Myth about the file name and class name in Java ❤️

The first lecture note given during java class is “In java file name and class name should be the same”. When the above law is violated a compiler error message will appear as below

Java

/***** File name: Trial.java ******/
public class Beginner { 
    public static void main(String[] args) 
    { 
        System.out.println("Hello world"); 
    } 
}

Output:

javac Trial.java
Trial.java:9: error: class Beginner is public, should be
                    declared in a file named Beginner.java
public class Beginner
^
1 error

But the myth can be violated in such a way to compile the above file.

Java

/***** File name: Trial.java ******/
class Beginner { 
    public static void main(String[] args) 
    { 
        System.out.println("Hello world"); 
    } 
} 

Step 1:

javac Trial.java

Step 1 will create a Beginner.class (byte code) without any error message since the class is not public.
Step 2:

java Beginner

Now the output will be Hello world
The myth about the file name and class name should be same only when the class is declared in public.
The above program works as follows:

Now, this .class file can be executed. By the above features, some more miracles can be done. It is possible to have many classes in a java file. For debugging purposes, this approach can be used. Each class can be executed separately to test their functionalities(only on one condition: Inheritance concept should not be used).
But in general, it is good to follow the myth.
Example 1:

Java

/*** File name: Trial.java ***/
class ForBeginner { 
    public static void main(String[] args) 
    { 
        System.out.println("For Beginner class"); 
    } 
} 
  
class BeginnerTest { 
    public static void main(String[] args) 
    { 
        System.out.println("Beginner Test class"); 
    } 
} 

When the above file is compiled as javac Trial.java this will create 2 .class files as ForBeginner.class and BeginnerTest.class .
Since each class has separate main() stub they can be tested individually.

When java ForBeginner is executed the output is For Beginner class.
When java BeginnerTest is executed the output is Beginner Test class.

Example 2:

Java

// Program to find area of triangle 
class Triangle { 
    int x, y; 
    void printArea() 
    { 
        System.out.println("Area of triangle is: " + x * y / 2); 
    } 
} 
  
class Demo { 
    public static void main(String args[]) 
    { 
        // Object creation 
        Triangle t = new Triangle(); 
        t.x = 10; 
        t.y = 13; 
        t.printArea(); 
    } 
} 

Note: There are two classes here, Triangle and Demo. Then which class name must be considered as the file name?
The class name Demo must be taken as the file name. The reason behind taking Demo as the file name is that it has the main method and execution begins from the main method.

Filename: Demo.java
Compilation: javac Demo.java
Run: java Demo
Output: Area of triangle:65

Myth about the file name and class name in Java

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