用JAVA代码实现验证邮箱地址是否符合

public class Test{
 public static void main(String[] args){
  Test  t = new Test();
  String email = "test@sina.com" ;
  boolean  isEmail = t.isEmail(email);
  if(isEmail){
   System.out.println(email +" is  a right email!!!");
  }else{
   System.out.println("error");
  }
 }
 public boolean isEmail(String email){
  //正则表达式
  /*
    String regex = "^[A-Za-z]{1,40}@[A-Za-z0-9]{1,40}\\.[A-Za-z]{2,3}$";
    return email.matches(regex);
   */

  //不适用正则
  if(email==null||"".equals(email)) return false ;
  if(!containsOneWord('@',email)||!containsOneWord('.',email)) return false;
  String prefix = email.substring(0,email.indexOf("@"));
  String middle = email.substring(email.indexOf("@")+1,email.indexOf("."));
  String subfix = email.substring(email.indexOf(".")+1);
  System.out.println("prefix="+prefix +"  middle="+middle+"  subfix="+subfix);

  if(prefix==null||prefix.length()>40||prefix.length()==0) return false ;
  if(!isAllWords(prefix)) return false ;
  if(middle==null||middle.length()>40||middle.length()==0) return false ;
  if(!isAllWordsAndNo(middle)) return false ;
  if(subfix==null||subfix.length()>3||subfix.length()<2) return false ;
  if(!isAllWords(subfix)) return false ;
  return true ;
 }
 //判断字符串只包含指定的一个字符c
 private boolean containsOneWord(char c , String word){
  char[] array = word.toCharArray();
  int count = 0 ;
  for(Character ch : array){
   if(c == ch) {
    count++;
   }
  }
  return count==1 ;
 }
 //检查一个字符串是否全部是字母
 private boolean isAllWords(String prefix){
  char[] array = prefix.toCharArray();
  for(Character ch : array){
   if(ch<'A' || ch>'z' || (ch<'a' && ch>'Z')) return false ;
  }
  return true;
 }
 //检查一个字符串是否包含字母和数字
 private boolean isAllWordsAndNo(String middle){
  char[] array = middle.toCharArray();
  for(Character ch : array){
   if(ch<'0' || ch > 'z') return false ;
   else if(ch >'9' && ch <'A') return false ;
   else if(ch >'Z' && ch <'a') return false ;
  }
  return true ;
 }
}