x = x + i;but this is not:x += i;
Solution: Objext x = "Buy "; Stirng i = "Effective java!"; x = x + i;//it's legal.显示：But Effective java! x += i; //it's illegal.因为+=操作符没有对String类型的定义。
TID: The += operator allows its right-hand operand to be of any type if the variable on
the left-hand side is of type String, in which case the operator performs string
concatenation.
