加法保持关联。以第一种情况
20+30+"abc"+(10+10)----- ------- 50 +"abc"+ 20 <--- here both operands are integers with the + operator, which is addition --------- "50abc" + 20 <--- + operator on integer and string results in concatenation ------------ "50abc20" <--- + operator on integer and string results in concatenation
在第二种情况下：
20+30+"abc"+10+10----- 50 +"abc"+10+10 <--- here both operands are integers with the + operator, which is addition --------- "50abc" +10+10 <--- + operator on integer and string results in concatenation ---------- "50abc10" +10 <--- + operator on integer and string results in concatenation ------------ "50abc1010" <--- + operator on integer and string results in concatenation
