Null-free "maps": Is a callback solution slower than tryGet()?

In comments to "How to implement List, Set, and Map in null free design?", Steven Sudit and I got into a discussion about using a callback, with handlers for "found" and "not found" situations, vs. a tryGet() method, taking an out parameter and returning a boolean indicating whether the out parameter had been populated. Steven maintained that the callback approach was more complex and almost certain to be slower; I maintained that the complexity was no greater and the performance at worst the same.

But code speaks louder than words, so I thought I'd implement both and see what I got. The original question was fairly theoretical with regard to language ("And for argument sake, let's say this language don't even have null") -- I've used Java here because that's what I've got handy. Java doesn't have out parameters, but it doesn't have first-class functions either, so style-wise, it should suck equally for both approaches.

(Digression: As far as complexity goes: I like the callback design because it inherently forces the user of the API to handle both cases, whereas the tryGet() design requires callers to perform their own boilerplate conditional check, which they could forget or get wrong. But having now implemented both, I can see why the tryGet() design looks simpler, at least in the short term.)

First, the callback example:

class CallbackMap<K, V> {
    private final Map<K, V> backingMap;

    public CallbackMap(Map<K, V> backingMap) {
        this.backingMap = backingMap;
    }

    void lookup(K key, Callback<K, V> handler) {
        V val = backingMap.get(key);
        if (val == null) {
            handler.handleMissing(key);
        } else {
            handler.handleFound(key, val);
        }
    }
}

interface Callback<K, V> {
    void handleFound(K key, V value);
    void handleMissing(K key);
}

class CallbackExample {
    private final Map<String, String> map;
    private final List<String> found;
    private final List<String> missing;
    private Callback<String, String> handler;

    public CallbackExample(Map<String, String> map) {
        this.map = map;
        found = new ArrayList<String>(map.size());
        missing = new ArrayList<String>(map.size());
        handler = new Callback<String, String>() {
            public void handleFound(String key, String value) {
                found.add(key + ": " + value);
            }

            public void handleMissing(String key) {
                missing.add(key);
            }
        };
    }

    void test() {
        CallbackMap<String, String> cbMap = new CallbackMap<String, String>(map);
        for (int i = 0, count = map.size(); i < count; i++) {
            String key = "key" + i;
            cbMap.lookup(key, handler);
        }
        System.out.println(found.size() + " found");
        System.out.println(missing.size() + " missing");
    }
}

Now, the tryGet() example -- as best I understand the pattern (and 开发者_StackOverflow中文版I might well be wrong):

class TryGetMap<K, V> {
    private final Map<K, V> backingMap;

    public TryGetMap(Map<K, V> backingMap) {
        this.backingMap = backingMap;
    }

    boolean tryGet(K key, OutParameter<V> valueParam) {
        V val = backingMap.get(key);
        if (val == null) {
            return false;
        }
        valueParam.value = val;
        return true;
    }
}

class OutParameter<V> {
    V value;
}

class TryGetExample {
    private final Map<String, String> map;
    private final List<String> found;
    private final List<String> missing;

    private final OutParameter<String> out = new OutParameter<String>();

    public TryGetExample(Map<String, String> map) {
        this.map = map;
        found = new ArrayList<String>(map.size());
        missing = new ArrayList<String>(map.size());
    }

    void test() {
        TryGetMap<String, String> tgMap = new TryGetMap<String, String>(map);

        for (int i = 0, count = map.size(); i < count; i++) {
            String key = "key" + i;
            if (tgMap.tryGet(key, out)) {
                found.add(key + ": " + out.value);
            } else {
                missing.add(key);
            }
        }
        System.out.println(found.size() + " found");
        System.out.println(missing.size() + " missing");
    }
}

And finally, the performance test code:

public static void main(String[] args) {
    int size = 200000;
    Map<String, String> map = new HashMap<String, String>();
    for (int i = 0; i < size; i++) {
        String val = (i % 5 == 0) ? null : "value" + i;
        map.put("key" + i, val);
    }

    long totalCallback = 0;
    long totalTryGet = 0;

    int iterations = 20;
    for (int i = 0; i < iterations; i++) {
        {
            TryGetExample tryGet = new TryGetExample(map);
            long tryGetStart = System.currentTimeMillis();
            tryGet.test();
            totalTryGet += (System.currentTimeMillis() - tryGetStart);
        }
        System.gc();

