In OOP (Object Oriented Programming), we use inheritance to achieve the polymorphism. The factory design pattern allows you to create/manage different types of classes in a single place.

Let’s say, we have a abstract class Animal that defines a walk method that indicates that any subclasses (any animals) can walk. The abstract keyword forbids the direct instantiation of the class Animal.

Also, we add to the prototype of the class Animal an abstract method: single that needs to be implemented in subclasses. Note that the following also uses the lombok plugin.

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import lombok.Data;
 
@Data
public abstract class Animal {
    protected String name;
 
    public Animal(String name) {
        this.name = name;
    }
 
    public void walk() {
        System.out.println(this.name + " Walks.");
    }
 
    public abstract void sing();
}

import lombok.Data;

@Data
public abstract class Animal {
    protected String name;

    public Animal(String name) {
        this.name = name;
    }

    public void walk() {
        System.out.println(this.name + " Walks.");
    }

    public abstract void sing();
}

And the Bird class inherits from the Animal. In the inherited class, we can invoke the constructor from the parent class using super constructor.

Not all animals can fly, thus we define the behavior that is only specific to bird e.g. fly().

Also, we need to implement the content of the sing which is the abstract method in the Animal.

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@Data
class Bird extends Animal {
    public Bird(String name) {
        super(name);
    }
 
    public void fly() {
        System.out.println(this.name + " Flies.");
    }
 
    @Override
    public void sing() {
        System.out.println(this.name + " Sings happily.");
    }
}

@Data
class Bird extends Animal {
    public Bird(String name) {
        super(name);
    }

    public void fly() {
        System.out.println(this.name + " Flies.");
    }

    @Override
    public void sing() {
        System.out.println(this.name + " Sings happily.");
    }
}

Similarly, a Fish can swim and has a different implementation of the sing

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@Data
class Fish extends Animal {
    public Fish(String name) {
        super(name);
    }
 
    public void swim() {
        System.out.println(this.name + " Swims.");
    }
 
    @Override
    public void sing() {
        System.out.println(this.name + " Sings sadly.");
    }
}

@Data
class Fish extends Animal {
    public Fish(String name) {
        super(name);
    }

    public void swim() {
        System.out.println(this.name + " Swims.");
    }

    @Override
    public void sing() {
        System.out.println(this.name + " Sings sadly.");
    }
}

Factory Pattern in Java

In our factory class, we have a static method that takes parameters of Animal Type and Animal Name, then return an instance of the target.

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class AnimalFactory {
    public static Animal createAnimal(String type, String name) {
        switch (type) {
            case "BIRD": return new Bird(name);
            case "FISH": return new Fish(name);
            default:
                throw new IllegalArgumentException("Invalid Animal");
        }
    }
}

class AnimalFactory {
    public static Animal createAnimal(String type, String name) {
        switch (type) {
            case "BIRD": return new Bird(name);
            case "FISH": return new Fish(name);
            default:
                throw new IllegalArgumentException("Invalid Animal");
        }
    }
}

The return type should be parent abstract Animal class, and it can return concrete instance of a Bird or Fish respectively.

Example usage:

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Animal bird1 = AnimalFactory.createAnimal("BIRD", "bird1");
// bird1 Sings happily.
bird1.sing();
Animal fish1 = AnimalFactory.createAnimal("FISH", "fish1");
// fish1 Sings sadly.
fish1.sing();

Animal bird1 = AnimalFactory.createAnimal("BIRD", "bird1");
// bird1 Sings happily.
bird1.sing();
Animal fish1 = AnimalFactory.createAnimal("FISH", "fish1");
// fish1 Sings sadly.
fish1.sing();

Both bird and fish are Animal type but their sing is different as implemented in each subclass. We can also override the parent’s implementation of the walk. All these are the polymorphism of the OOP.

Another usage is to do the type casting – but this is usually a hint of code smell.

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Bird bird2 = (Bird)AnimalFactory.createAnimal("BIRD", "bird2");
// bird2 Flies.
bird2.fly();
Fish fish2 = (Fish)AnimalFactory.createAnimal("FISH", "fish2");
// fish2 swims
fish2.swim();

Bird bird2 = (Bird)AnimalFactory.createAnimal("BIRD", "bird2");
// bird2 Flies.
bird2.fly();
Fish fish2 = (Fish)AnimalFactory.createAnimal("FISH", "fish2");
// fish2 swims
fish2.swim();

The Animal type is casted into either Bird or Fish. Usually, it makes more sense to type casting a child class to its parent (if they belong to the same inheritance tree).

Obviously, you can’t convert Bird to Fish or vice versa.

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Bird bird2 = (Bird) AnimalFactory.createAnimal("BIRD", "bird2");
Animal fish2 = bird2;
((Fish)fish2).sing();

Bird bird2 = (Bird) AnimalFactory.createAnimal("BIRD", "bird2");
Animal fish2 = bird2;
((Fish)fish2).sing();

This will result a runtime exception:

Exception in thread “main” java.lang.ClassCastException: com.company.Bird cannot be cast to com.company.Fish

–EOF (The Ultimate Computing & Technology Blog) —