처음 일부분만 보여주고 나머지는 * 로 마스킹

MaskString.Java

public class Main {
    public static void main(String[] args)  {
      System.out.println(maskString("ChangJoo Park", 3, '*'));
   }
   
   public static String maskString(String targetString, int numberOfPrefix, char maskCharacter) {
      int stringLength = targetString.length();
      int postfixLength = stringLength - numberOfPrefix;
      
      if (stringLength < numberOfPrefix) {
        return targetString;
      }
      
      String prefix = targetString.substring(0, numberOfPrefix);
      
      if (postfixLength <= 0) {
        return prefix;
      }
      
      String postfix = new String(new char[postfixLength]).replace('\0', maskCharacter);
      return prefix.concat(postfix);
   }
}

https://repl.it/@ChangJoo_Park/Masking-String

Display masked email ID only

 private static final String REGEX_EMAIL = "[a-zA-Z0-9\\+\\.\\_\\%\\-\\+]{1,256}" +
            "\\@" +
            "[a-zA-Z0-9][a-zA-Z0-9\\-]{0,64}" +
            "(" +
            "\\." +
            "[a-zA-Z0-9][a-zA-Z0-9\\-]{0,25}" +
            ")+";

public String mask(final String targetString, final int numberOfPrefix, final char maskCharacter) {
        String targetTemp = targetString;

        if (targetTemp.matches(REGEX_EMAIL)) {
            targetTemp = targetTemp.substring(0, targetTemp.indexOf('@'));
        }

        int stringLength = targetTemp.length();
        int postfixLength = stringLength - numberOfPrefix;

        if (stringLength < numberOfPrefix) {
            return new String(new char[stringLength]).replace('\0', maskCharacter);
        }

        String prefix = targetTemp.substring(0, numberOfPrefix);

        if (postfixLength <= 0) {
            return prefix;
        }

        String postfix = new String(new char[postfixLength]).replace('\0', maskCharacter);
        return prefix.concat(postfix);
}