import java.util.HashMap;
 
public class PasswordValidator{	
	private static HashMap _legal_chars;
	
	static{
		String[] legalLetters = {"a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"};
		String[] legalNumbers = {"0","1","2","3","4","5","6","7","8","9"};
		
		_legal_chars = new HashMap();
		String character;
		
		for(int i=legalLetters.length-1; i>=0; i--){
			character = legalLetters[i];
			_legal_chars.put(character,null);
			character = character.toUpperCase();
			_legal_chars.put(character,null);
		}
		
		for(int i=legalNumbers.length-1; i>=0; i--){
			character = legalNumbers[i];
			_legal_chars.put(character,null);
		}
		
		_legal_chars.put("_",null);
	}
	
	public static boolean isPasswordValid(String password){
		if(password==null){
			throw new IllegalArgumentException("password cannot be null");
		}
		
		boolean isValid = true;
		char[] chars = password.toCharArray();
		
		for(int i=chars.length-1;i>=0; i--){
			if(!_legal_chars.containsKey(String.valueOf(chars[i]))){
				isValid = false;
				break;
			}
		}
		
		return isValid;
	}
}

public class Test {
 
	public static void main (String[] args) throws Exception {
 
 
		String str[] = { "Hello", "abc_y2u_2", "_", "  ", "ab-hy", "hj^lk_90*" } ;
 
		for ( int i = 0 ; i < str.length; i++ ) {
 
			System.out.println ( "\"" + str[i] + "\" is " + isValidPassword ( str[i] ) ) ;
		}
	}
 
	public static boolean isValidPassword ( String pass ) {
		String regex = "[A-Za-z0-9_]+";
 
		boolean valid = false;
 
		if ( pass != null ) {
			valid = pass.matches ( regex );
		}
 
		return valid ;
 
	}
 
}