How to run main in java

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 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

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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Java how to run main method in java

If your entry point is the class, only the main method of that class will be executed (unless you manually execute main methods of other classes). Put them in a package, make sure the directory tree matches the package tree, and use the fully qualified name of the class: Solution 1: You can run static methods as needed Solution 2: Based off your question I assume you want the main function in Exe to essentially run the main function in PackageDemoDriver: I think that’ll provide the functionality you’re after, if your PackageDemo and PackageDemoDriver are in different classes Solution 3: Is this what you’re looking for?

Why doesn’t the main method run?

Your application has one entry point, and that entry point is the single main method that gets executed. If your entry point is the theGame class, only the main method of that class will be executed (unless you manually execute main methods of other classes).

Creating an instance of lineTest class doesn’t cause its main method to be executed.

I’ve made a start below. It looks like you might want to invest some time in following a more basic java tutorial or course to get your basic java knowledge up to speed.

What happens in the code below is that the class theGame has a main entry for the program. The JVM will invoke the main method at the start of your program. From there, it will execute the instructions you give. So most of the times, two main methods do not make sense in a single project. Exception to this rule is if you want to have two separate application entry points two the same program (for instance a command-line application and a GUI application that use the same logic but are controlled differently).

So with the code below, you will have to specify the TheGame class as a main entry point when starting your JVM for this application.

public class TheGame < private final LineTest theBoard; public TheGame() < theBoard = new LineTest(); >public void run() < JFrame frame = new JFrame("Points"); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame.add(theBoard); frame.setSize(250, 200); frame.setLocationRelativeTo(null); frame.setVisible(true); >/** * Main entry for the program. Called by JRE. */ public static void main(String[] args) < TheGame instance = new TheGame(); instance.run(); >> 

public class LineTest extends JPanel < public void paintComponent(Graphics g) < super.paintComponent(g); Graphics2D g2d = (Graphics2D) g; g2d.setColor(Color.red); g2d.drawLine(100, 100, 100, 200); >> 

When a java application is executed, it is executed by invoking the main method on one particular class. This main method will be whichever main method is on the class that was selected to executed.

In your case, you are selecting to execute the main method on the class theGame .

When another class is constructed in the application, the constructor of that class is automatically executed, but the main method on that class is not automatically executed.

Learn Java Programming — Main Method Parameter and Arguments, The main method is the entry point for a class invoked from the java command line tool. In Duration: 6:00

Java Main Method Tutorial — Everything You Need to Know

Full Java Course: https://course.alexlorenlee.com/courses/learn-java-fastIf you want to be a Duration: 9:50

JAVA MAIN METHOD Explained | public static void main(String[] args)

6.15 Different Ways of writing main method in java

When we start JVM by running java command we also provide name of class which contains
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How to run the main method of a Java class in a jar? [duplicate]

you can do something like this

java -cp jarName.jar packageName.ClassName argumentsIfAny 

java -cp the\path\to\jarFile.jar SecondLevelJsonCreator 3 "C:/path/filename.xls" 

You should generally avoid putting classes in the default package. Put them in a package, make sure the directory tree matches the package tree, and use the fully qualified name of the class:

java -cp the\path\to\jarFile.jar com.mycompany.myproject.SecondLevelJsonCreator 3 "C:/path/filename.xls" 

How to run a main method method without building the WHOLE, But now instead of running it, it started to build the maven project as a whole. enter image description here. Is there a way to «fix» this? java intellij-idea

How do I run the main method in a different file in Java?

You can run static methods as needed

public static void main(String[] args)

Based off your question I assume you want the main function in Exe to essentially run the main function in PackageDemoDriver:

I think that’ll provide the functionality you’re after, if your PackageDemo and PackageDemoDriver are in different classes

Is this what you’re looking for?

package exercise; import jav.PackageDemoDriver; class Exe < public static void main(String[] args) < PackageDemoDrive.main(args); >> 

What is the difference for run or main function in java class, main method is entry point of a program but run method of Runnable interface is entry

Java 11 doesn’t run main method if the public class is not declared first

The class to be executed is the first top-level class found in the source file. It must contain a declaration of the standard public static void main(String[]) method.

i.e. you need to make public class Chimpanzee as the top-level class as follows:

public class Chimpanzee extends Ape < public Chimpanzee() < super(2); System.out.print("Chimpanzee-"); >public static void main(String[] args) < new Chimpanzee(); >> class Primate < public Primate() < System.out.print("Primate-"); >> class Ape extends Primate < public Ape(int fur) < System.out.print("Ape1-"); >public Ape() < System.out.print("Ape2-"); >> 

Run a Java Main Method in Maven, And we want to execute its main method from the command line via Maven. In order to do this, we can use the exec-maven-plugin. To be more

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