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Java’s duplicate class error means two top-level types with the same name are declared in the same package. A mismatched-filename error is different: it means a public type’s source file does not use the expected name, or the source file is not organized where the compiler expects it. Check the diagnostic, declarations, package statements, filenames, and build inputs to identify which problem you have.
What the two Java errors mean
“Duplicate class” means a same-package name collision
The Java Language Specification makes it a compile-time error for a top-level type’s name to also be the name of another top-level class or interface declared in the same package. In practical terms, look for two top-level declarations such as class Person and public class Person that belong to the same package. The rule concerns top-level types; a nested class is not itself a second top-level declaration. See the Java SE 26 Language Specification, Chapter 7.
A filename error concerns source-file organization
A compiler may report that a public class should be declared in a correspondingly named file—for example, class Person is public, should be declared in a file named Person.java. This is a filename complaint, not proof that another Person declaration exists. The JLS describes filesystem-based naming restrictions that a host may enforce for public types and types referenced from other compilation units. In a conventional source tree, a public type named Toad in package wet.sprocket belongs in Toad.java under the matching package directory.
OpenJDK’s compiler resources include distinct diagnostic text for duplicate class: {0} and bad file name: {0}. The wording varies by diagnostic and compiler context, so use the actual message and named file to guide the investigation. See OpenJDK’s compiler diagnostic resources.
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How to tell which problem you have
| Clue | Duplicate top-level declaration | Filename or source-layout mismatch |
|---|---|---|
| Typical diagnostic | duplicate class: Person |
A message that a public class should be declared in a file named Person.java, or a bad-file-name diagnostic |
| What to verify | Whether another top-level type with that name is declared in the same package | Whether the public type’s spelling and capitalization match the .java filename |
| Package and path | Compare package declarations; same simple names in different packages do not trigger this same-package rule | Check that the source file is in the directory corresponding to its declared package in a conventional source tree |
Find and fix the cause
- Start with the complete compiler message. Note the reported type name, source file, and line. Begin at the named location, but do not assume that the first file mentioned is the only relevant source.
- Search all compiled sources for another top-level declaration. Look for a class, interface, enum, or record with the same name. Confirm that it is top-level, rather than nested inside another type.
- Compare package declarations. A package statement determines a compilation unit’s package membership; a file with no package statement is in the unnamed package. If a type
Tis in packageP, its fully qualified name isP.T. Two types with the same simple name in different packages are not the same-package collision described by the JLS. - For a filename complaint, compare exact names. If the public top-level declaration is
public class Person, make the source filePerson.java, including the same capitalization. Also check that its folder matches its package in your source tree. - Inspect build inputs and source roots. Check whether the build compiles a second copy of the source or includes the same source through more than one input or source root. Remove the unintended duplicate input or consolidate the declarations as appropriate. The particular build configuration is project-specific; it is not a separate language-specification rule.
- Recompile after one targeted change. If another diagnostic appears, investigate it on its own. Renaming a file can address a filename complaint without removing a duplicate declaration.
Common causes and corrections
Two declarations share a package
Search for repeated top-level declarations and decide which one should remain. If both types are needed, give them distinct names or put them in different packages, then update references accordingly.
The package statements do not match the folders you expect
A source file’s package statement—not merely its folder name—determines its package membership. Correct an unintended package declaration or move the file into the appropriate package directory and keep the project’s source-root configuration consistent.
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The public type and filename differ
Rename the source file to match the public top-level type, preserving capitalization, or rename the type and its references to match the intended filename. Check the actual filesystem spelling, particularly when code is moved between systems with different filename case behavior.
The build sees another copy of the source
When declarations appear unique in the files you opened, inspect the build’s source roots and compiler inputs for generated, copied, or otherwise unintended source files. Fix the input set rather than changing an unrelated declaration.
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Why one change can reveal two errors
The diagnostics answer different questions: whether a top-level name is declared twice in a package, and whether a source file is named or organized as expected. A project can have both problems, or fixing one can expose the other. After each edit, compile again and follow the new diagnostic instead of treating every message as a filename issue or every repeated name as a duplicate-class error.
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