The main method

The declaration of the main method

Java is primary an object-oriented language. It means a Java program can be considered as a collection of objects that communicate via calling each other's methods. A typical Java program includes a lot of classes, interfaces, objects and other concepts from the object-oriented programming.

Even the simplest "procedural-style" program should have at least one class and the main method inside to start the program. The main method is the entry point for any applications. Ever since Java 7 there has been no other way to start an application without this method (excluding the case when you start your application inside a special container for applications but it is not considered in this course).

Let's see an example of the simplest application that prints the text "Hello, Java" in the standard output:

public class Main {

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

Here is a class named Main. The class contains the main method for starting the program.

It is important to mention that a class containing the main method can have any name, but the main method should always have the same name.

Let's take a closer look at the declaration of the main method:

public static void main(String[] args)

• the keyword public indicates that the method can be invoked from everywhere;
• the keyword static indicates the method can be invoked without creating an instance of the class;
• the keyword void indicates the method doesn't return any value;
• the array variable args contains arguments entered at the command line if no arguments the array is empty.

As you can see, even the simplest Java application contains a lot of concepts. All of them will be studied in the next topics related to methods and the object-oriented programming. Now you should just understand how to write and run a simple java program with the main method.

Invalid declarations of the main method

If the main method has an invalid declaration, two cases are possible:

• your program cannot be compiled
• your program is successfully compiled but can't be started

Your program cannot be compiled. It is a case when the main method declaration breaks the syntax of Java.

Examples:

• invalid method declaration: no returning value (even void).

public static main(String[] args)

• invalid method declaration: a mistake in the keyword (pubic instead of public).

pubic static void main(String[] args)

A program can be compiled but cannot be run. It is a case when the main method has a correct declaration as a regular method but doesn't satisfy the specific requirement of the main method.

Examples:

• invalid arguments (should be String[] args)

public static void main(String args) {
    System.out.println("Hello, Java");
}

• the method declaration has no the keyword static

public void main(String args) { 
    System.out.println("Hello, Java");
} 

In both cases, an error happens at runtime.

Conclusion

So, the main method is the entry point of any Java programs. It has a very specific syntax which you need to remember.