A DESCRIPTION OF THE REQUEST :
The code:
    s = s + a + b; // (1)
is compiled into the following equivalent code:
    s = (new StringBuilder()).append(s).append(a).append(b).toString();

In contrast, the code:
    s += a; // (2)
    s += b;
compiles into:
    s = (new StringBuilder()).append(s).append(a).toString();
    s = (new StringBuilder()).append(s).append(b).toString();

The latter case could be optimized by the compiler/code generator into the same code as the first case (1) above. This would reduce the number of objects allocated and improve performance.

Related bug: 4059189

JUSTIFICATION :
Improved performance.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
(See above.)
ACTUAL -
(See above.)

---------- BEGIN SOURCE ----------
(See code fragements above.)
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Use sequences of '+' operations in a single (large) statement, instead of coding separate '+=' statements.