main method in java

Java main method
Anyone who is familiar with java must be knowing main method. Whenever a class is executed using java command(or through an IDE), java main method is called first.
In other words, main method is the entry point of a class or an application having multiple classes.

This post focuses on the detailed explanation of signature of this method and it will also answer some of the most commonly framed interview questions on public static void main.

1. public is the access specifier,
2. static is an access modifier,
3. void is the return type and
4. main is the name of method.

All of these keywords of main method are explained in detail below.
public
This keyword makes main method exposed to outside this class.
Since this method is always called from outside a class, it should be declared as public else it will not be visible external to the class.

If main method is preceded with any other access specifier(private or protected), then it would not be visible to JVM and an error will be thrown.
Example,

public class MainNotPublic {
     
   // main method has protected access
   protected static void main(String[] args) {
      System.out.print("Main is protected");
   }

}

Above class declares main method as protected and gets the following message when executed.

This is a common interview question based on main method
“Why main is public in java”

static
Applying static keyword to a field or method binds it to the class.
This means that in order to access a static field or method, instance of the class is not required and it can be directly accessed using class name.
Also, there is only one copy of a static field or method and it is shared by all instances of the class.
When a class is executed, there is no instance of it. Hence, in order to call a method without an instance, it should be marked as static.

This is also an answer to an important interview question
“why main method is static in java”.

What should happen when main is not declared static as in the class below

public class MainNotStatic {
     
   // main method is non-static
   public void main(String[] args) {
      System.out.print("Main is not static");
   }

}

You are right, an error is received when this class is executed.

void
Return type of a method indicating that the method does not return any value.
In java, main method does not return any value, hence it should be preceded with void.
If the main method is given any other return type, then an Error will be thrown at execution time.
Example,
Suppose a class has following main method.

public class MainWithReturn {
     
   // main method returning an integer  
   public static int main(String[] args) {
      return 0;
   }
}

This class compiles fine but when executed shows the following.

String[] args
main method in java accepts an array of String arguments.
This array argument is used for holding parameters sent to the class.
Any command line parameters supplied while executing a class are automatically populated in this array. There is no limit to the length of this array.
This also means that you can pass any number of arguments while running a java program. Example,
If a class is executed as java MyClass John 20 Student
then the array argument of main method of MyClass will have below elements

Can I create a method with name main in a class ?
Off course you can create a method which has name as main but it will be considered as an overloaded method and will be treated as just like any other method which has its name as main.

Look at the below example,

public class SecureImp{
   public static void main(String[] args) {
      System.out.println("Entry method of class");
      // create an object of this class
      SecureImp obj = new SecureImp();
      // call method 
      obj.main(""); 
   }
  /**
   * Another method with name as main 
   **/
   public void main(String args) {
      System.out.println("Another main method");
   }
}

When the class is executed, main method runs which creates an object of this class and calls another method which also has name as main.
Output is :

Note that if argument count and types of both methods will be same, then the class will not compile.

Let’s tweak in

1. String[] args can also be written as String args[].
2. It is not necessary to name the argument String array to args. It can be any name as per java variable naming rules.
3. public and static are interchangeable. This means that main method can also be declared as static public void main(String[] a).
4. Any parameters passed to a class are automatically populated in the String array parameter of the main method.

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