String s = "abc";
  String ss = "bcd"+s+"def"+5;
  System.out.println(ss);

系統幫我們做了優化，建立了一個StringBuilder物件然後進行了append的拼接，但是Stringbuilder的父類AbstractStringBuilder會建立一個char[]陣列，並且toString()會建立一個String物件

String s = "a" + "b" + "c" + "d" + "e";
System.out.println(s);

    String a = "0";
    for (int i = 1; i < 10; i++) {
        a = a + "M";
    }
    System.out.println(a);