Java: how to get Iterator from String [duplicate]

Answer 1

Not sure if there is a more direct way but you could do something like;

Arrays.asList(string.toCharArray()).iterator();

Scratch that; Arrays.asList doesn't do what I seem to remember it doing.

Edit 2: Seems like it last worked this way in 1.4

Answer 2

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 3

With java 8 or newer you can use the stream facility. With the chars() method you can access an IntStream. The IntStream supports the method iterator() that returns an OfInt iterator. OfInt implements Iterator<Integer>.

String str = "foobar";
OfInt ofit = str.chars().iterator();
Iterator<Integer> it = ofit;

It is not a perfect answer, since you asked for Iterator<Character>.

Btw: With str.codePoints() you can also access a code point IntStream.

Answer 4

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 5

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;
    }

Answer 6

This feels dirty, but you could use Scanner with empty string delimiter:

Scanner scanner = new java.util.Scanner(myInput).useDelimiter("");

Scanner implements Iterator, so scanner is now an Iterator of length-1 strings, which is close.

To continue with the (very?) dirty, in Java 8 you can then do this to succinctly iterate by chars:

for (String s: (Iterable<String>)() -> scanner) { 
  char c = s.charAt(0);
  System.out.println(c); 
}

For details on why () -> scanner works (and why it may be dangerous, though not in this use case), see Explain how this lambda can be assigned to an Iterable.