In Android, determine that strings must contain letters or numbers

public static boolean isLetterDigit(String str){
  boolean isDigit = false;//Define a boolean value to indicate whether it contains a number     boolean isLetter = false;//Define a boolean value to indicate whether it contains letters     for(int i=0 ; i
       if(((i))){   //Judge each character by judging numbers in the char wrapper class         isDigit = true; 
       }
       if(((i))){  //Judge each character using the method of judging letters in the char wrapper class         isLetter = true;
       }
     }
     String regex = "^[a-zA-Z0-9]+$";
    boolean isRight = isDigit && isLetter&&(regex);
 return isRight;
 
 }

String txt = ().toString();

  Pattern p = ("[0-9]*"); 
   Matcher m = (txt); 
   if(() ){
   (,"The input is a number", Toast.LENGTH_SHORT).show();
   } 
   p=("[a-zA-Z]");
   m=(txt);
   if(()){
   (,"The letters are entered", Toast.LENGTH_SHORT).show();
   }
   p=("[\u4e00-\u9fa5]");
   m=(txt);
   if(()){
   (,"The Chinese characters are entered", Toast.LENGTH_SHORT).show();
   }