+ operator does use StringBuilder. But it will use a StringBuilder for each string expression. Take the following piece of code: String foo = ""; for (int

+ operator does use StringBuilder. But it will use a StringBuilder for each string expression. Take the following piece of code: String foo = ""; for (int i = 0; i < 1000; i++) { foo += "x"; } The inside of the for loop can be replaced with: StringBuilder tmp = new StringBuilder(foo.length()); tmp.append("x"); foo = tmp.toString() As you will notice, we are still copying over the string foo each time we concatenate. So building strings with + inside a loop will still give you the feared copy-string-each-time behavior. The new String.concat() trick is only useful for crafting long strings in a single go, i.e. in a single expression.