Java Interview Questions on main() Method

The main() method is the entry point for Java applications, and having a solid grasp of its nuances can greatly enhance your performance during technical discussions. In this article, we will explore more than 25 Java interview questions related to the main() method, along with detailed explanations and full code examples, ensuring that you're well-prepared to tackle any interview scenario.

1) What is the signature of the main() method?

The main() method's signature is: public static void main(String[] args)

This signature indicates that the method is public (accessible from anywhere), static (belongs to the class rather than an instance), returns void (no return value), and accepts a single argument of type String array.

Example:

public class MainMethodExample {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

Output:

Hello, World!

2) Can the main() method be overloaded?

Yes, the main() method can be overloaded just like any other method in Java. However, only the standard main() method with the String[] args parameter will be executed when the program is launched.

Example:

public class OverloadedMain {
    public static void main(String[] args) {
        System.out.println("Standard main() method");
    }
    public static void main(String arg) {
        System.out.println("Overloaded main() method");
    }
}

Output:

Standard main() method

3) Can the main() method be made non-static?

No, the main() method must be declared as static since it is called before any instances of the class are created. This allows it to be invoked without creating an object of the class.

4) How is command-line input passed to the main() method?

Command-line arguments are passed as an array of String to the main() method. They are available through the args parameter.

Example:

public class CommandLineArgs {
    public static void main(String[] args) {
        System.out.println("Number of arguments: " + args.length);
        for (String arg : args) {
            System.out.println(arg);
        }
    }
}

Output:

Number of arguments: 3
arg1
arg2
arg3

5) What happens if the main() method is declared with a return type other than void?

If the main() method has a return type other than void, the program will compile successfully, but it won't execute when launched. The standard main() method with void return type will be the entry point.

6) How can you prevent a Java program with a main() method from terminating immediately?

To prevent a program from terminating immediately, you can use various techniques like waiting for user input or using an infinite loop.

Example:

public class StayAlive {
    public static void main(String[] args) {
        System.out.println("Program started. Press Enter to exit.");
        try {
            System.in.read();
        } catch (Exception e) {
            e.printStackTrace();
        }
        System.out.println("Program terminated.");
    }
}

Output:

Program started. Press Enter to exit.
[You press the Enter key]
Program terminated.

7) Is it necessary to have a main() method in every Java class?

No, only the class containing the entry point of the program needs a main() method. Other classes can exist without it.

8) How can you pass arguments to the main() method of an executable JAR file?

When executing a JAR file from the command line, you can pass arguments after the JAR filename.

Run: java -jar MyJarFile.jar arg1 arg2

9) Can you use a different name for the main() method?

No, the Java Virtual Machine (JVM) specifically looks for a method with the signature public static void main(String[] args) as the entry point.

10) What happens if you omit the String[] args parameter in the main() method?

If you omit the String[] args parameter, the code will still compile, but you won't be able to access command-line arguments within the main() method.

Example:

public class NoArgsMain {
    public static void main() {
        System.out.println("Hello from main()!");
    }
}

Output:

Hello from main()!

11) How can you terminate the Java program explicitly?

You can use the System.exit(int status) method to terminate the program explicitly. The argument status is used to indicate the exit status of the program.

Example:

public class TerminateProgram {
    public static void main(String[] args) {
        System.out.println("Before termination");
        System.exit(0); // Normal termination
        System.out.println("After termination"); // This won't be executed
    }
}

Output:

Before termination

12) What is the default value of the args parameter if no command-line arguments are provided?

If no command-line arguments are provided, the args parameter will be an empty array.

Example:

public class DefaultArgs {
    public static void main(String[] args) {
        if (args.length == 0) {
            System.out.println("No arguments provided.");
        } else {
            System.out.println("Arguments provided:");
            for (String arg : args) {
                System.out.println(arg);
            }
        }
    }
}

Output:

No arguments provided.

13) Can the main() method be declared as final?

Yes, the main() method can be declared as final. However, it won't have any impact on the program's execution since the JVM directly invokes the main() method and doesn't follow normal inheritance rules.

14) How can you specify JVM options when launching a Java program?

JVM options can be specified using the -D flag followed by the option in the command line.

Run: java -DmyProperty=value MyProgram

15) What is the purpose of the throws clause in the main() method?

The throws clause in the main() method's signature indicates that the method might throw a particular type of exception during its execution. However, it is uncommon to use the throws clause in the main() method since uncaught exceptions can lead to program termination.

16) Can you use a variable-argument (varargs) parameter in the main() method?

No, the main() method cannot have a variable-argument parameter. It must accept exactly one argument of type String[].

17) What is the role of the public access modifier in the main() method?

The public access modifier allows the main() method to be accessed from outside the class, which is essential since the JVM needs to invoke it externally to start the program.

18) How can you debug the main() method using command-line tools?

You can use the jdb (Java Debugger) command-line tool to debug the main() method. It allows you to set breakpoints, inspect variables, and step through the code.

19) What happens if you declare the main() method as private?

If the main() method is declared as private, the program will compile successfully. However, the JVM won't be able to access the method to start the program, resulting in an error.

20) Can you invoke the main() method explicitly within your program?

Yes, you can invoke the main() method explicitly like any other static method. However, it won't act as the entry point, and the program won't execute from there.

Example:

public class ExplicitMainCall {
    public static void main(String[] args) {
        System.out.println("Main method invoked explicitly.");
    }  
    public void start() {
        main(new String[]{});
    }
}

21) How can you access environment variables from the main() method?

You can use the System.getenv(String name) method to access environment variables from the main() method.

Example:

public class EnvVariables {
    public static void main(String[] args) {
        String javaHome = System.getenv("JAVA_HOME");
        System.out.println("JAVA_HOME: " + javaHome);
    }
}

22) What happens if the main() method throws an exception?

If the main() method throws an uncaught exception, the program will terminate, and the exception details will be displayed.

Example:

public class MainException {
    public static void main(String[] args) {
        throw new RuntimeException("Exception from main()");
    }
}

Output:

Exception in thread "main" java.lang.RuntimeException: Exception from main()
	at MainException.main(MainException.java:3)

23) Can you use the main() method to create new threads?

Yes, you can create new threads within the main() method just like in any other method. However, remember that the main() method itself is already running on the main thread.

24) How can you measure the execution time of the main() method?

You can measure the execution time of the main() method by recording the start and end times using System.currentTimeMillis() or other time-related classes.

Example:

public class ExecutionTime {
    public static void main(String[] args) {
        long startTime = System.currentTimeMillis(); 
        // Code to be measured    
        long endTime = System.currentTimeMillis();
        long executionTime = endTime - startTime;
        System.out.println("Execution time: " + executionTime + " ms");
    }
}

25) Can you override the main() method?

Yes, you can technically override the main() method in a subclass, but the overridden method won't act as the entry point. The JVM directly invokes the main() method of the class specified during execution.

In Summary, Mastering the main() method and its related concepts is essential for any Java developer aspiring to excel in interviews and real-world programming scenarios. By thoroughly understanding the behavior, syntax, and potential applications of the main() method, you'll be well-equipped to tackle various interview questions and demonstrate your expertise in Java programming. Remember, practice makes perfect, so experiment with these concepts and build your confidence in Java programming.