Question

I would like to convert the string containing abc to a list of characters and a hashset of characters. How can I do that in Java ?

List<Character> charList = new ArrayList<Character>("abc".toCharArray());

Answer 1

You will have to either use a loop, or create a collection wrapper like Arrays.asList which works on primitive char arrays (or directly on strings).

List<Character> list = new ArrayList<Character>();
Set<Character> unique = new HashSet<Character>();
for(char c : "abc".toCharArray()) {
    list.add(c);
    unique.add(c);
}

Here is an Arrays.asList like wrapper for strings:

public List<Character> asList(final String string) {
    return new AbstractList<Character>() {
       public int size() { return string.length(); }
       public Character get(int index) { return string.charAt(index); }
    };
}

This one is an immutable list, though. If you want a mutable list, use this with a char[]:

public List<Character> asList(final char[] string) {
    return new AbstractList<Character>() {
       public int size() { return string.length; }
       public Character get(int index) { return string[index]; }
       public Character set(int index, Character newVal) {
          char old = string[index];
          string[index] = newVal;
          return old;
       }
    };
}

Analogous to this you can implement this for the other primitive types.
Note that using this normally is not recommended, since for every access you
would do a boxing and unboxing operation.

The Guava library contains similar List wrapper methods for several primitive array classes, like Chars.asList, and a wrapper for String in Lists.charactersOf(String).