The Java `main()` Method: Interview Questions and Answers

This article explores common interview questions about the Java main() method, the entry point for Java applications.

`main()` Method Signature

The signature of the main method is:

public static void main(String[] args)

public: Accessible from any other class.
static: Can be called without creating an object of the class.
void: Doesn't return a value.
main: The method's name (required by the JVM).
String[] args: An array of strings containing command-line arguments.

Overloading `main()`

Yes, you can overload the main() method (create multiple methods with the same name but different parameters). However, only the method with the standard signature public static void main(String[] args) serves as the program's entry point.

Non-Static `main()`

No, the main() method *must* be static because the JVM calls it before any objects of the class are created.

Command-Line Arguments

Command-line arguments are passed to the main() method as a String array (the args parameter).

public class CommandLineArgs {
    public static void main(String[] args) {
        for (String arg : args) {
            System.out.println(arg);
        }
    }
}

Non-Void Return Type for `main()`

A main() method with a return type other than void will compile but won't be executed by the JVM as the program's entry point.

Preventing Immediate Program Termination

To keep a program running, you can:

• Wait for user input (using System.in.read()).
• Use an infinite loop (while(true) { ... }).

public class StayAlive {
    public static void main(String[] args) throws IOException {
        System.out.println("Press Enter to exit...");
        System.in.read();
        System.out.println("Program exited.");
    }
}

`main()` Method in Every Class?

No, only the class containing the program's entry point needs a main() method.

Passing Arguments to JAR Files

When running a JAR file, pass arguments after the JAR filename:

java -jar MyJar.jar arg1 arg2

Renaming `main()`

No, the JVM specifically searches for public static void main(String[] args).

Omitting `args` Parameter

Omitting String[] args still compiles, but you lose access to command-line arguments within the main() method.

Terminating a Java Program

Use System.exit(int status). The status code indicates the program's exit status (0 for normal termination).

Default Value of `args`

If no command-line arguments are given, args is an empty array.

`final` `main()` Method

Declaring main() as final has no practical effect on execution, as the JVM directly calls it.

Specifying JVM Options

Use the -D flag: java -DmyOption=myValue MyProgram

`throws` Clause in `main()`

While technically allowed, using a throws clause in main() is generally avoided. Unhandled exceptions in main() cause program termination.

Varargs in `main()`

No, main() must accept exactly one String[] argument.

Role of public in `main()`

public makes main() accessible from any class, crucial for JVM invocation.

Debugging `main()` with `jdb`

The jdb (Java Debugger) allows command-line debugging of the main() method.

Private `main()` Method

A private main() method compiles but can't be executed by the JVM.

Explicit `main()` Call

You can explicitly call main() within your code, but this is not the program's entry point (the JVM still looks for the standard main method).

Accessing Environment Variables

Use System.getenv("variableName") to access environment variables.

Exceptions in `main()`

Uncaught exceptions in main() terminate the program.

Creating Threads in `main()`

You can create and start new threads within the main() method.

Measuring `main()` Execution Time

Use System.currentTimeMillis() (or similar) to record the start and end times, then calculate the difference.

Overriding `main()`

While technically possible, overriding main() in a subclass doesn't make the overridden method the entry point; the JVM always uses the original class's `main()` method.