Task: Range expansion
Go: package main

import (
    "fmt"
    "strconv"
    "strings"
)

const input = "-6,-3--1,3-5,7-11,14,15,17-20"

func main() {
    fmt.Println("range:", input)
    var r []int
    var last int
    for _, part := range strings.Split(input, ",") {
        if i := strings.Index(part[1:], "-"); i == -1 {
            n, err := strconv.Atoi(part)
            if err != nil {
                fmt.Println(err)
                return
            }
            if len(r) > 0 {
                if last == n {
                    fmt.Println("duplicate value:", n)
                    return
                } else if last > n {
                    fmt.Println("values not ordered:", last, ">", n)
                    return
                }
            }
            r = append(r, n)
            last = n
        } else {
            n1, err := strconv.Atoi(part[:i+1])
            if err != nil {
                fmt.Println(err)
                return
            }
            n2, err := strconv.Atoi(part[i+2:])
            if err != nil {
                fmt.Println(err)
                return
            }
            if n2 < n1+2 {
                fmt.Println("invalid range:", part)
                return
            }
            if len(r) > 0 {
                if last == n1 {
                    fmt.Println("duplicate value:", n1)
                    return
                } else if last > n1 {
                    fmt.Println("values not ordered:", last, ">", n1)
                    return
                }
            }
            for i = n1; i <= n2; i++ {
                r = append(r, i)
            }
            last = n2
        }
    }
    fmt.Println("expanded:", r)
}

Java: import java.util.*;

class RangeExpander implements Iterator<Integer>, Iterable<Integer> {

    private static final Pattern TOKEN_PATTERN = Pattern.compile("([+-]?\\d+)-([+-]?\\d+)");

    private final Iterator<String> tokensIterator;

    private boolean inRange;
    private int upperRangeEndpoint;
    private int nextRangeValue;

    public RangeExpander(String range) {
        String[] tokens = range.split("\\s*,\\s*");
        this.tokensIterator = Arrays.asList(tokens).iterator();
    }

    @Override
    public boolean hasNext() {
        return hasNextRangeValue() || this.tokensIterator.hasNext();
    }

    private boolean hasNextRangeValue() {
        return this.inRange && this.nextRangeValue <= this.upperRangeEndpoint;
    }

    @Override
    public Integer next() {
        if (!hasNext()) {
            throw new NoSuchElementException();
        }

        if (hasNextRangeValue()) {
            return this.nextRangeValue++;
        }

        String token = this.tokensIterator.next();

        Matcher matcher = TOKEN_PATTERN.matcher(token);
        if (matcher.find()) {
            this.inRange = true;
            this.upperRangeEndpoint = Integer.valueOf(matcher.group(2));
            this.nextRangeValue = Integer.valueOf(matcher.group(1));
            return this.nextRangeValue++;
        }

        this.inRange = false;
        return Integer.valueOf(token);
    }

    @Override
    public Iterator<Integer> iterator() {
        return this;
    }

}

class RangeExpanderTest {
    public static void main(String[] args) {
        RangeExpander re = new RangeExpander("-6,-3--1,3-5,7-11,14,15,17-20");
        for (int i : re) {
            System.out.print(i + " ");
        }
    }
}