Java: how to get Iterator<Character> from String

Question

Hi,

I need a Iterator<Character> from a String object. Is there any available function in Java that provides me this or do I have to code my own?

Answer 1

One option is to use Guava:

ImmutableList<Character> chars = Lists.charactersOf(someString);
UnmodifiableListIterator<Character> iter = chars.listIterator();

This produces an immutable list of characters that is backed by the given string (no copying involved).

If you end up doing this yourself, though, I would recommend not exposing the implementation class for the Iterator as a number of other examples do. I'd recommend instead making your own utility class and exposing a static factory method:

public static Iterator<Character> stringIterator(final String string) {
  // Ensure the error is found as soon as possible.
  if (string == null)
    throw new NullPointerException();

  return new Iterator<Character>() {
    private int index = 0;

    public boolean hasNext() {
      return index < string.length();
    }

    public Character next() {
      /*
       * Throw NoSuchElementException as defined by the Iterator contract,
       * not IndexOutOfBoundsException.
       */
      if (!hasNext())
        throw new NoSuchElementException();
      return string.charAt(index++);
    }

    public void remove() {
      throw new UnsupportedOperationException();
    }
  };
}

Answer 2

CharacterIterator it = new StringCharacterIterator("abcd"); 
// Iterate over the characters in the forward direction 
for (char ch=it.first(); ch != CharacterIterator.DONE; ch=it.next())
// Iterate over the characters in the backward direction 
for (char ch=it.last(); ch != CharacterIterator.DONE; ch=it.previous()) 

Answer 3

The Iterator iterate over a collection or whatever implements it. String class does nost implement this interface. So there is no direct way.

To iterate over a string you will have to first create a char array from it and then from this char array a Collection.

Answer 4

It doesn't exist, but it's trivial to implement:

class CharacterIterator implements Iterator<Character> {

    private final String str;
    private int pos = 0;

    public CharacterIterator(String str) {
        this.str = str;
    }

    public boolean hasNext() {
        return pos < str.length();
    }

    public Character next() {
        return str.charAt(pos++);
    }

    public void remove() {
        throw new UnsupportedOperationException();
    }
}

The implementation is probably as efficient as it gets.

Answer 5

Short answer: No, you have to code it.

Long answer: List and Set both have a method for obtaining an Iterator (there are a few other collection classes, but probably not what your looking for). The List and Set interfaces are a part of the Collections Framework which only allow for adding/removing/iterating Objects like Character or Integer (not primitives like char or int). There is a feature in Java 1.5 called auto-boxing that will hide this primitive to Object conversion but I don't recommend it and it won't provide what you want in this case.

An alternative would be to wrap the String in a class of your own that

implements Iterator<Character>

but that might be more work than it is worth.

Here is a code snippet for doing what you want:

String s = "";
List<Character> list = new ArrayList<Character>(s.length());
for (int i = 0; i < s.length(); i++) {
    // note that Character.valueOf() is preferred to new Character()
    // you can omit the Character.valueOf() method
    // and Java 1.5+ will auto-box the primitive into an Object
    list.add(Character.valueOf(s.charAt(i)));
}
Iterator<Character> iterator = list.iterator();

Answer 6

No direct way. Not difficult to code, though:

public static Iterator<Character> gimmeIterator(final String x) {
        Iterator<Character> it = new Iterator<Character>() {
            String str = x == null ? "" : x;
            int pos = -1; // last read
            public boolean hasNext() {  return(pos+1 <  str.length());  }
            public Character next() { pos++;  return str.charAt(pos);       }
            public void remove() {
                throw new UnsupportedOperationException("remove unsupported for this iterator");
            }
        };  
        return it;
    }