String concatenation: concat() vs "+" operator?

Question

Assuming String a and b: a += b a = a.concat(b) Under the hood, are they the same thing?

Answer 1

In the nutshell, they are the same, copying char[] from input the current char array.

char buf[] = Arrays.copyOf(value, len + otherLen);

For the + operator, the compiler converts "x = "+x; into a StringBuilder internally and uses .append(int) to "add" the integer to the string. If you look at the append implementation, it's copying compiled char array.

System.arraycopy(value, srcBegin, dst, dstBegin, srcEnd - srcBegin);