假设s1和s2是两个字符串，下面哪些语句或表达式是错误的？
String s = new String("new String");
String s3 = s1 + s2;
String s3 = s1 - s2;
s1 == s2;
s1 >= s2;
s1.compareTo(s2);
int i = s1.length();
char c = s1(0);
char c = s1.charAt(s1.length());

String s3 = s1 - s2;s1 >= s2;就这两个不对吧