Java String Concatenation with + operator

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.

Solution 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

Solution 2:

Adding to the concept of associativity, you could ensure that two integers are never added together by using parentheses to always pair a string with an integer so the desired concatenation operation will take place rather than an addition.

String s4 = ((20 + (30 + "abc")) + 10)+10;

would produce:

2030abc1010

Solution 3:

Also, to add to this topic, as the wrong part of the answer of Jonathan Schober tipped me off on a thing to keep in mind:

a+=something is not equal to a=a+<something> : the += evaluates the right side first, and only then adds it to the left side. So it has to be rewritten, it is equivalent to:

a=a+(something); //notice the parentheses!

Showing the difference

public class StringTest {
  public static void main(String... args){
    String a = "";
    a+=10+10+10;

    String b = ""+10+10+10;

    System.out.println("First string, with += : " + a);
    System.out.println("Second string, with simple =\"\" " + b);

  }
}