public static void main(String[] args) {String str1 = "aaa";    String str2 = "aaa";    String str3 = new String("aaa");    String str4 = str1+"";    String str5 = "aa"+"a";    System.out.PRintln(str1 == str3);    System.out.println(str1.equals(str3));    System.out.println(str1==str2);    System.out.println(str1 == str4);    System.out.println(str3 == str4);    System.out.println(str1 == str5);/** * String 相加 ==> str4 = str1+"";     *//*    String str1 = "aaa";    StringBuilder sb = new StringBuilder();    sb.append(str1).append("");    String str4 = sb.toString();*/}
結果：
false
true
true
false
false
true
所以有大量“+”的拼接操作時用StringBuilder或者StringBuffer會省下很多內存;
StringBuilder和StringBuffer作用類似,只是StringBuffer的由于增加了同步鎖是線程安全的，但是會稍微影響性能;