Interface

[Laurie carrera]

I wrote a simple program to exercise the interface functionality in java.
I encountered one error. I wanted to make the interface coderanch, but this gave me the following error: "class Driveable is coderanch, should be declared in a file named Driveable.java." Why?

Here is the code:

   
package tutorial;
 
import java.util.*;
import java.lang.*;
import java.io.*;
import java.util.Scanner;
 
 interface Drivable {
    public void DriveAge(int a);             
}
public class test implements Drivable{                   
   
    public static void main (String []args ){
        System.out.print("Please enter your age\n");
        Scanner input = new Scanner (System.in);
        new test().DriveAge(input.nextInt());
    }
        
    public void DriveAge(int a){      
         boolean b;        
         if (a>=18)
         {  
         b=true;
         System.out.println("Eligible for a drivers license:"+b);
         }
         else
         {
         b=false;
         System.out.println("Eligible for a drivers license:"+b);
         }           
    }
}

Any help would be appreciated.

[wise owen]

The .java file name must be the same as the public class' (or interface's) name.

[marc weber]

To expand on wise owen's explanation...

A .java file can contain at most one public class or interface definition. If the file includes a public class or interface definition, then the name of the .java file must exactly match the name of that public class or interface.

Note that a .java file is not required to contain any public class or interface definitions, and can contain an unlimited number of non-public class or interface definitions.

Or to be precise...

When packages are stored in a file system (�7.2.1), the host system may choose to enforce the restriction that it is a compile-time error if a type is not found in a file under a name composed of the type name plus an extension (such as .java or .jav) if either of the following is true:

The type is referred to by code in other compilation units of the package in which the type is declared.

The type is declared public (and therefore is potentially accessible from code in other packages).

This restriction implies that there must be at most one such type per compilation unit.

...

When packages are stored in a database (�7.2.2), the host system must not impose such restrictions.

Ref: JLS - 7.6 Top Level Type Declarations.
[ October 25, 2006: Message edited by: marc weber ]

[Laurie carrera]

Marc,
If I understand you right, a .java file can have only one public declaration of the class/interface kind. The public interface therefore would have to be declared in a seperate .java file (if I am not wrong, the file should named after the interface, hence Drivable.java) and implemented from the test.java file.True/false?

[marc weber]

Originally posted by Laurie carrera:
... True/false?

True.

If Drivable is going to be a top-level public interface, then it needs to be in a file called Drivable.java, which cannot include any other top-level public class or interface definitions.

Similarly, if Test is going to be a top-level public class, then it needs to be in a file called Test.java, which cannot include any other top-level public class or interface definitions.
[ October 25, 2006: Message edited by: marc weber ]

[marc weber]

Originally posted by marc weber:
... A .java file can contain at most one public class or interface definition. If the file includes a public class or interface definition, then the name of the .java file must exactly match the name of that public class or interface.

Note that a .java file is not required to contain any public class or interface definitions, and can contain an unlimited number of non-public class or interface definitions...

I should add that I'm talking about top-level (non-nested) classes and interfaces here.

[Abhijit Kolhatkar]

To extend the question:
I think, if we are declaring any interface/class, it is not mandetory that the file name of the .java file should match the class/interface name.
I.e. for the interface drivable, the name can be MyInterface.java. (we keep the name of the .java file and class inside it same for our convinience and to streamline the coding methodology)

Are there any exceptions to this?
I.e. are there any situations when it is enforced that the name of the .java file should exactly match the name of the class/interface in it? I think not...

Thanks in advance.

[Ilja Preuss]

Originally posted by Abhijit Kolhatkar:
To extend the question:
I think, if we are declaring any interface/class, it is not mandetory that the file name of the .java file should match the class/interface name.
I.e. for the interface drivable, the name can be MyInterface.java. (we keep the name of the .java file and class inside it same for our convinience and to streamline the coding methodology)

Are there any exceptions to this?
I.e. are there any situations when it is enforced that the name of the .java file should exactly match the name of the class/interface in it? I think not...

Thanks in advance.

Your question has already been answered in this thread. If you reread it carefully, you will learn that there is a quite important exception...