Iterating over the "letters" (codepoints, not chars) of a string.

IterateCodepoints.java

package com.emmanueloga.cracking.arrays;

import java.util.Iterator;

import com.google.common.collect.ImmutableList;
import com.google.common.collect.Lists;

class StringCodepointsIterable implements Iterable<String> {
    public class StringCodepointsIterator implements Iterator<String> {
        private int index = 0;

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

        @Override
        public boolean hasNext() {
            return index < StringCodepointsIterable.this.string.length();
        }

        @Override
        public String next() {
            int codePoint = StringCodepointsIterable.this.string.codePointAt(index);
            index += Character.charCount(codePoint);
            return new String(Character.toChars(codePoint));
        }
    }

    private final String string;

    public StringCodepointsIterable(final String string) {
        this.string = string;
    }

    @Override
    public Iterator<String> iterator() {
        return new StringCodepointsIterator();
    }
}

public class IterateCodepoints {

    public static void main(String... args) {
    	// Uses the beautiful Mathematical Capital Script C char:
    	// http://unicode-table.com/en/1D49E/
        String unicode = new String(Character.toChars(0x1d49e)) + ":Hi";

        System.out.print("String: " + unicode + "(length " + unicode.length()
                + ", " + unicode.codePointCount(0, unicode.length())
                + " codepoints)");

        System.out.print("\ncharAt for each index in (0...str.length() - 1): ");
        for (int i = 0; i < unicode.length(); i++) {
            System.out.print(unicode.charAt(i));
            System.out.print("_");
        }

        System.out.print("\nGuava's Lists.charactersOf: ");
        ImmutableList<Character> chars = Lists.charactersOf(unicode);
        for (Character c : chars) {
            System.out.print(c);
            System.out.print("_");
        }

        System.out.print("\nCODEPOINTS: ");
        for (int i = 0; i < unicode.length();) {
            int cp = unicode.codePointAt(i);
            System.out.print(Character.toChars(cp));
            System.out.print("_");
            i += Character.isSupplementaryCodePoint(cp) ? 2 : 1;
        }

        System.out.print("\nCODEPOINTS INTSTREAM: ");
        unicode.codePoints().forEach(c -> {
            System.out.print(Character.toChars(c));
            System.out.print("_");
        });

        System.out.print("\nStringCodepointsIterable: ");
        for (String stringOfSingleCodepoing : new StringCodepointsIterable(
                unicode)) {
            System.out.print(stringOfSingleCodepoing);
            System.out.print("_");
        }

    }
}

// Output:
// String: 𝒞:Hi(length 5, 4 codepoints)
// charAt for each index in (0...str.length() - 1): ?_?_:_H_i_
// Guava's Lists.charactersOf: ?_?_:_H_i_
// CODEPOINTS: 𝒞_:_H_i_
// CODEPOINTS INTSTREAM: 𝒞_:_H_i_
// StringCodepointsIterable: 𝒞_:_H_i_

I've replaced the code in Apache POI with a new implementation - https://svn.apache.org/viewvc?view=revision&revision=1893385