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Java main() Method – public static void main(String[] args)

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  • Difficulty Level : Easy
  • Last Updated : 03 Jun, 2022

In Java programs, the point from where the program starts its execution or simply the entry point of Java programs is the main() method. Hence, it is one of the most important methods of Java and having a proper understanding of it is very important.

The Java compiler or JVM looks for the main method when it starts executing a Java program. The signature of the main method needs to be in a specific way for the JVM to recognize that method as its entry point. If we change the signature of the method, the program compiles but does not execute.

The execution of the Java program, the java.exe is called. The Java.exe inturn makes Java Native Interface or JNI calls, and they load the JVM. The java.exe parses the command line, generates a new String array, and invokes the main() method. A daemon thread is attached to the main method, and this thread gets destroyed only when the Java program stops execution.

Java main() Method

Syntax: Most common in defining main() method

Java




// Java Program to demonstrate the
// syntax of the main() function
 
class GeeksforGeeks {
    public static void main(String[] args)
    {
        System.out.println("I am a Geek");
    }
}


Output

I am a Geek

Output explanation:  Every word in the public static void main statement has got a meaning to the JVM. 

1. Public 

It is an Access modifier, which specifies from where and who can access the method. Making the main() method public makes it globally available. It is made public so that JVM can invoke it from outside the class as it is not present in the current class.

Java




// Java Program to demonstrate the
// use of any other access modifier
// other than public
 
class GeeksforGeeks {
    private static void main(String[] args)
    {
        System.out.println("I am a Geek");
    }
}


Error: Main method not found in class, please define the main method as:
public static void main(String[] args)
or a JavaFX application class must extend javafx.application.Application

2. Static

It is a keyword that is when associated with a method, making it a class-related method. The main() method is static so that JVM can invoke it without instantiating the class. This also saves the unnecessary wastage of memory which would have been used by the object declared only for calling the main() method by the JVM.

Java




// Java Program to demonstrate the
// error occurred when we dont use the
// static keyword in the main() method
 
class GeeksforGeeks {
    public void main(String[] args)
    {
        System.out.println("I am a Geek");
    }
}


Error: Main method is not static in class test, please define the main method as:
public static void main(String[] args)

3. Void 

It is a keyword and is used to specify that a method doesn’t return anything. As the main() method doesn’t return anything, its return type is void. As soon as the main() method terminates, the java program terminates too. Hence, it doesn’t make any sense to return from the main() method as JVM can’t do anything with the return value of it.

Java




// Java Program to demonstrate the
// error occurred when we dont use the
// void return type in the main() method
 
class GeeksforGeeks {
    public static int main(String[] args)
    {
        System.out.println("I am a Geek");
        return 1;
    }
}


Error: Main method not found in class, please define the main method as:
public static void main(String[] args)
or a JavaFX application class must extend javafx.application.Application

4. main 

It is the name of the Java main method. It is the identifier that the JVM looks for as the starting point of the java program. It’s not a keyword.

Java




// Java Program to demonstrate the
// error occurred when we name the
// main() method as main.
 
class GeeksforGeeks {
    public static void myMain(String[] args)
    {
        System.out.println("I am a Geek");
    }
}


Error: Main method not found in class, please define the main method as:
public static void main(String[] args)
or a JavaFX application class must extend javafx.application.Application

5. String[] args 

It stores Java command-line arguments and is an array of type java.lang.String class. Here, the name of the String array is args but it is not fixed and the user can use any name in place of it. 

Java




// Java Program to demonstrate
// the working of String[] args
// in the main() method
 
class GeeksforGeeks {
   
      // Commamd-Line Code ->
    // javac GeeksforGeeks.java
    // java GeeksforGeeks 1 2 3
   
    public static void main(String[] args)
    {
        for (String elem : args)
            System.out.println(elem);
    }
}


Output:

1
2
3 

Apart from the above-mentioned signature of main, you could use public static void main(String args[]) or public static void main(String… args) to call the main function in Java. The main method is called if its formal parameter matches that of an array of Strings.

Can the main method be int? If not, why?

Java




class GeeksforGeeks {
 
public static int main(String[] args)
    {
 
        System.out.println("GeeksforGeeks");
    }
}


 
Java does not return int implicitly, even if we declare the return type of main as int. We will get a compile-time error: 

prg1.java:6: error: missing return statement

   }

   ^

1 error

Java




class GeeksforGeeks {
 
   public static int main(String[] args) {
 
       System.out.println("GeeksforGeeks");
 
       return 0;
   }
 
}


Now, even if we do return 0 or integer explicitly ourselves, from int main. We get a run time error.

Error: Main method must return a value of type void in class GeeksforGeeks, please  
define the main method as:
  public static void main(String[] args)

Explanation: 

The C and C++ programs which return int from main are processes of Operating System. The int value returned from main in C and C++ is exit code or exit status. The exit code of the C or C++ program illustrates, why the program was terminated. Exit code 0 means successful termination. However, the non-zero exit status indicates an error. 

For Example exit code 1 depicts Miscellaneous errors, such as “divide by zero”.

The parent process of any child process keeps waiting for the exit status of the child. And after receiving the exit status of the child, cleans up the child process from the process table and free the resources allocated to it. This is why it becomes mandatory for C and C++ programs(which are processes of OS) to pass their exit status from the main explicitly or implicitly.

However, the Java program runs as a ‘main thread’ in JVM. The Java program is not even a process of Operating System directly. There is no direct interaction between the Java program and Operating System. There is no direct allocation of resources to the Java program directly, or the Java program does not occupy any place in the process table. Whom should it return an exit status to, then? This is why the main method of Java is designed not to return an int or exit status.

But JVM is a process of an operating system, and JVM can be terminated with a certain exit status. With help of java.lang.Runtime.exit(int status) or System.exit(int status).

Can we execute a java program without main method?

Yes, we can execute a java program without a main method by using a static block.

A static block in Java is a group of statements that gets executed only once when the class is loaded into the memory by ClassLoader, It is also known as a static initialization block, and it goes into the stack memory.

class StaticBlock {
    static
    {
        System.out.println(
            "This class can be executed without main");
        System.exit(0);
    }
}


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