Remove custom Utf8.toString implementation

`String::new` is now faster for both ASCII and Unicode strings:

```
Utf8::toString of ascii string took 132 ms
String::new of ascii string took 59 ms
Change = -55.30%
Utf8::toString of unicode string took 410 ms
String::new of unicode string took 280 ms
Change = -31.71%
```

There's at least two reasons for this:

* Java 9 introduced compact strings, which means that `String` is now backed by
a byte array to reduce the memory footprint of ASCII strings.
* Detection of Unicode strings may use HotSpot intrinsics.

1 files changed, 4 insertions, 21 deletions

diff --git a/vespajlib/src/main/java/com/yahoo/text/Utf8.java b/vespajlib/src/main/java/com/yahoo/text/Utf8.java
index cb8ca244fe2..a8a0adf1a7d 100644
--- a/vespajlib/src/main/java/com/yahoo/text/Utf8.java
+++ b/vespajlib/src/main/java/com/yahoo/text/Utf8.java
@@ -48,8 +48,7 @@ public final class Utf8 {
      * @return String decoded from UTF-8
      */
     public static String toString(byte[] data, int offset, int length) {
-        String s = toStringAscii(data, offset, length);
-        return s != null ? s : toString(ByteBuffer.wrap(data, offset, length));
+        return toString(ByteBuffer.wrap(data, offset, length));
     }
 
     /**
@@ -118,14 +117,14 @@ public final class Utf8 {
         return utf8 != null ? utf8 : string.getBytes(StandardCharsets.UTF_8);
     }
     /**
-     * Will try an optimistic approach to utf8 decoding.
+     * Decode a UTF-8 string.
      *
      * @param utf8 The string to encode.
      * @return Utf8 encoded array
      */
     public static String toString(byte [] utf8) {
-        String s = toStringAscii(utf8, 0, utf8.length);
-        return s != null ? s : new String(utf8, StandardCharsets.UTF_8);
+        // This is just wrapper for String::new now. Pre-Java 9 this had an more efficient approach for ASCII strings.
+        return new String(utf8, StandardCharsets.UTF_8);
     }
 
     /**
@@ -145,22 +144,6 @@ public final class Utf8 {
         return utf8;
     }
 
-    private static String toStringAscii(byte [] b, int offset, int length) {
-        if (length > 0) {
-            char [] s = new char[length];
-            for (int i=0; i < length; i++) {
-                if (b[offset + i] >= 0) {
-                    s[i] = (char)b[offset+i];
-                } else {
-                    return null;
-                }
-            }
-            return new String(s);
-        } else {
-            return "";
-        }
-    }
-
     /**
      * Utility method as toBytes(String).
      *