as far as I understand x+=1 works as x=x+1, but why it doesn't work in String?
String str = "";
str = str + null + true; // fine
str += null + true; // Error message: The operator + is undefined for the argument type(s) null, boolean
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1 Answer
In Java, expression are evaluated from left to right. Thus
str = str + null + true;
is the same as
str = (str + null) + true;
and null and true are implicitly converted to Strings. This works because in str + null, the compiler knows that str is a String and converts null to a String. This is possible because every value can be converted to a String in Java. By the same line of argumentation, the compiler knows that (str + null) is a String and thus coverts true to a String.
On the other hand,
str += null + boolean;
is equivalent to
str = str + (null + boolean);
hence, null + boolean is evaluated first. Since the operator + is not defined for types null, boolean, a compiler-error is generated.
