using a lot of these. • } } • The ﬁrst closing bracket denotes the end of the “main” method, the second the end of the HelloWorld class.

Primitive Types

Building Blocks • Primitives are the most basic kinds of “type” in Java (a building block!) • You can also think of them as like atoms in chemistry. • A “type” is where we say what kind of thing a variable is. • Objects are made up of other objects and primitives.

For Example... • Whole numbers, like 42, or 926 are of type int (or short, or long). • Decimal numbers, like 2.34376 or 1.203 are ﬂoat, or double. • True or False are boolean. • A character like ‘a’ or ‘c’ is a char. Notice the single quotes? Those are important.

Declaring Variables • We can declare a variable of a primitive type in Java as follows: • int i = 42; • double d = 735.27; • boolean b = true; • char c = ‘h’; • So: • type variable_name = value;

See how we can use the “+” sign to include it in our output?

Strings

Strings • A String is not a primitive, it’s an Object, but we can declare it like a primitive. • It’s a little more complex to declare Objects (but we’ll look at that later) • String s = “hello world”; • String s = “hello world” + “nhow are you?”

Conditions

Comparing Things • We compare things in • Equals: == Java using conditional logic. • Greater Than: > • We can put this in an “if • Less Than: < statement” • Greater Than or • if (a == b ) { ... } Equal To: >= • else if (a < b) { ... } • Less Than or Equal To: <= • else { ... }

Try this for different values of a and b

Repeating Things • Loops are helpful for sections of code that we want to repeat. • There are three kinds of loop. • while • do while • for

For Loops • For when we know how many times we want to repeat something. • 10 times • for(int i = 0; i < 10; i++) • For each character in a string • for(int i = 0; i < stringname.length(); i++)

Repeat Something 10 Times

For each character in a string. s.charAt(i) gets the character in the string at position i.

While and Do-While Loops • When we want to repeat something until a condition changes. • In a while loop we check that condition at the start of the loop • while(a == b) { ... } • In a do-while loop we check that condition at the end of the loop. • do { ... } while (a == b)

Example While Loop

Example Do-While

Whitespace

Tidy Code • Tidy code is much easier to read (and debug!) • Debug - ﬁx when it’s not working. • As a rule, indent in one inside each set of {}. • In longer sections of code, we can use // to denote a comment. • A comment is code that is ignored by the compiler. • The Java compiler ignores whitespace, so use line breaks wherever you think it will make your code clearer.

Finally...

Finally • This slide deck covers the very basics of Java - the building blocks. • It’s important to understand these, because everything else builds upon them. • Next, we’re going to look at Processing.

@kittenthebad http://kittenthebad.wordpress.com/ catehuston@googlewave.com

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Java Building Blocks

by Cate Huston on Nov 17, 2009

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Java Building Blocks — Presentation Transcript