Can You Run Java Program Without a Main Method? [Interview Question Answer] (original) (raw)

Hello guys, the first thing Java programmers learn is that they need a main method to run, but when they go to any Interview or college viva and ask can run a Java program without a main method, they are surprised like hell. Well, there are actually different types of execution models available in Java; for example, Applets, which run on the browser don't have the main method. Instead, they have life-cycle methods like init(), start() and stop(), which controls their execution. Since Applet is a Java program, you can answer this question is Yes. Similarly, we have Servlet, which runs in a Servlet container, comes as bundled in a web server like Tomcat, or Jetty.

The Servlet also works on callback mechanism, it has methods like init(), service(), and destroy(). Container calls init() if Servlet is a first-time loaded into memory, calls service() if there is a request to process, and calls destroy() before removing it from memory.

Since Servlet is also a Java program, we can say that it runs without the main method. The third one in this category is MIDlet, which runs on mobile devices like Nokia, Samsung, and others. MIDlet has life-cycle methods like startApp(), pauseApp() and destroyApp(), to start, pause and shut-down mobile applications.

Ironically J2ME has an app in its core method, but it was made popular by Android and iPhone. Since MIDlets are also Java programs, you can say they run without the main method.

Now, if the Interviewer persists with something like this, Can you run a Java program without the main method in Core Java, not on the managed environment like the case of Applet, Servlet, and MIDlet. The answer is NO, Why? I will explain that in the next paragraph.

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Java program without the Main method

Many Java programmer gives you answer that they can run a Java program without a main method by writing code in static initializer block, which is half true. Yes, code written in static initializer block is executed before calling the main method, but you won't be able to run a class by using Java command, or Eclipse, or anything else, until it got public static void main(String args[]) method on it.

If you try to run such programs, you will get the following error :

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

Though you can run a Java program with an empty Main method, in which case only code executed will be from the static initializer block. Following is a simple Java program with some code written on a static initializer block, including a print statement, variable initialization, and starting a thread.

As soon as you remove the main method, it will compile fine but will throw the above error when you try to run this program from the command line.

By the way, this is one of the many questions related to the main method in Java; you can see other questions for your interview preparation then I suggest you check out this Java Interview Courses, which contains important topics and more than 200+ Interview Questions and Answers for beginners and sufficient to crack any Java interview.

/**

Java application to demonstrate whether you can run a program without a main method
or not.
@author http://java67.blogspot.com */ public class JavaAppWithoutMain {

static { System.out.println("HelloWorld, Java progarm without main method"); int x = 20; // Can initialize static variables System.out.println("Variable x : " + x); Thread t = new Thread() { @Override public void run() { System.out.println("Started thread from static initializer block"); System.out.println("Thread Finished"); } }; t.start(); }

public static void main(String args[]) { // Empty main method }

}

Here is how the output looks like when you run this program from the command prompt with an empty main method.

That's all about whether you can run a Java program without the main method in Java or not. In short, Yes, you can run a Java program without a main method in a managed environment like Applet, Servlet, and MIDlet, which runs under control of browser, server, and mobile device, but can't run a core Java program without public static void main(string args[]){} method. JVM will not allow you to execute those methods.

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