That’s because the language designers overloaded the arithmetic operator (+) to perform string concatenation when the operands can be cast as Strings.

Without overloading the + operator, string concatenation which is a common operation, would have been unnecessarily verbose. Your example without the + operator would look like below:

   String a = new String(“a”).concat(1); // String object concat
   String a = “a”.concat(1);             // String literal concat