How to get the file extension in Java?
答案：使用lastIndexOf和substring方法可提取文件扩展名。例如，String fileName = "document.pdf"; int dotIndex = fileName.lastIndexOf('.'); String extension = dotIndex > 0 && dotIndex
To get the file extension in Java, you can extract it from the file's name by finding the last occurrence of a dot (.) and taking the substring that follows it. Here's how you can do it reliably.
Using String Methods
If you have a file name as a string, you can use lastIndexOf and substring to retrieve the extension.
- Find the last index of the dot character.
- Ensure the dot is not at the start or end (to avoid incorrect extensions).
- Extract the substring after the dot.
Example:
String fileName = "document.pdf";int lastIndex = fileName.lastIndexOf('.');
String extension = "";
if (lastIndex > 0 && lastIndex extension = fileName.substring(lastIndex 1);
}
System.out.println(extension); // Output: pdf
Using java.nio.file.Path
You can also work with Path if you're dealing with file paths.
- Get the file name from the path using getFileName().
- Apply the same logic as above on the file name string.
Example:
import java.nio.file.Path;import java.nio.file.Paths;
Path path = Paths.get("C:/files/document.pdf");
String fileName = path.getFileName().toString();
int dotIndex = fileName.lastIndexOf('.');
String ext = (dotIndex > 0 && dotIndex ? fileName.substring(dotIndex 1) : "";
System.out.println(ext); // Output: pdf
Handling Edge Cases
Be cautious with files that:
- Have no extension (e.g., file).
- Start with a dot but aren't extensions (e.g., .gitignore).
- Contain multiple dots (e.g., version.2.1.jar – extension is jar).
The logic using lastIndexOf handles these correctly by focusing on the part after the final dot, only if it makes sense.
Basically just check where the last dot is and pull out what comes after — as long as it's a valid position.
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