从别的地方copy来的，用着还可以。
private static final int SIZE = 6;
private static final String SYMBOL = "*";
public static void main(String[] args) {
String s = "";
String conceal = toConceal(s);
System.out.println(conceal);
}
public static String toConceal(String value) {
if (null == value || "".equals(value)) {
return value;
}
int len = value.length();
int pamaone = len / 2;
int pamatwo = pamaone - 1;
int pamathree = len % 2;
StringBuffer stringBuilder = new StringBuffer();
if (len <= 2) {
if (pamathree == 1) {
return SYMBOL;
}
stringBuilder.append(SYMBOL);
stringBuilder.append(value.charAt(len - 1));
} else {
if (pamatwo <= 0) {
stringBuilder.append(value.substring(0, 1));
stringBuilder.append(SYMBOL);
stringBuilder.append(value.substring(len - 1, len));
} else if (pamatwo >= SIZE / 2 && SIZE + 1 != len) {
int pamafive = (len - SIZE) / 2;
stringBuilder.append(value.substring(0, pamafive));
for (int i = 0; i < SIZE; i++) {
stringBuilder.append(SYMBOL);
}
if ((pamathree == 0 && SIZE / 2 == 0) || (pamathree != 0 && SIZE % 2 != 0)) {
stringBuilder.append(value.substring(len - pamafive, len));
} else {
stringBuilder.append(value.substring(len - (pamafive + 1), len));
}
} else {
int pamafour = len - 2;
stringBuilder.append(value.substring(0, 1));
for (int i = 0; i < pamafour; i++) {
stringBuilder.append(SYMBOL);
}
stringBuilder.append(value.substring(len - 1, len));
}
}
return stringBuilder.toString();
}