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import java.io.*;
import javax.imageio.*;
import java.util.*;
import javax.imageio.stream.*;

public class Test {
    public static void main(String[] args) {
        File f = new File("c://test//3.jpg");
        if (f.exists()) {
            System.out.println(getFormatInFile(f));
        }
    }

    // Returns the format of the image in the file 'f'.
    // Returns null if the format is not known.
    public static String getFormatInFile(File f) {
        return getFormatName(f);
    }

    // Returns the format name of the image in the object 'o'.
    // Returns null if the format is not known.
    private static String getFormatName(Object o) {
        try {
            // Create an image input stream on the image
            ImageInputStream iis = ImageIO.createImageInputStream(o);

            // Find all image readers that recognize the image format
            Iterator iter = ImageIO.getImageReaders(iis);
            if (!iter.hasNext()) {
                // No readers found
                return null;
            }

            // Use the first reader
            ImageReader reader = (ImageReader) iter.next();

            // Close stream
            iis.close();

            // Return the format name
            return reader.getFormatName();
        } catch (IOException e) {
            //
        }

        // The image could not be read
        return null;
    }
}