JDK 12's Files.mismatch Method

JDK 12 introduces a new method to the Files class. The method, Files.mismatch(Path,Path), has been introduced to JDK 12 via JDK-8202302 and is available in JDK 12 Early Access Build 20 (same early access build that supports the new {@systemProperty} Javadoc tag).

JDK-8202302 ["(fs) New Files.mismatch method for comparing files"] adds the Files.mismatch(Path,Path) method "to compare the contents of two files to determine whether there is a mismatch between them" and can be used to determine "whether two files are equal." There was talk at one time of adding a Files.isSameContent() method, but it was decided to use Files.mismatch(Path,Parh) because of its consistency "with the Arrays.mismatch and Buffer.mismatch methods."

The next code listing contains a simple Java class that demonstrates the new Files.mismatch(Path,Path) and contrasts it with Files.isSameFile(Path,Path). The full source code is available on GitHub.

package dustin.examples.jdk12.files;

import java.nio.file.Files;
import java.nio.file.Path;

import static java.lang.System.out;

/**
 * Demonstrate {@code Files.mismatch(Path,Path)} introduced with JDK 12
 * and useful for determining if two files have the same content even
 * if they're not the same files.
 */
public class FilesDemo
{
   public static void main(final String[] arguments) throws Exception
   {
      if (arguments.length < 2)
      {
         out.println("USAGE: FilesDemo <file1Name> <file2Name>");
         return;
      }

      final String file1Name = arguments[0];
      final Path file1Path = Path.of(file1Name);
      final String file2Name = arguments[1];
      final Path file2Path = Path.of(file2Name);

      out.println("\nFiles '" + file1Name + "' and '" + file2Name + "' are "
         + (Files.isSameFile(file1Path, file2Path) ? "the" : "NOT the")
         + " same.\n\n");
      out.println("\nFiles '" + file1Name + "' and '" + file2Name + "' are "
         + (Files.mismatch(file1Path, file2Path) == -1 ? "the" : "NOT the")
         + " same content.\n\n");
   }
}

When the above code is executed against various combinations of files, it provides results that are captured in the next table. Note that the Files.mismatch method does not itself return a boolean, but the code shown above adapts its response to the appropriate boolean.

Results of Code Snippet Using Files.isSameFile() and Files.mismatch()
Files Relationship	Files.isSameFile(Path,Path)	Files.mismatch(Path,Path)
Same File	"Same" (true)	"Same" (-1)
Copied File	"Not Same" (false)	"Same" (-1)
Different Files	"Not Same" (false)	"Not Same" (positive integer)
Soft-linked	"Same" (true)	"Same" (-1)
Hard-linked	"Same" (true)	"Same" (-1)
Either File Not Found	FileNotFoundException	FileNotFoundException

The addition of Files.mismatch(Path,Path) is another step in accomplishing JDK-6852033 ["Inputs/Outputs methods to make common I/O tasks easy to do"] and makes it easier to determine when two files that are not the same file are still "equal" or have the same content.