1. CompSci 230 Software Construction
Lecture Slides #2: Hello World! S1 2016

2. Agenda Topics: Java Basics Getting started – our first .java file
“Hello world!” in Java and Python
Backward and forward compatibility
Syntax and semantics
COMPSCI 230: OOD

3. Java Basics –source code
In Java, each program is a class.
In source code, each class sits in a test file that has the same name as the class and the extension .java
E.g., if our class is called HelloWorld, we’ll put it into a file called HelloWorld.java
We can edit the .java file with any text editor
In 230, we will be using an integrated development environment (Eclipse) to do this
COMPSCI 230: OOD

4. Java Basics - compilation
We compile the class with the Java compiler javac:
On the command line: javac HelloWorld.java
Compilation results in a bytecode file with the extension .class (if we don’t have any syntax errors in the source code)
E.g., HelloWorld.class
Note: In Eclipse, we can invoke javac differently – more on this shortly
COMPSCI 230: OOD

5. Java Basics – running a program
Once we have the .class file, we can run the program using the Java VM, e.g.:
java HelloWorld
Note: We omit the .class – the Java VM knows what to look for
In Eclipse, we just hit the “Play” button (green circle with white triangle)
This takes care of compilation AND running of the program in the VM
That’s provided we’re building our program as part of an Eclipse project, in which case we get a little extra complexity thrown in.
COMPSCI 230: OOD

6. Java Basics – application structure
A program may use additional classes (we’ll get to that later), which also sit in their own files (except in the case of nested classes, which we’ll discuss much later)
The class that “starts” the program must have a class method (also called static method) which returns nothing (void).
OK, and now it’s time to look at our HelloWorld.java
COMPSCI 230: OOD

7. “Hello World!” in Java HelloWorld.java (Java source code):
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello World!"); }
What does all this mean?
COMPSCI 230: OOD

8. “Hello World!” in Java HelloWorld.java (Java source code):
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello World!"); }
public is our first nod to “development in the large”
It’s a so-called access modifier. public means that code from outside the application can see this class (and use it)
This allows the developers of other classes to use the HelloWorld class in their code (if they want to)
It also allows our program to compile (in this case)
COMPSCI 230: OOD

9. “Hello World!” in Java HelloWorld.java (Java source code):
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello World!"); }
class HelloWorld means that what follows is a class declaration of a class named “HelloWorld”
In that respect, Java’s no different from Python
COMPSCI 230: OOD

10. “Hello World!” in Java HelloWorld.java (Java source code):
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello World!"); }
Curly braces in Java have the same purpose as whitespace indentation in Python.
An opening curly brace marks the beginning of a block of code
A closing curly brace marks the end of a block of code
Curly braces must match, i.e., for every opening brace, there must be a closing brace
The concept is very similar to parentheses in mathematical formulas
Here, the curly braces marked by the boxes indicate the block that contains the class declaration
Note: javac ignores whitespace indentation completely – we use it only to make the code easier to read for humans!
COMPSCI 230: OOD

11. “Hello World!” in Java HelloWorld.java (Java source code):
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello World!"); }
This is the declaration of our main() method
It’s also public, meaning code from outside the class can invoke the method
The static means that we can invoke the method without having to build an object from the blueprint that the HelloWorld class represents
The void means that the method returns nothing
Note there’s no def as you have it in Python
COMPSCI 230: OOD

12. “Hello World!” in Java HelloWorld.java (Java source code):
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello World!"); }
In the declaration of our main() method
The method signature is very similar to a Python method (method name followed by parentheses with a parameter list)
New here: the String[] in front of the parameter args
Remember that all variables (and function return values) in Java need a type (unlike in Python, where the Python interpreter figures this out at runtime)
Here, the type of the parameter args is an array (“[]”) whose elements are of type String
This parameter contains an array of arguments passed in from the command line (if any)
COMPSCI 230: OOD

13. “Hello World!” in Java HelloWorld.java (Java source code):
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello World!"); }
In the declaration of our main() method
Note again the curly braces that enclose the block with the code of the method
COMPSCI 230: OOD

14. “Hello World!” in Java HelloWorld.java (Java source code):
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello World!"); }
In the declaration of our main() method
Guess what this does!
See system-out-println-in-java for the gory details
COMPSCI 230: OOD

15. In Eclipse File -> New -> Java Project
Enter HelloWorld as the project name and save
You’ll see HelloWorld pop up in the Package Explorer on the left
Add a class (File -> New -> Class) named HelloWorld and Eclipse will create almost everything else for you.
All you need to do is add the line with the code: System.out.println("Hello World!");
COMPSCI 230: OOD

16. In Eclipse
COMPSCI 230: OOD

17. Review questions Which form does a program take in Java?
What are the essential elements of any Java program?
How do we store the source code for classes in Java?
How do we name Java class files?
What does the modifier “public” do?
What is the purpose of curly braces in Java?
Can you give a few examples of types in Java?
What does the “static” mean in front of a method declaration?
How do we compile a Java program, and how do we run it?
What is a key difference in the way we specify method parameters in Java and in Python?
COMPSCI 230: OOD