        {
            CallbackExample callback = new CallbackExample(map);
            long callbackStart = System.currentTimeMillis();
            callback.test();
            totalCallback += (System.currentTimeMillis() - callbackStart);
        }
        System.gc();
    }

    System.out.println("Avg. callback: " + (totalCallback / iterations));
    System.out.println("Avg. tryGet(): " + (totalTryGet / iterations));
}

On my first attempt, I got 50% worse performance for callback than for tryGet(), which really surprised me. But, on a hunch, I added some garbage collection, and the performance penalty vanished.

This fits with my instinct, which is that we're basically talking about taking the same number of method calls, conditional checks, etc. and rearranging them. But then, I wrote the code, so I might well have written a suboptimal or subconsicously penalized tryGet() implementation. Thoughts?

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Updated: Per comment from Michael Aaron Safyan, fixed TryGetExample to reuse OutParameter.

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I would say that neither design makes sense in practice, regardless of the performance. I would argue that both mechanisms are overly complicated and, more importantly, don't take into account actual usage.

Actual Usage
If a user looks up a value in a map and it isn't there, most likely the user wants one of the following:

• To insert some value with that key into the map
• To get back some default value
• To be informed that the value isn't there

Thus I would argue that a better, null-free API would be:

• has(key) which indicates if the key is present (if one only wishes to check for the key's existence).
• get(key) which reports the value if the key is present; otherwise, throws NoSuchElementException.
• get(key,defaultval) which reports the value for the key, or defaultval if the key isn't present.
• setdefault(key,defaultval) which inserts (key,defaultval) if key isn't present, and returns the value associated with key (which is defaultval if there is no previous mapping, otherwise prev mapping).

The only way to get back null is if you explicity ask for it as in get(key,null). This API is incredibly simple, and yet is able to handle the most common map-related tasks (in most use cases that I have encountered).

I should also add that in Java, has() would be called containsKey() while setdefault() would be called putIfAbsent(). Because get() signals an object's absence via a NoSuchElementException, it is then possible to associate a key with null and treat it as a legitimate association.... if get() returns null, it means the key has been associated with the value null, not that the key is absent (although you can define your API to disallow a value of null if you so choose, in which case you would throw an IllegalArgumentException from the functions that are used to add associations if the value given is null). Another advantage to this API, is that setdefault() only needs to perform the lookup procedure once instead of twice, which would be the case if you used if( ! dict.has(key) ){ dict.set(key,val); }. Another advantage is that you do not surprise developers who write something like dict.get(key).doSomething() who assume that get() will always return a non-null object (because they have never inserted a null value into the dictionary)... instead, they get a NoSuchElementException if there is no value for that key, which is more consistent with the rest of the error checking in Java and which is also a much easier to understand and debug than NullPointerException.

Answer To Question
To answer original question, yes, you are unfairly penalizing the tryGet version.... in your callback based mechanism you construct the callback object only once and use it in all subsequent calls; whereas in your tryGet example, you construct your out parameter object in every single iteration. Try taking the line:

OutParameter out = new OutParameter();

Take the line above out of the for-loop and see if that improves the performance of the tryGet example. In other words, place the line above the for-loop, and re-use the out parameter in each iteration.

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David, thanks for taking the time to write this up. I'm a C# programmer, so my Java skills are a bit vague these days. Because of this, I decided to port your code over and test it myself. I found some interesting differences and similarities, which are pretty much worth the price of admission as far as I'm concerned. Among the major differences are:

1. I didn't have to implement TryGet because it's built into Dictionary.
2. In order to use the native TryGet, instead of inserting nulls to simulate misses, I simply omitted those values. This still means that v = map[k] would have set v to null, so I think it's a proper porting. In hindsight, I could have inserted the nulls and changed (_map.TryGetValue(key, out value)) to (_map.TryGetValue(key, out value) && value != null)), but I'm glad I didn't.
3. I want to be exceedingly fair. So, to keep the code as compact and maintainable as possible, I used lambda calculus notation, which let me define the callbacks painlessly. This hides much of the complexity of setting up anonymous delegates, and allows me to use closures seamlessly. Ironically, the implementation of Lookup uses TryGet internally.
4. Instead of declaring a new type of Dictionary, I used an extension method to graft Lookup onto the standard dictionary, much simplifying the code.

