Today we will see how to name, compile and running java files containing more than one class definitions.
1). Consider the following program.
| class ClassOne
{
public static void main(String[] args)
{
ClassTwo.methodOfClassTwo();
}
}
class ClassTwo
{
static void methodOfClassTwo()
{
System.out.println("From Class Two");
}
}
|
You can save this program with any name. It can be
ClassOne.java or it can be
ClassTwo.java or it can be
anything.java.
Compile : You have to compile this program with the name you have given like
>javac ClassOne.java or
>javac ClassTwo.java or
>javac anything.java.
When you compile a java file, the number of .class files
generated will be equal to number of class definitions in it. i.e If a
java file has one class definition, one .class file will be generated.
If it has two class definitions, two .class file will be generated and
so on.
Running : That means for the above program, two .class files will be generated. Then which one to run? is it
>java ClassOne or is it
>java ClassTwo….. It must be
>java ClassOne, because execution of any java program start with main() method. If you try to run
>java ClassTwo, you will get an
error: Main method not found in class ClassTwo, please define the main method as public static void main(String[] args).
2). Now consider same example with litte modification, just declare ClassOne as
public.
| public class ClassOne
{
public static void main(String[] args)
{
ClassTwo.methodOfClassTwo();
}
}
class ClassTwo
{
static void methodOfClassTwo()
{
System.out.println("From Class Two");
}
}
|
: The name of above java file must be and only
“ClassOne.java”. You can’t give any other name. If you give any other
name you will get compile time error :
class ClassOne is public, should be declared in a file named ClassOne.java.
Compile : Only one name is allowed here so you have to compile with that name i.e
>javac ClassOne.java.
Running : That must be
>java ClassOne. because this is only class that has main() method.
3). Now make little more modifications to the program. Declare ClassTwo as public and ClassOne as default.
class ClassOne
{
public static void main(String[] args)
{
ClassTwo.methodOfClassTwo();
}
}
public class ClassTwo
{
static void methodOfClassTwo()
{
System.out.println("From Class Two");
}
}
|
: You have to save this file with name as
“ClassTwo.java”. If you give any other name you will get compile time error becuase ClassTwo is public.
Compile : It must be
>javac ClassTwo.java.
Running : You must name this program as
ClassTwo.java, you must compile this program as
>javac ClassTwo.java but you have to run it as
>java ClassOne not as >java ClassTwo. Because only ClassOne has main() method. ClassTwo doesn’t have main() method. If you run it as
>java ClassTwo, you will get run time error
: Main method not found in class ClassTwo, please define the main method as public static void main(String[] args).
4). Now make little more modifications to the program. Declare both the classes as public.
| public class ClassOne
{
public static void main(String[] args)
{
ClassTwo.methodOfClassTwo();
}
}
public class ClassTwo
{
static void methodOfClassTwo()
{
System.out.println("From Class Two");
}
}
|
: Whatever you give the name, whether it is ClassOne.java or
ClassTwo.java or anything.java, you will get compile time error.
Because
One java file should contain only one or zero public class. It should not contain more than one public class.
5) Look at the following program.
| class ClassOne
{
public static void main(String[] args)
{
System.out.println("From Class One");
}
}
class ClassTwo
{
public static void main(String[] args)
{
System.out.println("From Class Two");
}
}
|
: You can save this program with any name. It can be
ClassOne.java or it can be
ClassTwo.java or it can be
anything.java as there is no public class
.
Compile : You have to compile this program with the name you have given i.e
>javac ClassOne.java or
>javac ClassTwo.java or
>javac anything.java.
Running : Notice that both the classes have main() method. You can run both the classes with their name. i.e If you trigger
>java ClassOne, you will get
From Class One as output. If you trigger
>java ClassTwo, you will get
From Class Two as output.
