Java new main method

Java main() method explained with examples

In this article, we will learn Java main() method in detail. As the name suggest this is the main point of the program, without the main() method the program won’t execute.

What is a main() method in Java?

The main() method is the starting point of the program. JVM starts the execution of program starting from the main() method.

Syntax of main() method:

public static void main(String args[])

public: We have already learned in the access specifier tutorial that public access specifier allows the access of the method outside the program, since we want the JVM to identify the main method and start the execution from it, we want it to be marked “public”. If we use other access modifier like private, default or protected, the JVM wouldn’t recognise the main() method and the program won’t start the execution.

static: The reason the main() method is marked static so that it can be invoked by JVM without the need of creating an object. In order to invoke the normal method, we need to create the object first. However, to invoke the static method we don’t need an object. Learn more about static method here.

void: This is the return type. The void means that the main() method will not return anything.

main(): This the default signature which is predefined by JVM. When we try to execute a program, the JVM first identifies the main() method and starts the execution from it. As stated above, the name of this method suggests that it is the “main” part of the program.

String args[]: The main method can also accepts string inputs that can be provided at the runtime. These string inputs are also known as command line arguments. These strings inputs are stored in the array args[] of String type.

Can we have main() method defined without String args[] parameter?

If we have a main() method without String args[] in a program, the program will throw no compilation error however we won’t be able to run the program as the JVM looks for the public main method with the String args[] parameter and if it doesn’t find such method, it doesn’t run the program.

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

As you can see that the program threw error at runtime.

Java – static block vs main method

As we learned in the previous article, static block is used to initialise the static data members. Let’s run a program with static block and main method (static method) to see in which order they run.

class JavaExample < //static block static < System.out.println("Static Block"); >//static method public static void main(String args[]) < System.out.println("Main Method"); >>

As we can see, the static block executed before the main method.

What if a program doesn’t have a main method?

Let’s write a program without the main method to see whether it runs or not.

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

Different ways to write main method in java

The following are the valid ways to write a main method in java:

public static void main(String[] args) //We can interchange static and public static public void main(String[] args) //We can place the square brackets at the different locations public static void main(String[] args) public static void main(String []args)

Overloading of main() method in Java

We can overload the main method in Java. This allows us to have more than one main() method in Java. However the signature of all the overloaded methods must be different. To learn more about overloading, refer this guide: Method overloading in Java.

class JavaExample < public static void main(String args[]) < System.out.println("main method"); main(100); main('A'); >//Overloaded int main method public static void main(int a) < System.out.println(a); >//Overloaded char main method public static void main(char ch) < System.out.println(ch); >>

Frequently Asked Questions

The main method is used to specify the starting point of the program. This is the starting point of our program from where the JVM starts execution of the program.

No, you can’t declare a method inside main() method.

Yes we have can more than one main methods in java, however JVM will always calls String[] argument main() method. Other main() methods will act as a Overloaded method. In order to invoke these overloaded methods, we have to call them explicitly.

No, we cannot override main method of java because it is a static method and we cannot override a static method. The static method in java is associated with class which is why we don’t need an object to call these. Therefore, it is not possible to override the main method in java.

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

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 in turn 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.

Syntax of main() Method

The most common in defining the main() method is shown in the below example.

Java

Every word in the public static void main statement has got a meaning in the JVM that is described below:

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

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

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

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

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

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

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.

FAQs on Java main() Method

Q1. Can the main method be int? If not, why?

Java

prg1.java:6: error: missing return statement > ^ 1 error

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

Java

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)

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

Explanation of the above programs:

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).

Q2. Can we execute a Java program without the 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.

Q3. Can we declare the main() method without String[] args?

Yes, we can declare the main() method without String[] args. Although it will generate an error message if we directly try to execute the main method inside the driver class as done in the below example.

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