Java – Why use arg type `class Object` instead of `Comparable[]`

interfacesjavaobject-oriented

In the code below, I believe it would look more appropriate to make the method argument be of type Comparable[] instead of Object[].

The first reason it would be more appropriate is that one can be safe from runtime exceptions because every passed object has to implement interface Comparable. Second, every object in Java inherits from class Object so one can typecast any object as (Object) to use the methods of class Object.

package java.util; 
public class Arrays {
.........
public static void sort(Object[] a) {
        Object[] aux = (Object[])a.clone();
        mergeSort(aux, a, 0, a.length, 0);
    }
.........
}

Is there a reason why type Object[] is preferred over type Comparable[] as an argument type?

Best Answer

If you look at the mergeSort code you will see that it casts each member of array a to Comparable. If any object in the array is not a Comparable, a ClassCastException will be generated unless you have created a Comparator which knows how compare your objects and passed that to the sort method. Example of the two different techniques (comparable v comparator) here.

The Java developers decided that you should not be forced to add a Comparable interface to objects just because you want to store them in an array and may choose to sort them at some point. This gives you greater flexibility (and less cruft on your classes) at the minor risk of runtime failures rather than compile-time type checking. The flexibility is a win, in my opinion.

Related Solutions

Java Syntax – Checking ‘instanceof’ in a Switch Statement

I fully realize that I am 2 years late to the party :-) but probably few clarifications will help other devs.

What the other answers missed to address was the mentioning of the "Kryonet" which adds additional twist to the problem: usually when kryonet is used it means that

a) the client is an android app and

b) the client is some game

If both conditions are met IMHO using the instanceof if-else sequence is the way to go.

The usual way of structuring client/server app in our case android game + "pure" java server is to use (at least) 3 projects:

"common" - which contains code common for the server and the client, most importantly all the message classes (which usually implement some tag/marker interface as GameMessage).
server - contains the server and includes the common project. Usually all messages are simply registered in Kryonet, i.e. no pooling is used (because on server GC is not considered a problem)
android client - contains the android app (game) and includes the common project. Because in java games creating objects during the game may trigger the GC it is considered the biggest mortal sin (which leads straight to the hell of missed frames) it is considered good practice¹ to use object pooling for the game messages (this requires little customization on the Kryonet/kryo in order to use object pools). All messages are (must be) registered in kryonet in the same order as on the server in order things to work.

First, lets look at the alternatives and see why they are not fit for the structure/implementation described above:

1. They say 'Use polymorphism and method overriding'

Take a look here for an example of the proposed solution (accepted answer by jmg). It looks very elegant but there there are at least 2 big problems:

You can't just use parameterless overrided method like do() because in real implementation you will some need external data to act upon. For example when receiving PlayerAbandonedGame message on the server you will need at least the particular game object as parameter in order to call removePlayer(Player pl) method. NewGameState message will require reference to the particular GameState variable that will be updated and so on. One very ugly solution is overrided method to have single parameter like TheWholeThing which contains all the objects that may be needed. It is needless to say that this not only violates the principle of encapsulation but also "spreads" the game logic across multiple message classes.
Remember that we have the common project and all the messages are defined there. Even if server-to-client messages are completely distinct set to client-to-server messages it will be simply stupid to put server or client logic in the common project.

Game message objects are just data containers. It is up to the server or the client to handle that data in a particular way (or even ignore it as often happens with out of order UDP messages).

2. They say 'Use reflection'

Our context is 'a) the client is an android app'. The reaction of every android developer hearing Use reflection should be to stand up, clear his/hers throat and with his/hers best british accent to say: 'Are you completely mAd?!' :-)

Reflection is very slow on older Android versions, it is slow even now (5.0). Also IMO using reflection is the ugliest possible solution not just for this particular problem. It should be avoided up in the stack, i.e. in app dev.

Way to go

Back to the original question: if one really insist on using switch the solution is to have base class like:

public abstract class GameMessage {
    byte mCode; // not private because of kryo

    public GameMessage(byte code) {
        super();
        mCode = code;
    }

    public byte getCode() {
        return mCode;
    }
}

concrete message will look like:

public class PlayerAbandonedGame extends GameMessage {
    // some fields

    public PlayerAbandonedGame() {
        super((byte) 42);
    }
}

and then message handler will look like:

public synchronized void onMessageReceived(GameMessage msg) {
    switch(msg.getCode()) {
        case 42:
            // do something
            break;
        default:
            mLogger.error("Unknown message code {}", msg.getCode());
            // may be even throw an exception?
            break;
    }

    // ...
}

Of course it is advisable to use enum values instead of hardcoded numbers...

Potential drawback is (as mentioned in other answers) if you create new message class and forget to add it to some of the switch statements. The compiler cannot catch this problem and you will hit it only runtime (in the default case).

Real (small) drawback is that code field is send each time and adds to the traffic. In reality this adds about 1-10% to the size of the messages (if you use byte for the code field).

How I do it

I personally use if-else sequence with instanceof test like:

public synchronized void onMessageReceived(GameMessage msg) {
    if (msg instanceof MyLetterAttemptResult) {
        handleMyLetterAttemptResult((MyLetterAttemptResult) msg);
        mPools.release(msg);
    } else if (msg instanceof InAutoMatchQueue) {
        handleInAutoMatchQueue((InAutoMatchQueue) msg);
    } else if (msg instanceof AutoMatchRetry) {
        handleAutomatchRetry();
    // more else-ifs
    } else {
        mLogger.error("Unknown message {}", msg.getClass().getSimpleName());
        // throw exception may be?
    }
}

Isn't that "code smell"? You bet it is, but after trying all other alternatives it seems to me that it is best. To reduce the smelliness it is better to try to keep code in the ifs short like above. Just call handle* method and release message back to the pool (if it is pooled).

Isn't that slow? It is a bit slower than the switch(msg.getCode()) but not that much. Here is some benchmarking (answer by Dan Mihai Ile). Advantage is that you don't have the code field send each time over the network. Keep the most frequently used messages at the top so they are found first.

Isn't that error prone? There is a risk if you create new message class and forget to add corresponding else if. The thing is that you have just two separate message handling methods - one on the server and one on the client. Both are the single most important code pieces repectively on the server and the client. You will simply have to not mess it up. Just add the new message class immediately to the unit test that tests onMessageReceived().

This is not OOP! I have a better solution! I will be very happy to discuss it and use it :-). btw, I wonder if @Cameron (the author of this question) found better solution. After all this question is 2 years old...

¹ For me avoiding the GC is more like "utter crap" than "best practice". As Eonil commented in 1: I regard avoiding GC on GC based language is a kind of insane trial.

Java Design Patterns – Forcing ‘Program to an Interface’ Without Using Interfaces

I don't see any way to "lock down the interface to a class" in your code, but I do think you need to consider two things:

Your clients would still need to re-compile their code, even though the client source code doesn't have to change: invoking a method on an interface uses a different bytecode operation then invoking a method on an instance.
What would prevent your coworkers from creating a class in the trusted package and invoking the constructor there?

Best Answer

Related Solutions

Java Syntax – Checking ‘instanceof’ in a Switch Statement

Java Design Patterns – Forcing ‘Program to an Interface’ Without Using Interfaces

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