Added FormattedDouble class to be able to cast as double

gson/src/main/java/com/google/gson/internal/FormattedDouble.java

@@ -0,0 +1,66 @@
+public class FormattedDouble extends Number {
+    private static final long serialVersionUID = 1L;
+
+    // ThreadLocal because DecimalFormat is not thread-safe
+    private static final ThreadLocal<DecimalFormat> format =
+            // Specify DecimalFormatSymbols to make code independent from default Locale
+            ThreadLocal.withInitial(() -> new DecimalFormat("0.00", DecimalFormatSymbols.getInstance(Locale.ENGLISH)));
+
+    private final Double delegate;
+
+    public FormattedDouble(Double value) {
+        Objects.requireNonNull(value, "Value should not be null");
+        this.delegate = value;
+    }
+
+    @Override
+    public byte byteValue() {
+        return delegate.byteValue();
+    }
+
+    @Override
+    public short shortValue() {
+        return delegate.shortValue();
+    }
+
+    @Override
+    public int intValue() {
+        return delegate.intValue();
+    }
+
+    @Override
+    public long longValue() {
+        return delegate.longValue();
+    }
+
+    @Override
+    public float floatValue() {
+        return delegate.floatValue();
+    }
+
+    @Override
+    public double doubleValue() {
+        return delegate.doubleValue();
+    }
+
+    @Override
+    public boolean equals(Object obj) {
+        if (obj == this) {
+            return true;
+        } else if (obj instanceof FormattedDouble) {
+            return ((FormattedDouble) obj).delegate.equals(delegate);
+        } else {
+            return false;
+        }
+    }
+
+    @Override
+    public int hashCode() {
+        return 31 * delegate.hashCode() + format.get().hashCode();
+    }
+
+    @Override
+    public String toString() {
+        return format.get().format((double) delegate);
+    }
+}