The Problem

When you compile a Java source file, you might encounter the following error message:

Error: a public class <ClassName> must be defined in a file called <ClassName>.java

This error occurs when you violate one of the fundamental rules of Java: a single source file (.java) can contain at most one public class, and the name of that public class must exactly match the source file’s name (including case).

This rule helps the Java compiler and runtime environment locate and load classes efficiently from the file system.

Example of Error-Prone Code

Let’s say you have a file named MyProgram.java with the following content:

File Name: MyProgram.java

// The file is named MyProgram.java, but the public class is named HelloWorld.
public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

When you try to compile this code, the compiler expects the public class HelloWorld to be in a file named HelloWorld.java. Since it’s in MyProgram.java, it will produce an error like this:

MyProgram.java:2: error: class HelloWorld is public, should be declared in a file named HelloWorld.java
public class HelloWorld {
       ^
1 error

How to Fix It

This problem is very simple to fix.

1. Rename the File to Match the Class Name

The most common solution is to rename the .java file to match the name of the public class.

  • In the example above, you would rename the file MyProgram.java to HelloWorld.java.

This is the preferred method when you want to keep the class name as it is.

2. Rename the Class to Match the File Name

Alternatively, if you want to keep the file name, you can change the name of the public class to match it.

  • You would modify the content of MyProgram.java as follows:

File Name: MyProgram.java

// Change the class name to match the file name, MyProgram.
public class MyProgram {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

If the class does not need to be accessed from other packages, you can remove the public access modifier. A non-public (package-private) class does not have the restriction of matching the file name.

File Name: MyProgram.java

// Removing the public keyword allows the class and file names to be different.
class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

However, this changes the accessibility of your class and should be done with caution. In most cases, keeping the class public and ensuring the names match is the best practice.

Conclusion

The Error: a public class ... must be defined in a file called ... .java is caused by a mismatch between the public class name and the file name. To resolve it, choose one of the following:

  1. Change the file name to match the public class name.
  2. Change the public class name to match the file name.

Adhering to this naming convention is a standard practice in Java that improves code organization and readability. It’s best to always follow this rule.

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