In this lesson, we will build our very first Java program. Our program will simply print "Hello, World!" on the screen (a tradition by most programmers when learning new languages). Our code may not seem too exciting at first, however we will learn about the basic template that all Java programs need to follow.
The Hello World program
There is Java code of this program:
public class Main {
public static void main(String[] args) {
System.out.println("Hello, World!");
}
} You can type this code in the Your Code section here and then press the execute button. In the result section, you will see:
Hello, World!The basic terminology
Now that you have seen the result, let's learn some basic terminology and then try to understand this program.
- Program – a sequence of instructions (called statements), which are executed one after another in a predictable manner. Sequential flow is the most common and straight-forward sequence of statements, in which statements are executed in the order that they are written - from top to bottom in a sequential manner;
- Statement – a single action (like print a text) terminated by semi-colon (
;); - Block – a group of zero, one or more statements enclosed by a pair of braces
{...}; There are two such blocks in the program above. - Syntax – a set of rules that define how a program needs to be written in order to be valid; Java has its own specific syntax that we will learn;
- Keyword – a word that has a special meaning in the programming language (
public, class, and many others). These words cannot be used as variable names for your own program; - Identifier or name – a word that refers to something in a program (such as a variable or a function name);
- Comment – a textual explanation of what the code does. Java comments start with
//. - Whitespace – all characters that are not visible (space, tab, newline, etc.).
The Hello World program under a microscope
The Hello World program illustrates the basic elements of Java programs. For now we will discuss only the most important elements.
1. The public class. It is the basic unit of a program. Every Java program must have at least one class. The definition of a class consists of the class keyword followed by the class name. A class can have any name, such as App, Main, or Program, but it must not start with a digit. A set of braces {...} encloses the body of a class.
public class Main {
// ...
}// is just a comment, not a part of the program. We will learn about comments in detail in later topics. main inside a class. It is the entry point for a Java program. Again, the braces {...} encloses the body of the method, which contains programming statements.public static void main(String[] args) {
// statements go here
}public, static, and void will be discussed later, so just remember them for now. The name of this method (main) is predefined and should always be the same. Capitalisation matters; if you name your first method like Main, MAIN or something else, the program cannot start.System.out.println("Hello, World!"); // each statement has to end with ;This is one of the most important things to understand from the Hello World program. We invoke a special method println to display a string followed by a new line on the screen. We will often use this approach to print something of interest to the screen. The text is printed without the double quotes. It’s important to note that "Hello, World!" is not a keyword or an identifier; it is just a text to be printed.
Conclusion
main method. Every Java program must have a main method as it is the first to be executed when the program runs. Don't worry about memorising every single term used in the lesson (syntax, statement, block). These terms will reappear in future lessons. Do not forget to use the provided Hello World program as a template for your own programs.