Improved Long-Double Number Policy (google#2674)

* Improved Long-Double Number Policy

The Parsing of a Double value was always executing a `Long.parseLong(value)`, which generated a `NumberFormatException`.

Identifying that a Number is a Double or a Long can be easily achieve (in a naive way) looking for the decimal separator.

This simple change avoids the extra `NumberFormatException`

A simple JUnit test, parsing a `Long` or a `Double` 10K times shows the next values:

* Double (old parsing): ~42 ms
* Double (new parsing): ~6 ms
* Long (old parsing): ~7 ms
* Long (new parsing): ~7 ms

As we can see, the parsing for `Long` values stays the same (±1ms), while the parsing for `Double` is dramatically improved.

Reducing the number of exceptions also has a positive side effect in memory consumption.

* Replace `contains(".")` by `indexOf('.') >= 0`

The usage of `indexOf(char)` is slightly faster

* Rename exception variables

gson/src/main/java/com/google/gson/ToNumberPolicy.java

@@ -68,22 +68,30 @@ public Number readNumber(JsonReader in) throws IOException {
     @Override
     public Number readNumber(JsonReader in) throws IOException, JsonParseException {
       String value = in.nextString();
-      try {
-        return Long.parseLong(value);
-      } catch (NumberFormatException longE) {
+      if (value.indexOf('.') >= 0) {
+        return parseAsDouble(value, in);
+      } else {
         try {
-          Double d = Double.valueOf(value);
-          if ((d.isInfinite() || d.isNaN()) && !in.isLenient()) {
-            throw new MalformedJsonException(
-                "JSON forbids NaN and infinities: " + d + "; at path " + in.getPreviousPath());
-          }
-          return d;
-        } catch (NumberFormatException doubleE) {
-          throw new JsonParseException(
-              "Cannot parse " + value + "; at path " + in.getPreviousPath(), doubleE);
+          return Long.parseLong(value);
+        } catch (NumberFormatException e) {
+          return parseAsDouble(value, in);
         }
       }
     }
+
+    private Number parseAsDouble(String value, JsonReader in) throws IOException {
+      try {
+        Double d = Double.valueOf(value);
+        if ((d.isInfinite() || d.isNaN()) && !in.isLenient()) {
+          throw new MalformedJsonException(
+              "JSON forbids NaN and infinities: " + d + "; at path " + in.getPreviousPath());
+        }
+        return d;
+      } catch (NumberFormatException e) {
+        throw new JsonParseException(
+            "Cannot parse " + value + "; at path " + in.getPreviousPath(), e);
+      }
+    }
   },
 
   /**