Thursday, August 25, 2011

Typesafe List in Java 5+ part two...

Looks like John's comment on yesterday's post had us both up half the night working on the same thing. I'm posting my version here because it has a parameter that lets the caller determine the trade-off between speed and safety. When debugging, you can use Check.ALL (or leave it null). In performance-critical code, you can later set it to NONE. Check.FIRST is for situations where you just a sanity check - it's fast and better than nothing.

I've also included some test code and a routine that takes advantage of the String.valueOf() methods to coerce each element of the original list to a String, if necessary, much the way that many dynamically typed languages do. I wonder if it would be worthwhile to further optimize this method for long lists by dividing the original list into runs of items that do not need casting and copying those runs with stringList.addAll(list.subList(fromIdx, toIdx)) instead of copying one item at a time?

John's solution uses B.class.isAssignableFrom(a.getClass()) while mine uses a instanceof B. I think either works in this case because a List can only hold objects, not primitives. If we were dealing with primitives, John's solution would handle more cases than mine. Not sure if there are performance considerations, but I doubt they would be significant.

import java.util.ArrayList;
import java.util.List;

public class TypeSafe {

    public enum Check {

    public static <T> List<T> typeSafeList(List list,
                                           Class<T> clazz,
                                           Check safety) {
        if (clazz == null) {
            throw new IllegalArgumentException("typeSafeList() requires a non-null class parameter");
        // Should we perform any checks?
        if (safety != Check.NONE) {
            if ( (list != null) && (list.size() > 0) ) {
                for (Object item : list) {
                    if ( (item != null) &&
                         !(item instanceof String) ) {
                        throw new ClassCastException(
                                "List contained a(n) " +
                                item.getClass().getCanonicalName() +
                                " which is not a(n) " +
                    // Should we stop on first success?
                    if (safety == Check.FIRST) {
                    // Default (Check.ALL) checks every item in the list.
        } // end if perform any checks
        return (List<T>) list;
    } // end typeSafeList()

    public static List<String> coerceToStringList(List list) {
        if (list == null) {
            return null;
        try {
            // Return the old list if it's already safe
            return typeSafeList(list, String.class, Check.ALL);
        } catch (ClassCastException cce) {
            // Old list is not safe.  Make new one.
            List<String> stringList = new ArrayList<String>();

            for (Object item : list) {
                if (item == null) {
                } else if (item instanceof String) {
                    stringList.add((String) item);
                } else {
                    // If this throws a classCastException, so be it.
            return stringList;
    } // end typeSafeList()

    private static List makeTestList() {
        List l = new ArrayList();
        l.add(new Integer(3));
        return l;
    } // end makeTestList()

    public static void main(String[] args) {
        List unsafeList = makeTestList();
        List<String> stringList = coerceToStringList(unsafeList);

        System.out.println("Coerced strings:");
        for (String s : stringList) {

        List<String> safeList = typeSafeList(unsafeList,

        System.out.println("Safe-casted list:");
        for (String s : safeList) {
    } // end main()

1 comment:

John Yeary said...

We were both up very late. Nicely done. I need to look at it in more detail late tonight! ;-)