从文件名获取文件扩展名

/**
 * The extension separator character.
 */
private static final char EXTENSION_SEPARATOR = '.';

/**
 * The Unix separator character.
 */
private static final char UNIX_SEPARATOR = '/';

/**
 * The Windows separator character.
 */
private static final char WINDOWS_SEPARATOR = '\\';

/**
 * The system separator character.
 */
private static final char SYSTEM_SEPARATOR = File.separatorChar;

/**
 * Gets the extension of a filename.
 * <p>
 * This method returns the textual part of the filename after the last dot.
 * There must be no directory separator after the dot.
 * <pre>
 * foo.txt      --> "txt"
 * a/b/c.jpg    --> "jpg"
 * a/b.txt/c    --> ""
 * a/b/c        --> ""
 * </pre>
 * <p>
 * The output will be the same irrespective of the machine that the code is running on.
 *
 * @param filename the filename to retrieve the extension of.
 * @return the extension of the file or an empty string if none exists.
 */
public static String getExtension(String filename) {
    if (filename == null) {
        return null;
    }
    int index = indexOfExtension(filename);
    if (index == -1) {
        return "";
    } else {
        return filename.substring(index + 1);
    }
}

/**
 * Returns the index of the last extension separator character, which is a dot.
 * <p>
 * This method also checks that there is no directory separator after the last dot.
 * To do this it uses {@link #indexOfLastSeparator(String)} which will
 * handle a file in either Unix or Windows format.
 * <p>
 * The output will be the same irrespective of the machine that the code is running on.
 *
 * @param filename  the filename to find the last path separator in, null returns -1
 * @return the index of the last separator character, or -1 if there
 * is no such character
 */
public static int indexOfExtension(String filename) {
    if (filename == null) {
        return -1;
    }
    int extensionPos = filename.lastIndexOf(EXTENSION_SEPARATOR);
    int lastSeparator = indexOfLastSeparator(filename);
    return (lastSeparator > extensionPos ? -1 : extensionPos);
}

/**
 * Returns the index of the last directory separator character.
 * <p>
 * This method will handle a file in either Unix or Windows format.
 * The position of the last forward or backslash is returned.
 * <p>
 * The output will be the same irrespective of the machine that the code is running on.
 *
 * @param filename  the filename to find the last path separator in, null returns -1
 * @return the index of the last separator character, or -1 if there
 * is no such character
 */
public static int indexOfLastSeparator(String filename) {
    if (filename == null) {
        return -1;
    }
    int lastUnixPos = filename.lastIndexOf(UNIX_SEPARATOR);
    int lastWindowsPos = filename.lastIndexOf(WINDOWS_SEPARATOR);
    return Math.max(lastUnixPos, lastWindowsPos);
}

学分

复制自Apache FileNameUtils Class - http://grepcode.com/file/repo1.maven.org/maven2/commons-io/commons-io/1.3.2/org/apache/commons/io/FilenameUtils.java#FilenameUtils.getExtension%28java.lang.String%29

JFileChooser怎么样?这并不简单，因为你需要解析它的最终输出…

JFileChooser filechooser = new JFileChooser();
File file = new File("your.txt");
System.out.println("the extension type:"+filechooser.getTypeDescription(file));

这是一个MIME类型…

好吧……我忘了你不想知道它的MIME类型。

下面链接中的有趣代码: http://download.oracle.com/javase/tutorial/uiswing/components/filechooser.html

/*
 * Get the extension of a file.
 */  
public static String getExtension(File f) {
    String ext = null;
    String s = f.getName();
    int i = s.lastIndexOf('.');

    if (i > 0 &&  i < s.length() - 1) {
        ext = s.substring(i+1).toLowerCase();
    }
    return ext;
}

相关问题: 我如何修剪一个文件扩展名从一个字符串在Java?

如果在Android上，你可以使用这个:

String ext = android.webkit.MimeTypeMap.getFileExtensionFromUrl(file.getName());

  @Test
    public void getFileExtension(String fileName){
      String extension = null;
      List<String> list = new ArrayList<>();
      do{
          extension =  FilenameUtils.getExtension(fileName);
          if(extension==null){
              break;
          }
          if(!extension.isEmpty()){
              list.add("."+extension);
          }
          fileName = FilenameUtils.getBaseName(fileName);
      }while (!extension.isEmpty());
      Collections.reverse(list);
      System.out.println(list.toString());
    }

Java有一个内置的方法来处理这个问题，在Java .nio.file. files类中，这可能适合你的需要:

File f = new File("/path/to/file/foo.txt");
String ext = Files.probeContentType(f.toPath());
if(ext.equalsIgnoreCase("txt")) do whatever;

注意，这个静态方法使用这里找到的规范来检索“内容类型”，而“内容类型”是可以变化的。

这个特别的问题给了我很多麻烦，然后我找到了一个非常简单的解决方案，我张贴在这里。

file.getName().toLowerCase().endsWith(".txt");

就是这样。