Java 8 convert integers to String with Lambda

Question

I'm trying to create a function which returns a random string of alphabetic characters, but I'm trying to do it using lambdas, my function looks like:

public static String nextString() {
  return IntStream.range(0, 10).map(i -> getRandomChar()).XXX
}

private static Character getRandomChar() {
  //returns a random Character object
}

I don't know what to set on XXX.

I fixed it in a similar way by doing:

public static String nextString() {
    StringBuilder randomString = new StringBuilder();
    IntStream.range(0, stringLenght).forEach(i -> randomString.append(getRandomChar()));
    return randomString.toString();
}

But I'd prefer not to create a new StringBuilder each time the method is used and keep states and variables for a simple method.

If I try return IntStream.range(0, strLenght).map(i -> getRandomChar()).collect(Collectors.joining(""));got compilation problems, I tried that before, because of this error I published my question — Eduardo
– Eduardo, Commented Apr 4, 2017 at 18:29

Oleksandr Bondarchuk · Accepted Answer · 2017-04-04 18:29:38Z

4

Something like this:

public static String nextString() {
    return IntStream.range(0, 10).boxed().map(i -> getRandomChar(i)).collect(Collectors.joining());
}

private static String getRandomChar(int i) {
    return String.valueOf((char)i);
}

answered Apr 4, 2017 at 18:29

Oleksandr Bondarchuk

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2 Comments

Eduardo Over a year ago

I don't get why this code works only if getRandomChar returns a String but not when returns Character

Oleksandr Bondarchuk Over a year ago

Because Collectors.joining() expected Stream<String>. You can create own Collector like this: Collector<Character, StringBuilder, String> collector = Collector.of(StringBuilder::new, StringBuilder::append, (r1, r2) -> { r1.append(r2); return r1; }, StringBuilder::toString);

Holger · Accepted Answer · 2017-04-07 11:44:22Z

It’s not clear what getRandomChar() does that it is worth building another Stream operation around it. It’s much simpler to stream over random values in the first place.

E.g., the following code generates random strings consisting of upper case letters in the A-Z range:

public static String nextString() {
    return ThreadLocalRandom.current().ints(stringLength, 'A', 'Z'+1)
        .collect(StringBuilder::new, StringBuilder::appendCodePoint, StringBuilder::append)
        .toString();
}

If the intended characters do not form a consecutive range, you can use a string constant naming the allowed characters, e.g.

public static String nextString() {
    String allowed = "XO+";
    return ThreadLocalRandom.current().ints(stringLength, 0, allowed.length())
        .map(allowed::charAt)
        .collect(StringBuilder::new, StringBuilder::appendCodePoint, StringBuilder::append)
        .toString();
}

Granted, this collector doesn’t hide the StringBuilder the same way, Collectors.joining() does, but on the other hand, this operation has no boxing overhead and doesn’t create temporary strings.

You can use a similar operation to built the character pool String:

static final String ASCII_LETTERS_AND_NUMBERS =
        IntStream.concat(IntStream.concat(
            IntStream.rangeClosed('A', 'Z'), IntStream.rangeClosed('a', 'z')),
            IntStream.rangeClosed('0', '9'))
        .collect(StringBuilder::new, StringBuilder::appendCodePoint, StringBuilder::append)
        .toString();
public static String nextString() {
    return ThreadLocalRandom.current()
        .ints(stringLength, 0, ASCII_LETTERS_AND_NUMBERS.length())
        .map(ASCII_LETTERS_AND_NUMBERS::charAt)
        .collect(StringBuilder::new, StringBuilder::appendCodePoint, StringBuilder::append)
        .toString();
}

or, to challenge the reader of the random string:

static final String ALL_LETTERS_AND_NUMBERS =
    IntStream.rangeClosed(32, Character.MAX_VALUE)
        .filter(Character::isLetterOrDigit)
        .collect(StringBuilder::new, StringBuilder::appendCodePoint, StringBuilder::append)
        .toString();
public static String nextString() {
    return ThreadLocalRandom.current()
        .ints(stringLength, 0, ALL_LETTERS_AND_NUMBERS.length())
        .map(ALL_LETTERS_AND_NUMBERS::charAt)
        .collect(StringBuilder::new, StringBuilder::appendCodePoint, StringBuilder::append)
        .toString();
}

I absolutely love your approach in answering. And this ThreadLocalRandom.current().ints(stringLength, 'A', 'Z'+1) you have no idea how many times I needed that until now. big thx
Well, I know, it can be easily adapted to pick from a List or array as well.

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