带+运算符的Java字符串串联

Question

我对String串联感到困惑。

String s1 = 20 + 30 + "abc" + (10 + 10);
String s2 = 20 + 30 + "abc" + 10 + 10;
System.out.println(s1);
System.out.println(s2);

输出为：

50abc20
50abc1010

我想知道为什么在两种情况下都将 20 + 30 加在一起，但是 10 + 10
需要加上括号（s1）而不是串联到String（s2）。请+在此处说明String运算符的工作方式。

Answer 1

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