浅谈java字符串比较到底应该用==还是equals

 System.out.println("a" == "a"); //true
 System.out.println("a".equals("a")); //true
 System.out.println("a".equals(new String("a"))); //true
 System.out.println("a" == new String("a")); //false
 System.out.println(new String("a") == new String("a")); //false
 System.out.println(new String("a").equals(new String("a"))); //true

 System.out.println("a".concat("a") == "aa"); //false
 System.out.println("aa".replaceAll("a", "b") == "bb"); //false
 System.out.println("A".toLowerCase() == "a"); //false
 System.out.println("1;2".split(";")[0] == "1"); //false

 System.out.println("a".concat("") == "a"); //ture
 //replaceAll由于用到了StringBuffer的toString方法，会一直返回false
 System.out.println("a".toLowerCase() == "a"); //ture
 System.out.println("1;2".split("3")[0] == "1;2"); //ture

 String str1 = "a";
 String str2 = "a";
 System.out.println("a".equals(str1)); //正确，不会出现空指针
 System.out.println(str1.equals("a")); //不建议，若str1为空，会报空指针
 System.out.println(((str1 != null) && str1.equals(str2)) || (str1 == null && str2 == null));//正确，但是建议拆成两句写。你难道不觉得这么多逻辑判断很头疼么？