In java I am trying to create an object for a class but it shows an error "The public type Add1 must be defined in its own file"

Here I am getting an error

The public type Add1 must be defined in its own file

in the class name "Add1"

     public class Testing_Main 
     {
	     public static void main(String[] args) 
	     {
		     System.out.println("Before Object is Created");
		     Add1 obj=new Add1();
		     obj.add1();
		     System.out.println("After Object is Created");
	     }
     }
     public class Add1 
     {
	     public int add1()
	     {
	     	return 5;
	     }
     }

This kind of error The public type className must be defined in its own file occures when we have two public classes in one file. It is a kind of java rule that you should have one public class per file.

Why Only One Public Class Per Source File

> According to Java Language Specification (Section 7.6)
>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. This restriction makes it easy for a Java compiler
> to find a named class within a package. In practice, many programmers
> choose to put each class or interface type in its own compilation
> unit, whether or not it is public or is referred to by code in other
> compilation units.
>
> For example, the source code for a public type wet.sprocket.Toad would
> be found in a file Toad.java in the directory wet/sprocket , and the
> corresponding object code would be found in the file Toad.class in the
> same directory.

To get a more clear picture, let's imagine there are two public classes public class A and public class B in the same source file, and class A has reference to the not-yet-compiled class B. And we are compiling (compiling-linking-loading) class A now while linking to class B the compiler will be forced to check each *.java files within the current package because class B doesn’t have it’s specific B.java file. So, in the above case, it is a bit time consuming for the compiler to find which class lies under which source file and in which class the main method lies.

So the reason behind keeping one public class per source file is to actually make the compilation process faster because it enables a more efficient lookup of source and compiled files during linking. The idea is if you know the name of a class, you know where it should be found for each classpath entry and no indexing will be required.

We cannot have two public classes in one .java file. You can solve this problem by creating a separate file for the class `Add1.

Testing_Main.java

 public class Testing_Main 
 {
     public static void main(String[] args) 
     {
         System.out.println("Before Object is Created");
         Add1 obj=new Add1();
         obj.add1();
         System.out.println("After Object is Created");
     }
 }

Add1.java

 public class Add1 
 {
     public int add1()
     {
        return 5;
     }
 }

Or If you want to keep Add1 class in Testing_Main, then you can make the Add1 class non-public.

In java, each class must be associated with its own file. The classes are strictly bound with the filename, so a class called Foo must be places in a file called Foo.java. Therefore, you have to place both the classes TestingMain and Add1 in their own files, namely TestingMain.java and Add1.java respectively.