Permuter

public final class Permuter
{
    private final String characters;

    private List<Character> current;

    public Permuter()
    {
        this("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789");
    }

    public Permuter(final String characters)
    {
        this.characters = characters;

        this.current = new ArrayList<>();
        this.current.add(characters.charAt(0));
    }

    public String next()
    {
        final String result = this.build();
        this.advance();
        return result;
    }

    private String build()
    {
        String result = "";
        for (final char c : this.current)
            result += c;
        return result;
    }

    private void advance()
    {
        this.advance(0);
    }

    private void advance(final int index)
    {
        if (index >= this.current.size())
        {
            this.current.add(this.characters.charAt(0));
        }
        else
        {
            final char current = this.current.get(index);
            final int indexOfCurrent = this.characters.indexOf(current);
            final int indexOfNext = indexOfCurrent + 1;

            if (indexOfNext >= this.characters.length())
            {
                this.current.set(index, this.characters.charAt(0));
                this.advance(index + 1);
            }
            else
            {
                this.current.set(index, this.characters.charAt(indexOfNext));
            }
        }
    }
}