JAVA学习笔记（四）substring、System.exit(1)、java.text.SimpleDateFormat、==、equals、StringBuffer、SQl动态查询

while(!password.equalsIgnoreCase("iweb")) {

if((un<'a'||un>'z')&&(un<'1'||un>'9')&&un!='_') {
System.out.println("用户名必须由字母数字下划线组成，注册失败");
System.exit(-1);
}

System.out.println("   a  bc  e".trim());

if(phone.length()!=11||!phone.substring(0, 3).equals("138")) {
System.out.println("手机号码格式错误");
System.exit(1);
}

String a = "a,bc,t";
String[] b = a.split(",");
System.out.println(a.length() + " " + b.length);
for(int i=0;i<b.length;i++) {
System.out.println(b[i]);
}

java.util.Date now = new java.util.Date();
System.out.println(now);
java.text.SimpleDateFormat sdf = new java.text.SimpleDateFormat("yyyy年MM月dd日 hh时mm分ss秒SSS毫秒  E");
System.out.println(sdf.format(now));

String s1 = "abc";
String s2 = "abc";
char[] c = {'a','b','c'};
String s3 = new String("abc");
String s4 = new String("abc");
System.out.println("\"abc\"==\"abc\"-----" + (s1==s2));
System.out.println("\"abc\"==new String(\"abc\")-----" + (s1==s3));
System.out.println("new String(\"abc\")==new String(\"abc\")-----" + (s3==s4));
System.out.println("\"abc\".equals(new String(\"abc\"))-----" + s1.equals(s3));
System.out.println("\"abc\".equals(new String({'a','b','c'}数组名))-----" + s1.equals(new String(c)));
System.out.println("new String(\"abc\").equals(new String(\"abc\"))-----" + s3.equals(s4));

StringBuffer sb = new StringBuffer("abcd");
System.out.println(sb);
sb.append("edf");
System.out.println(sb);

String sql = "select * from users where 1=1";
java.util.Scanner san = new java.util.Scanner(System.in);
System.out.print("请输入用户名：");
String username = san.nextLine();
System.out.print("请输入电话：");
String phone = san.nextLine();
if(!username.equals("")) {
sql += " and uname='" + username + "'";
}
if(!phone.equals("")) {
sql += " and uphone='" + phone + "'";
}
System.out.println(sql);