Java collections convert a string to a list of characters

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

        

Answer 1

The most straightforward way is to use a for loop to add elements to a new List:

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

for (char c : abc.toCharArray()) {
  charList.add(c);
}

Similarly, for a Set:

String abc = "abc";
Set<Character> charSet = new HashSet<Character>();

for (char c : abc.toCharArray()) {
  charSet.add(c);
}

Answer 2

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).

Answer 3

List<String> result = Arrays.asList("abc".split(""));

Answer 4

IntStream can be used to access each character and add them to the list.

String str = "abc";
List<Character> charList = new ArrayList<>();
IntStream.range(0,str.length()).forEach(i -> charList.add(str.charAt(i)));

Answer 5

You can do this without boxing if you use Eclipse Collections:

CharAdapter abc = Strings.asChars("abc");
CharList list = abc.toList();
CharSet set = abc.toSet();
CharBag bag = abc.toBag();

Because CharAdapter is an ImmutableCharList, calling collect on it will return an ImmutableList.

ImmutableList<Character> immutableList = abc.collect(Character::valueOf);

If you want to return a boxed List, Set or Bag of Character, the following will work:

LazyIterable<Character> lazyIterable = abc.asLazy().collect(Character::valueOf);
List<Character> list = lazyIterable.toList();
Set<Character> set = lazyIterable.toSet();
Bag<Character> set = lazyIterable.toBag();

Note: I am a committer for Eclipse Collections.

Answer 6

Using Java 8 - Stream Funtion:

Converting A String into Character List:

ArrayList<Character> characterList =  givenStringVariable
                                                         .chars()
                                                         .mapToObj(c-> (char)c)
                                                         .collect(collectors.toList());

Converting A Character List into String:

 String givenStringVariable =  characterList
                                            .stream()
                                            .map(String::valueOf)
                                            .collect(Collectors.joining())