Can we create an Empty
java file?
Yes, it is valid to
create an empty Java file. The compilation will be successful but no class file
will be created. It’s just that the compiler won’t throw any error.
As there is no class
file gets generated, we can’t even try running the empty file.
We get a classNotFoundException.The
JVM when tries to run an file, tries to find the public class present in the
file and then tries to search for a valid main method in that particular class.
As, in this case, we
don’t have any main method, JVM complains that it didn’t find any public class
which has a valid main method.
The below will be the exception stack trace
Exception in thread "main"
java.lang.NoClassDefFoundError:
Caused by: java.lang.ClassNotFoundException:
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
Could not find the main class:. Program will exit.
Can a file contain more
than one class?
Yes, but only one public
class is allowed in a file.
When a file is executed
using the java command, it searches for a main method in the public class to
start its execution. So, if the class have two public class, it won’t be able
to figure out in which class it needs to search for the main method.
But it is perfectly
possible to have more than one class in a file until that particular file has
only one public class.
Screenshot showing two
classes in a file:
Once this file is
compiled, two class files will be generated, one for each
Compiler giving error when having more than one public class in a file:
How many class files are
generated when you compile a java file having two classes?
Two class files. One for
each class is generated.
Can the main method have
any return type other than void?
The JVM searches for the
public class in the file specified to run and then tries to find the main
method which is static having String[] as arguments and whose return type is
void.
If the return type is
changed from void, an exception will be throws saying Main method is not found.
For example, compiling
and running the below file:
Will result in the below
exception when we try to execute.
Exception in thread
"main" java.lang.NoSuchMethodError: main
But when another main
method is also present which is the original one, then the compiler shows an
error saying duplicate method.
Can we change the
argument list of the main method?
Yes, but JVM won’t treat
it as an original main method. See the above question for explanation.
Can we overload the main
method?
Once the file is run
using the java command, it goes to the public class in that file and searches
for the main method having void as return type and which is static and which
takes String[] as arguments. So, it does not bother to have another main method
with different signature. The JVM calls only the original main method.
The other main method
should be called manually in the code, the same way it applies for other normal
methods.
Can we have multiple
methods with the name “main” in a single class?
Yes, But the method
which is static and have the return type as void and having Sting[] as
arguments only will be called by the JVM.
For example, the below
is a valid class file which don’t give any compilation or Run time error.
The above program on execution,
prints the below output:
From the Original main
method
Can we have two main
methods in our application in two different classes?
Yes, any application can
have multiple classes having the main method. It is up to the user to choose
which one he wants to execute.
What is a class?
A class is a blue print
to create objects.
What is an object?
Object is an instance of
the class.
