[SPARK-9483] Fix UTF8String.getPrefix for big-endian.

Previous code assumed little-endian.

Author: Matthew Brandyberry <mbrandy@us.ibm.com>

Closes #7902 from mtbrandy/SPARK-9483 and squashes the following commits:

ec31df8 [Matthew Brandyberry] [SPARK-9483] Changes from review comments.
17d54c6 [Matthew Brandyberry] [SPARK-9483] Fix UTF8String.getPrefix for big-endian.

unsafe/src/main/java/org/apache/spark/unsafe/types/UTF8String.java

@@ -20,6 +20,7 @@ package org.apache.spark.unsafe.types;
 import javax.annotation.Nonnull;
 import java.io.Serializable;
 import java.io.UnsupportedEncodingException;
+import java.nio.ByteOrder;
 import java.util.Arrays;
 
 import org.apache.spark.unsafe.PlatformDependent;
@@ -53,6 +54,8 @@ public final class UTF8String implements Comparable<UTF8String>, Serializable {
     5, 5, 5, 5,
     6, 6};
 
+  private static ByteOrder byteOrder = ByteOrder.nativeOrder();
+
   public static final UTF8String EMPTY_UTF8 = UTF8String.fromString("");
 
   /**
@@ -175,18 +178,35 @@ public final class UTF8String implements Comparable<UTF8String>, Serializable {
     // If size is greater than 4, assume we have at least 8 bytes of data to fetch.
     // After getting the data, we use a mask to mask out data that is not part of the string.
     long p;
-    if (numBytes >= 8) {
-      p = PlatformDependent.UNSAFE.getLong(base, offset);
-    } else  if (numBytes > 4) {
-      p = PlatformDependent.UNSAFE.getLong(base, offset);
-      p = p & ((1L << numBytes * 8) - 1);
-    } else if (numBytes > 0) {
-      p = (long) PlatformDependent.UNSAFE.getInt(base, offset);
-      p = p & ((1L << numBytes * 8) - 1);
+    long mask = 0;
+    if (byteOrder == ByteOrder.LITTLE_ENDIAN) {
+      if (numBytes >= 8) {
+        p = PlatformDependent.UNSAFE.getLong(base, offset);
+      } else if (numBytes > 4) {
+        p = PlatformDependent.UNSAFE.getLong(base, offset);
+        mask = (1L << (8 - numBytes) * 8) - 1;
+      } else if (numBytes > 0) {
+        p = (long) PlatformDependent.UNSAFE.getInt(base, offset);
+        mask = (1L << (8 - numBytes) * 8) - 1;
+      } else {
+        p = 0;
+      }
+      p = java.lang.Long.reverseBytes(p);
     } else {
-      p = 0;
+      // byteOrder == ByteOrder.BIG_ENDIAN
+      if (numBytes >= 8) {
+        p = PlatformDependent.UNSAFE.getLong(base, offset);
+      } else if (numBytes > 4) {
+        p = PlatformDependent.UNSAFE.getLong(base, offset);
+        mask = (1L << (8 - numBytes) * 8) - 1;
+      } else if (numBytes > 0) {
+        p = ((long) PlatformDependent.UNSAFE.getInt(base, offset)) << 32;
+        mask = (1L << (8 - numBytes) * 8) - 1;
+      } else {
+        p = 0;
+      }
     }
-    p = java.lang.Long.reverseBytes(p);
+    p &= ~mask;
     return p;
   }