Use the command line to make new objects

Question

In my program, the user needs to input what type of players the game will have. The players are "human", "good" (for a good AI), "bad" (for a bad AI) and "random" (for a random AI). Each of these players have their own class that extend one abstract class called PlayerType.

My struggle is mapping a String to the object so I can A) create a new object using the String as sort of a key and B) get the related String from an object of its subclass

Ultimately, I just want the implicit String to only appear once in the code so I can change it later if needed without refactoring.

I've tried using just a plain HashMap, but that seems clunky with searching the keys via the values. Also, I'm guessing that I'll have to use the getInstance() method of Class, which is a little less clunky, which is okay if it's the only way.

Register your classes with a factory using the classes' string key as the key, and the class itself as the value. — Dave Newton, Sep 16 '15 at 16:59
probably some sort of switch statement based on the names available, and then just return a new instance of whichever matches. — CasualT, Sep 16 '15 at 17:04
@CasualT Can't do that because OP wants to use strings contained within the classes themselves as the keys. — Dave Newton, Sep 16 '15 at 17:06
well, you can still reference those classes in a switch... otherwise, would need to use reflection to scan dynamically. — CasualT, Sep 16 '15 at 17:08

EpicPandaForce · Accepted Answer · 2015-09-16T18:30:38.430

What I would do is create an enum which essentially functions as a factory for the given type.

public enum PlayerTypes {
    GOOD { 
        @Override
        protected PlayerType newPlayer() { 
            return new GoodPlayer();
        }
    }, 
    BAD {
        @Override
        protected PlayerType newPlayer() { 
            return new BadPlayer();
        }
    },
    RANDOM {
        @Override
        protected PlayerType newPlayer() { 
            return new RandomPlayer();
        }
    };

    protected abstract PlayerType newPlayer();

    public static PlayerType create(String input) {
        for(PlayerTypes player : PlayerTypes.values()) {
             if(player.name().equalsIgnoreCase(input)) {
                 return player.newPlayer();
             }
        }
        throw new IllegalArgumentException("Invalid player type [" + input + "]");
    }
)

Because then you can just call it like so:

String input = getInput();
PlayerTypes.create(input);

Of course, you'll get an IllegalArgumentException which you should probably handle by trying to get the input again.

EDIT: Apparently in this particular case, you can replace that loop with just merely

return PlayerTypes.valueOf(input).newPlayer();

And it'll do the same thing. I tend to match for additional constructor parameters in the enum, so I didn't think of using valueOf(), but it's definitely cleaner.

EDIT2: Only way to get that information back is to define an abstract method in your PlayerType class that returns the PlayerTypes enum for that given type.

public class PlayerType {
    public abstract PlayerTypes getType();
}

public class GoodPlayer extends PlayerType {
    @Override
    public PlayerTypes getType() {
        return PlayerTypes.GOOD;
    }
}

If you are willing to picky about upper/lower-casing, you just `return PlayerTypes.valueOf(input).newPlayer();`. You'll get the IllegalArgumentException for free since that's the exception that `valueOf()` throws if it can't match up the type. — azurefrog, Sep 16 '15 at 17:10
@azurefrog valid point, I edited it in now that I'm not on a pesky phone soft keyboard, thanks for the info — EpicPandaForce, Sep 16 '15 at 17:32
This answers my problem to A). Could it be possible to have an instance of, say, a `GoodPlayer` and then grab name it's tied to? (for my problem B) I guess the enum has to be bidirectional — Kyefer, Sep 16 '15 at 18:21

score 1 · Answer 2 · answered Sep 16 '15 at 17:19

1

I like the answer provided by Epic but I don't find maps to be clunky. So it's possible to keep a map and get the constructor call directly.

Map<String, Supplier<PlayerType> map = new HashMap<>();
map.put("human", Human::new);
Human h = map.get("human").get();

answered Sep 16 '15 at 17:19

ChiefTwoPencils

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Would this be possible to do with a non default constructors? – Kyefer Sep 16 '15 at 18:34
No, you'd have to use a slightly different interface that allows for parameters depending on how many you have. Or provide an init method to call on the instance. @Kyefer – ChiefTwoPencils Sep 16 '15 at 18:59

score 0 · Answer 3 · edited May 23 '17 at 12:03

The two main options I can think of:

Using Class.newInstance(), as you mentioned (not sure if you had this exact way in mind):

// Set up your map
Map<String, Class> classes = new HashMap<String, Class>();
classes.put("int", Integer.class);
classes.put("string", String.class);

// Get your data
Object s = classes.get("string").newInstance();

You could use Class.getDeclaredConstructor.newInstance if you want to use a constructor with arguments (example).

Another option is using switch:

Object getObject(String identifier) {
    switch (identifier) {
        case "string": return new String();
        case "int": return new Integer(4);
    }
    return null; // or throw an exception or return a default object
}

The `::new` (from Java 8) as suggested in [another answer](http://stackoverflow.com/a/32614878/1711796) is perhaps better than my first option. — Bernhard Barker, Sep 16 '15 at 17:30

score 0 · Answer 4 · edited Sep 16 '15 at 17:31

One potential solution:

public class ForFunFactory {

    private ForFunFactory() {
    }

    public static AThing getTheAppropriateThing(final String thingIdentifier) {
        switch (thingIdentifier) {
            case ThingImplApple.id:
                return new ThingImplApple();
            case ThingImplBanana.id:
                return new ThingImplBanana();
            default:
                throw new RuntimeException("AThing with identifier "
                        + thingIdentifier + " not found.");
        }
    }
}

public interface AThing {
    void doStuff();
}

class ThingImplApple implements AThing {

    static final String id = "Apple";

    @Override
    public void doStuff() {
        System.out.println("I'm an Apple.");
    }

}

class ThingImplBanana implements AThing {

    static final String id = "Banana";

    @Override
    public void doStuff() {
        System.out.println("I'm a Banana.");
    }

}

Use the command line to make new objects

4 Answers4

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