Java String Concatenation with + operator

Question

I got confused with the String concatenation.

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

The output is:

50abc20
50abc1010

I wonder why 20 + 30 are added together in both cases, but 10 + 10 require parenthese in order to be added (s1) instead of concatenated to the String (s2). Please explain how the String operator + works here.

Answer 1

Addition is left associative. Taking the first case

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

In the second case:

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