Double.toHexString(double d) String manipulation with + in an append of StringBuilder

A DESCRIPTION OF THE REQUEST :
Just a slight fix for performance when doing Double.toHexString(double d) method :P

Under the method :
public static String toHexString(double d) {

It uses StringBuilder for the output however, at line 306, it's done using + syntax in a StringBuilder.

JUSTIFICATION :
Performance enchantment...

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The String buffer should be done in a new StringBuilder.append() instead of using +.
ACTUAL -
answer.append("p" + (subnormal ?
DoubleConsts.MIN_EXPONENT:
FpUtils.getExponent(d) ));

---------- BEGIN SOURCE ----------
answer.append("p" + (subnormal ?
DoubleConsts.MIN_EXPONENT:
FpUtils.getExponent(d) ));
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
answer.append("p");
                answer.append((subnormal ?
DoubleConsts.MIN_EXPONENT:
FpUtils.getExponent(d) );