PalindromeGenerator.java

package math.recreational.generators;

import java.util.Iterator;

public class PalindromeGenerator implements Iterator<Long> {

    private Long current = 0l;

    @Override
    public boolean hasNext() {
        return current >= 0;
    }

    @Override
    public Long next() {
        if (!hasNext()) {
            throw new ArithmeticException(current.getClass().getSimpleName()
                    + " is not large enough to represent the next palindrome.");
        }
        Long tempValue = current;
        calculateNextValue();
        return tempValue;
    }

    private void calculateNextValue() {
        try {
            String s = String.valueOf(current);
            int length = s.length();
            int half = length / 2;
            if (isAllNines(s)) {
                current += 2l;
            } else if (length % 2 != 0) {
                String firstHalf = s.substring(0, half)
                        + s.substring(half, half + 1);
                String incrementedHalf = String
                        .valueOf(Long.valueOf(firstHalf) + 1l);
                String reversedHalf = reverse(incrementedHalf
                        .substring(0, half));
                current = Long.valueOf(incrementedHalf + reversedHalf);
            } else {
                String firstHalf = String.valueOf(Long.valueOf(s.substring(0,
                        half)) + 1);
                current = Long.valueOf(firstHalf + reverse(firstHalf));
            }
        } catch (NumberFormatException x) {
            current = Long.MIN_VALUE;
        }
    }

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

    private boolean isAllNines(String s) {
        return s.replace("9", "").isEmpty();
    }

    private String reverse(String firstHalf) {
        return new StringBuffer(firstHalf).reverse().toString();
    }

}