如何(使用Java 1.5 RegEx):

    String[] split = fullFileName.split("\\.");
    String ext = split[split.length - 1];

你真的需要一个“解析器”吗?

String extension = "";

int i = fileName.lastIndexOf('.');
if (i > 0) {
    extension = fileName.substring(i+1);
}

假设您正在处理简单的类似windows的文件名，而不是像archive.tar.gz这样的文件名。

顺便说一下，对于目录可能有一个'。'，但文件名本身没有(像/path/to.a/file)，你可以这样做

String extension = "";

int i = fileName.lastIndexOf('.');
int p = Math.max(fileName.lastIndexOf('/'), fileName.lastIndexOf('\\'));

if (i > p) {
    extension = fileName.substring(i+1);
}

  @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());
    }

这是一种经过测试的方法

public static String getExtension(String fileName) {
    char ch;
    int len;
    if(fileName==null || 
            (len = fileName.length())==0 || 
            (ch = fileName.charAt(len-1))=='/' || ch=='\\' || //in the case of a directory
             ch=='.' ) //in the case of . or ..
        return "";
    int dotInd = fileName.lastIndexOf('.'),
        sepInd = Math.max(fileName.lastIndexOf('/'), fileName.lastIndexOf('\\'));
    if( dotInd<=sepInd )
        return "";
    else
        return fileName.substring(dotInd+1).toLowerCase();
}

测试用例:

@Test
public void testGetExtension() {
    assertEquals("", getExtension("C"));
    assertEquals("ext", getExtension("C.ext"));
    assertEquals("ext", getExtension("A/B/C.ext"));
    assertEquals("", getExtension("A/B/C.ext/"));
    assertEquals("", getExtension("A/B/C.ext/.."));
    assertEquals("bin", getExtension("A/B/C.bin"));
    assertEquals("hidden", getExtension(".hidden"));
    assertEquals("dsstore", getExtension("/user/home/.dsstore"));
    assertEquals("", getExtension(".strange."));
    assertEquals("3", getExtension("1.2.3"));
    assertEquals("exe", getExtension("C:\\Program Files (x86)\\java\\bin\\javaw.exe"));
}

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

我发现了一个更好的方法来找到扩展混合以上所有的答案

public static String getFileExtension(String fileLink) {

        String extension;
        Uri uri = Uri.parse(fileLink);
        String scheme = uri.getScheme();
        if (scheme != null && scheme.equals(ContentResolver.SCHEME_CONTENT)) {
            MimeTypeMap mime = MimeTypeMap.getSingleton();
            extension = mime.getExtensionFromMimeType(CoreApp.getInstance().getContentResolver().getType(uri));
        } else {
            extension = MimeTypeMap.getFileExtensionFromUrl(fileLink);
        }

        return extension;
    }

public static String getMimeType(String fileLink) {
        String type = CoreApp.getInstance().getContentResolver().getType(Uri.parse(fileLink));
        if (!TextUtils.isEmpty(type)) return type;
        MimeTypeMap mime = MimeTypeMap.getSingleton();
        return mime.getMimeTypeFromExtension(FileChooserUtil.getFileExtension(fileLink));
    }