How to replace all characters in a Java string with stars

Question

I want to replace all the characters in a Java String with * character. So it shouldn\'t matter what character it is, it should be replaced with a *

Answer 1

public String allStar(String s) {
    StringBuilder sb = new StringBuilder(s.length());
    for (int i = 0; i < s.length(); i++) {
        sb.append('*');
    }
    return sb.toString();
}

Answer 2

Recursive method

String nCopies(String s, int n) {
    return n == 1 ? s.replaceFirst(".$", "") : nCopies(s + s, --n);
}

Answer 3

    String text = "Hello World";
    System.out.println( text.replaceAll( "[A-Za-z0-9]", "*" ) );

output : ***** *****

Answer 4

There may be other faster/better ways to do it, but you could just use a string buffer and a for-loop:

public String stringToAsterisk(String input) {
    if (input == null) return "";

    StringBuffer sb = new StringBuffer();
    for (int x = 0; x < input.length(); x++) {
        sb.append("*");
    }
    return sb.toString();
}

If your application is single threaded, you can use StringBuilder instead, but it's not thread safe.

I am not sure if this might be any faster:

public String stringToAsterisk(String input) {
    if (input == null) return "";

    int length = input.length();
    char[] chars = new char[length];
    while (length > 0) chars[--length] = "*";
    return new String(chars);
}

Answer 5

Java 11 and later

str = "*".repeat(str.length());

Note: This replaces newlines \n with *. If you want to preserve \n, see solution below.

Java 10 and earlier

str = str.replaceAll(".", "*");

This preserves newlines.

To replace newlines with * as well in Java 10 and earlier, you can use:

str = str.replaceAll("(?s).", "*");

The (?s) doesn't match anything but activates DOTALL mode which makes . also match \n.

Answer 6

Without any external library and without your own loop, you can do:

String input = "Hello";
char[] ca = new char[input.length()];
Arrays.fill(ca, '*');
String output = new String(ca);

BTW, both Arrays.fill() and String(char []) are really fast.