Hello World · COS 212 Resources

The structure of this crash course was borrowed from learnjavaonline.org

The Java Crash Course slides cover more areas but in less detail.

A good place to start and test the Java Installation is with the classic Hello World.

Main.java

public class Main {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

Compiling and running the above code will output Hello, World!.

Lets analyse the parts of the code.

public class Main {...

Defines a class with the name Main.
The name of source file has to match the name of the class defined in it. In this case it is Main.java.
Only one class can be defined per source file.
Methods can only be defined in a class.
Statements can only be written in a class definition.
Unlike C++, there are no header files. One source file will contain your class and method definitions.
public indicates the class is accessible from other Java Packages. In the context of COS 212 only public class will be used.

public static void main(String[] args) {...

This is the entry point for our program. The signature has to match exactly as above.
The Access Level Modifier public specifies that the method is visible to all classes.
- private specifies that is is accessible only within the class.
static specifies that the method can be called without instantiating an instance of the class.
- i.e Main.main()
void specifies that the method returns no value.
String[] is the type for an array of Strings. We will go over the different types soon.
args will contain the command line arguments used to start the program.

System.out.println("Hello, World!");

System is a Java class providing useful class fields and methods.
out is a static member variable that represents the "standard" output stream.
println is a method of out that prints the given string along with a new line character.