Our first complete Java program

To create a program, Java requires us to define at least one class. For now we’re not going to worry much about what exactly is a class. Let’s just think of it as a construct that contains all our Java statements (our code). A class must have a name, and since this is our first program will output a quote from Albert Einstein we’ll call the class EinsteinQuote. Another essential element needed to create a program is what’s called the main method or main for short. The main method designates the starting point of our program’s execution; the first Java statement in main is the first one to execute, and so forth. Both the class and main require that we clearly define their boundaries, and in Java this is done using a set of curly braces, ‘{‘ and ‘}’. These symbols allow Java to separate groups of statements into what’s called a code block and are used extensively with many constructs that we’ll encounter later on. As you might guess the opening curly brace designates the start of a code block and the closing brace the end of a block. We’re now ready to write our first program. Here we go (you should be typing this in the Editor pane of DrJava – the top right-hand pane):

public class EinsteinQuote

{

public static void main(String[] args)

{

System.out.println(“The important thing is not to stop questioning.”);

System.out.println(“Curiosity has its own reason for existing.”);

System.out.println(“ Albert Einstein.”);

}

}

After typing the code, you should save it into a Java file (the file must have a .java extension), and the name of the file must match that of the class. In this case, the file will be EinsteinQuote.java. Before we can run or execute the program, we need to compile it. Compilation is the process of checking for errors and converting Java code into machine understandable code (remember that computers represent all information as 0’s and 1’s). We refer to the program that compiles the code as the compiler. In DrJava we can invoke the compiler by clicking the Compile button at the top of the window. Any error messages will be displayed at the bottom tab labeled Compiler Output. If all goes well and there are no errors, you can click on the Run button at the top to execute the program. You can view the output of your program by selecting the Console tab at the bottom.

Let’s go back to all the code we typed in for our first program. Some of the terms used (such as static) seem a bit puzzling, and you’re right to think that. But these will be more meaningful as we learn more about Java. At this point, our focus should be on the Java statements inside the main method (the ones between the inner set of braces). After writing your first few programs all this extra stuff will be difficult to forget.