How to Generate Random Strings Using Stream - Java

It is sometimes useful to create random Strings, maybe as Session-ID for a web-service or an initial password after registration for an application. This can be easily achieved using Streams.

First, we need to initialize a random number generator. To enhance security for the generated Strings, it is a good idea to use SecureRandom.

private static final SecureRandom rng = new SecureRandom(SecureRandom.generateSeed(20)); 
//20 Bytes as a seed is rather arbitrary, it is the number used in the JavaDoc example

When creating random Strings, we usually want them to use only certain characters (e.g. only letters and digits). Therefore we can create a method returning a boolean which can later be used to filter the Stream.

//returns true for all chars in 0-9, a-z and A-Z
boolean useThisCharacter(char c){
    //check for range to avoid using all unicode Letter (e.g. some chinese symbols)
    return c >= '0' && c <= 'z' && Character.isLetterOrDigit(c);
}

Next we can utilize the RNG to generate a random String of specific length containing the charset which pass our useThisCharacter check.

public String generateRandomString(long length){
    //Since there is no native CharStream, we use an IntStream instead
    //and convert it to a Stream<Character> using mapToObj.
    //We need to specify the boundaries for the int values to ensure they can safely be cast to char
    Stream<Character> randomCharStream = rng.ints(Character.MIN_CODE_POINT, Character.MAX_CODE_POINT).mapToObj(i -> (char)i).filter(c -> this::useThisCharacter).limit(length);

    //now we can use this Stream to build a String utilizing the collect method.
    String randomString = randomCharStream.collect(StringBuilder::new, StringBuilder::append, StringBuilder::append).toString();
    return randomString;
}

Notes

• Creating a SecureRandom is quite expensive, so it is best practice to only do this once and call one of its setSeed() methods from time to time to reseed it.