Method "Main" in Java

The Java main method serves as the gateway to the world of Java programming. It's the starting point for every Java application, the place where execution begins, and the foundation upon which your code is built. In this topic, we'll explore the main method's significance, structure, and how it kick-starts your Java programs.

The declaration of the main method

Java is primarily an object-oriented programming 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 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 application. It means that the execution of any program begins right here. Ever since Java 7, there has been no other way to start an application than with this method (excluding the case in which you start your application inside a special container for applications but this is not considered in our materials).

Let's see an example of the simplest application that prints the text "Hello, Java" to 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 name main.

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 args contains arguments entered at the command line, the array is empty if there are no arguments.

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 Java methods and object-oriented programming. For now, you should just understand how to write and run a simple Java program with the main method.

Shortcut key: 4 letters psvm and pressing the Tab key in the IntelliJ IDEA development environment will automatically create a main method with args arguments.

Invalid declarations of the main method

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

• Either your program cannot be compiled;
• Or your program is successfully compiled but can't be started.

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

Examples:

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

public static main(String[] args)

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

pulic static void main(String[] args)

A program can be compiled but cannot be run — This is the 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 keyword static

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

In both these cases, an error occurs at runtime.

Conclusion

So, the main method is the entry point of any Java program. It has a very specific syntax which you need to remember. The declaration of the main method must adhere to certain rules, including the keywords public, static, and void, as well as the parameter String[] args. Incorrect declaration of the main method can lead to the program being unable to compile.