How to create an object of an specific class given a key String in Java?

Question

I'm working on an example of a Zoo that has several different types of Animal. The user enters commands such "add tiger" and then a Tiger is added to the Zoo.

In my command interpreter class I have some code like this:

String animalName...
Animal newAnimal;
if (animalName.equals("tiger"))
    newAnimal = new Tiger();
else if (animalName.equals("elephant"))
    newAnimal = new Elephant();

The problem here is that when a new kind of animal is added to the program, this piece of code must also be changed. I'd like to add new animals just by subclassing Animal without changing anything in the already existing classes.

The name given by the user in her command isn't necessarily identical to the class name of the animal (for example, "add Bengal tiger" would add a BengalTiger object).

If possible, I'd prefer to avoid using reflection.

This is the final code:

private static String getClassName(String name) {
    char ch;
    int i;
    boolean upper = true;
    StringBuilder s = new StringBuilder("animals.");

    for (i=0; i<name.length(); i++) {
        ch = name.charAt(i);
        if (ch!=' ') {
            if (upper)
                s.append(Character.toUpperCase(ch));
            else
                s.append(Character.toLowerCase(ch));
            upper = false;
        } else
            upper = true;

    }
    return s.toString();
}

@Override
public Animal addAnimal(String s) {
    try {
        Animal animal = (Animal)Class.forName(getClassName(s)).newInstance();
        addAnimal(animal);
        return animal;
    } catch (InstantiationException e) {
        throw new IllegalArgumentException("There are no animals called like this");
    } catch (IllegalAccessException e) {
        throw new IllegalArgumentException("There are no animals called like this");
    } catch (ClassNotFoundException e) {
        throw new IllegalArgumentException("There are no animals called like this");
    } catch (ClassCastException e) {
        throw new IllegalArgumentException("There are no animals called like this");
    }
}

Why do you "prefer to avoid using reflection"? It's the solution to your problem. — Richard JP Le Guen
– Richard JP Le Guen, Commented Nov 29, 2012 at 22:15
Because reflection is very Java specific and I'd like a more technological independent solution. — joanq
– joanq, Commented Nov 29, 2012 at 22:41
Yes, it is an example and I have to use some language, but the idea is to show some OOP design problems and solutions. — joanq
– joanq, Commented Nov 29, 2012 at 22:49

AlexR · Accepted Answer · 2012-11-29 22:20:03Z

3

You should use dynamic class loading: Class.forName(name), e.g.:

String animal = ... // e.g. elephant
String className = "com.mycompany.animals." + animal.substring(0, 1).toUpperCase() + animal.subString(1);
Animal obj = (Animal)Class.forName(className);

answered Nov 29, 2012 at 22:20

AlexR

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2 Comments

tckmn Over a year ago

Wouldn't it be Animal a = (Animal)Class.forName(className).newInstance();?

joanq Over a year ago

Although it uses reflection, I'm using this solution due to its simplicity. The only problem I see here is that all animals must be in the same package, so it's not possible for someone to add a new package with some new kinds.

CodeDreamer · Accepted Answer · 2012-11-29 22:17:42Z

2

You need to use Class.forName(X) to make this happen. Class.forName will help you get the load the actual class as string value. Refer to http://docs.oracle.com/javase/1.4.2/docs/api/java/lang/Class.html#forName(java.lang.String).

answered Nov 29, 2012 at 22:17

CodeDreamer

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Comments

tckmn · Accepted Answer · 2012-11-29 22:24:13Z

1

Object animalObj = Class.forName(animalString);

But it has to be the exact name (with the same capitalization).

edited Nov 29, 2012 at 22:24

tckmn

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answered Nov 29, 2012 at 22:17

AlexWien

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Comments

Victor Sorokin · Accepted Answer · 2012-11-29 22:29:25Z

1

For completeness sake, here's rather verbose solution based on AbstractFactory pattern. You will need to subclass two interfaces instead of one (animal type's factory and animal type itself), but, hey, there's no reflection!

interface Animal {
    String whoAmI();
}

class Tiger implements Animal {
    @Override
    public String whoAmI() {
        return "Tiger";
    }
}

interface AnimalFactory {
    Animal create();
}

abstract class AbstractAnimalFactory implements AnimalFactory {
    protected AbstractAnimalFactory(String animalName, FactoriesRegistry registry) {
        registry.register(animalName, this);
    }
}

class TigersFactory extends AbstractAnimalFactory {
    TigersFactory(FactoriesRegistry registry) {
        super("tiger", registry);
    }

    @Override
    public Animal create() {
        return new Tiger();
    }
}

class FactoriesRegistry {
    HashMap<String, AnimalFactory> byAnimalName = new HashMap<String, AnimalFactory>();

    Animal produce(String id) {
        AnimalFactory factory = byAnimalName.get(id);
        if (factory != null)
            return factory.create();
        else
            throw new IllegalArgumentException("Unknown animal " + id);
    }

    public void register(String animalName, AnimalFactory factory) {
        byAnimalName.put(animalName, factory);
    }
}

public class SOSample {
    public static void main(String[] args) {
        FactoriesRegistry registry = new FactoriesRegistry();
        TigersFactory tigersFactory = new TigersFactory(registry);
        System.out.println(registry.produce("tiger").whoAmI());
    }
}

answered Nov 29, 2012 at 22:29

Victor Sorokin

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4 Comments

Richard JP Le Guen Over a year ago

Code still has to be changed when a new animal is added: someone has to write code which will instantiate the new ElephantFactory... no?

joanq Over a year ago

But here you need to modify main every time you add a new kind of animal, in order to create the factory.

Victor Sorokin Over a year ago

Yes, but the fragment which creates animals remains unchanged.

tuga Over a year ago

Agreed. This is a good solution. And it follows the open/closed principle. You just have to code a new Factory for each type of Animal, rather than changing existing code to support new animal types.

tuga · Accepted Answer · 2012-11-29 22:57:13Z

Reflection isn't the only Solution for this Problem like @Victor Sorokin mentioned. But it's the most simple/clean one to code!!!

Solution with reflection:

public interface Animal {
    // put your contract here
}

public class AnimalFactory {
    public static final AnimalFactory INSTANCE = new AnimalFactory();

    private Map<String, Class<? extends Animal>> animalTypeMap;

    private AnimalFactory() {};

    public Animal createAnimal(String key) {
        Animal animal = null;
        if (animalTypeMap != null) {
            Class<? extends Animal> animalType = animalTypeMap.get(key);
            if (animalType != null) {
                try {
                    animal = animalType.newInstance();
                } catch (Throwable error) {
                    throw new RuntimeException("Unable to create animal with key='" + key + "'", error);
                }
            }
        }
        return animal;
    }

    public Map<String, Class<? extends Animal>> getAnimalTypeMap() {
        if (animalTypeMap == null) {
            animalTypeMap = new HashMap<String, Class<? extends Animal>>();
        }
        return animalTypeMap;
    }

    public void setAnimalTypeMap(Map<String, Class<? extends Animal>> animalTypeMap) {
        this.animalTypeMap = animalTypeMap;
    }

    public void addAnimalType(String key, Class<? extends Animal> animalType) {
        getAnimalTypeMap().put(key, animalType);
    }

    public void removeAnimalType(String key, Class<? extends Animal> animalType) {
        if (animalTypeMap != null) {
            animalTypeMap.remove(key);
        }
    }    
}

And then, you use it like this:

Animal myAnimal = AnimalFactory.INSTANCE.createAnimal("myAnimalKey");

With this solution you just need to fill in that Map with the Animal types you need! The other solution it's also a clean/good solution, but is very verbose...

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