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_

http://stackoverflow.com/questions/3925130/java-how-to-get-iteratorcharacter-from-string/3925162#3925162

Also, http://stackoverflow.com/questions/196830/what-is-the-easiest-best-most-correct-way-to-iterate-through-the-characters-of-a#comment41307496_196830

@EmmanuelOga - we've been using code based on this gist in https://poi.apache.org/ (an open source project) for a while. A query has come up about whether we should be doing this without your approval. If it all possible, would you be able to Apache license this gist (by maybe editing the code snippet and prepending a license as a comment). If you object, we can look at removing the code from POI at the earliest possible juncture.

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