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.
+ can represent addition or concatenation.
Also addition and concatenation is left associative, so a+b+c is same as (a+b)+c (the b is associated with left +).
Taking the first case
20+30+"abc"+(10+10) <--- here both operands are integers with the + operator, which is addition
----- -------
50 +"abc"+ 20 <--- + operator on integer and string results in concatenation
---------
"50abc" + 20 <--- + operator on integer and string results in concatenation
------------
"50abc20"
In the second case:
20+30+"abc"+10+10 <--- here both operands are integers with the + operator, which is addition
-----
50 +"abc"+10+10 <--- + operator on integer and string results in concatenation
---------
"50abc" +10+10 <--- + operator on integer and string results in concatenation
----------
"50abc10" +10 <--- + operator on integer and string results in concatenation
------------
"50abc1010"
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);
}
}
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
You will need to start with an empty string.
So, this might work:
String s2 = ""+20+30+"abc"+10+10;
Or this:
String s2 ="";
s2 = 20+30+"abc"+10+10;
System.out.println(s2);
You need to know some rules:
1, Java operator priority,most of the left-to-right
2, Brackets priority than + sign priority.
3, The result is sum,if both sides of + sign are integer, else is concatenation.
