--- /dev/null	2018-05-07 23:51:01.519000000 -0700
+++ new/src/java.base/share/classes/java/nio/file/FilesMismatch.java	2018-09-18 19:47:12.936485688 -0700
@@ -0,0 +1,133 @@
+/*
+ * Copyright (c) 2018, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.  Oracle designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Oracle in the LICENSE file that accompanied this code.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+package java.nio.file;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.nio.file.attribute.BasicFileAttributes;
+import java.util.Arrays;
+
+/**
+ * Utility methods for Files.mismatch.
+ */
+class FilesMismatch {
+    // see Files.mismatch for the specification
+    static long mismatch(Path path, Path path2) throws IOException {
+        long result = mismatchByAttrs(path, path2);
+        if (result != -2) {
+            return result;
+        }
+
+        try (InputStream in1 = Files.newInputStream(path);
+             InputStream in2 = Files.newInputStream(path2);) {
+            result = mismatch(in1, in2);
+            return result;
+        }
+    }
+
+    // check mismatch by file attributes, return -1 for no mismatch, 0 for mismatch
+    // at the beginning of the file, or -2 to indicate undetermined
+    private static long mismatchByAttrs(Path path, Path path2) throws IOException {
+        // initial values: -1 -- no mismatch
+        long result = -1;
+        try {
+            // the following checks NPE as well
+            path = path.toRealPath();
+            path2 = path2.toRealPath();
+            if (path.compareTo(path2) == 0) {
+                return result;
+            }
+            // readAttributes checks whether the file exists, throws IOE if not
+            BasicFileAttributes attrs1 = Files.readAttributes(path, BasicFileAttributes.class);
+            BasicFileAttributes attrs2 = Files.readAttributes(path2, BasicFileAttributes.class);
+            long s1 = attrs1.size();
+            long s2 = attrs2.size();
+
+            if (s1 == 0 && s2 == 0) {
+                return result;
+            }
+            if (s1 == 0 && s2 > 0 || s1 > 0 && s2 == 0) {
+                return 0;
+            }
+            if (s1 == s2) {
+                Object k1 = attrs1.fileKey();
+                if (k1 != null && k1.equals(attrs2.fileKey())) {
+                    return result;
+                }
+            }
+        } catch (ProviderMismatchException e) {
+            throw new UnsupportedOperationException(e);
+        }
+        return -2;
+    }
+
+    /**
+     * Finds and returns the index of the first mismatching byte in the content
+     * of two files.
+     *
+     * @param in1 an inputstream
+     * @param in2 another inputstream
+     * @return the mismatch index if a mismatch is found, otherwise -1 to indicate
+     *         no mismatch
+     * @throws IOException
+     */
+    private static long mismatch(InputStream in1, InputStream in2)
+        throws IOException {
+        byte[] buffer1 = new byte[Files.BUFFER_SIZE];
+        byte[] buffer2 = new byte[Files.BUFFER_SIZE];
+        int totalRead = 0;
+        while (true) {
+            int nRead1 = in1.readNBytes(buffer1, 0, Files.BUFFER_SIZE);
+            int nRead2 = in2.readNBytes(buffer2, 0, Files.BUFFER_SIZE);
+
+            if (nRead1 == 0 || nRead2 == 0) {
+                if (nRead1 == 0 && nRead2 == 0) {
+                    // both files reach EOF
+                    return -1;
+                } else {
+                    // one of the files reaches EOF
+                    return totalRead;
+                }
+            } else if (nRead1 != nRead2) {
+                int len = Math.min(nRead1, nRead2);
+                // there's always a mismatch when nRead1 != nRead2
+                return totalRead +
+                    Arrays.mismatch(buffer1, 0, len, buffer2, 0, len);
+
+            } else {
+                int i = Arrays.mismatch(buffer1, 0, nRead1, buffer2, 0, nRead1);
+                if (i > -1) {
+                    return totalRead + i;
+                }
+                if (nRead1 < Files.BUFFER_SIZE) {
+                    // we've reached the end of the files, but found no mismatch
+                    return -1;
+                }
+                totalRead += nRead1;
+            }
+        }
+    }
+}