With apologies for the less-than-professional quality of the code, here it is:

using System;
using System.Collections.Generic;
using System.Linq;

namespace ConsoleApplication1
{
    static class CallbackDictionary
    {
        public static void Lookup<K, V>(this Dictionary<K, V> map, K key, Action<K, V> found, Action<K> missed)
        {
            V v;
            if (map.TryGetValue(key, out v))
                found(key, v);
            else
                missed(key);
        }
    }

    class TryGetExample
    {
        private Dictionary<string, string> _map;
        private List<string> _found;
        private List<string> _missing;

        public TryGetExample(Dictionary<string, string> map)
        {
            _map = map;
            _found = new List<string>(_map.Count);
            _missing = new List<string>(_map.Count);
        }

        public void TestTryGet()
        {
            for (int i = 0; i < _map.Count; i++)
            {
                string key = "key" + i;
                string value;
                if (_map.TryGetValue(key, out value))
                    _found.Add(key + ": " + value);
                else
                    _missing.Add(key);
            }

            Console.WriteLine(_found.Count() + " found");
            Console.WriteLine(_missing.Count() + " missing");
        }

        public void TestCallback()
        {
            for (int i = 0; i < _map.Count; i++)
                _map.Lookup("key" + i, (k, v) => _found.Add(k + ": " + v), k => _missing.Add(k));

            Console.WriteLine(_found.Count() + " found");
            Console.WriteLine(_missing.Count() + " missing");
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            int size = 2000000;
            var map = new Dictionary<string, string>(size);
            for (int i = 0; i < size; i++)
                if (i % 5 != 0)
                    map.Add("key" + i, "value" + i);

            long totalCallback = 0;
            long totalTryGet = 0;

            int iterations = 20;
            TryGetExample tryGet;
            for (int i = 0; i < iterations; i++)
            {
                tryGet = new TryGetExample(map);
                long tryGetStart = DateTime.UtcNow.Ticks;
                tryGet.TestTryGet();
                totalTryGet += (DateTime.UtcNow.Ticks - tryGetStart);
                GC.Collect();

                tryGet = new TryGetExample(map);
                long callbackStart = DateTime.UtcNow.Ticks;
                tryGet.TestCallback();
                totalCallback += (DateTime.UtcNow.Ticks - callbackStart);
                GC.Collect();
            }

            Console.WriteLine("Avg. callback: " + (totalCallback / iterations));
            Console.WriteLine("Avg. tryGet(): " + (totalTryGet / iterations));
        }
    }
}

My performance expectations, as I said in the article that inspired this one, would be that neither one is much faster or slower than the other. After all, most of the work is in the searching and adding, not in the simple logic that structures it. In fact, it varied a bit among runs, but I was unable to detect any consistent advantage.

Part of the problem is that I used a low-precision timer and the test was short, so I increased the count by 10x to 2000000 and that helped. Now callbacks are about 3% slower, which I do not consider significant. On my fairly slow machine, callbacks took 17773437 while tryget took 17234375.

Now, as for code complexity, it's a bit unfair because TryGet is native, so let's just ignore the fact that I had to add a callback interface. At the calling spot, lambda notation did a great job of hiding the complexity. If anything, it's actually shorter than the if/then/else used in the TryGet version, although I suppose I could have used a ternary operator to make it equally compact.

On the whole, I found the C# to be more elegant, and only some of that is due to my bias as a C# programmer. Mainly, I didn't have to define and implement interfaces, which cut down on the plumbing overhead. I also used pretty standard .NET conventions, which seem to be a bit more streamlined than the sort of style favored in Java.