CODEWITHSUNDEEP
javainheritance
Jeremy
Jeremy

Reputation: 5433

Java -- How to cleanly constuct lists of subclasses from a list of the superclass?

Specifically, I have a list of objects. Each subclass of that class of objects is associated with a specific piece of functionality.

Here is my naive solution to this problem, and it is ugly:

List<SuperClass> superClasses = ...

List<Subclass1> obj1 = Lists.newArrayList();
List<Subclass1> obj2 = Lists.newArrayList();
List<Subclass1> obj3 = Lists.newArrayList();
List<Subclass1> obj4 = Lists.newArrayList();
...

for (SuperClass obj : superClasses) {
  if (obj.getClass().equals(Subclass1.class)) {
    obj1.add((Subclass1) obj);
  }
}

//repeat for each subclass

I also tried something like this:

public Map<Class, List<SuperClass>> constructMap(List<SuperClass> superClasses);

but this couldn't be used:

(List<Subclass1>) constructMap(superClasses).get(Subclass1.class)
cannot cast from List<SuperClass> to List<Subclass1>.

Am I doomed to the naive code here? Is there really no smart way to take in a list of super-class objects and handle them based on their actual class?

Here is the problem I am trying to solve:

public Driver {

    List<Fruit> fruits = collectAllFruit();

    StemHandler.pullStem(fruitsThatAreApples, fruitsThatArePears);
    ColorHandler.getColor(fruitsThatAreApples, fruitsThatAreOranges, fruitsThatArePears);


public interface Fruit {

    getColor();
}

public class Apple implements Fruit {

    getColor();
    pullStem();
}

public class Orange implements Fruit {

    getColor();
    peel();

}

public class Pear implements Fruit {

    getColor();
    pullStem();

}


public interface FruitHandler {


}

public class StemHandler extends FruitHandler {

    pullStem(List<Apple>, List<Pear>)

}

public class ColorHandler extends FruitHandler {

    getColor(List<Apple>, List<Orange>, List<Pear>)
}

Upvotes: 0

Views: 86

Answers (1)

Ruan Mendes
Ruan Mendes

Reputation: 92334

The correct thing is to use polymorphism using your list of the base type (List<SuperClass>). You shouldn't handle them differently in your loop that iterates through the different sub classes. What you should do is call a specific method that is defined on the super class (better yet, an interface) and implement the method differently in each sub class (implementation). See Returning an extended class

interface MyInteface{
    public void doSomething();
}

class Sub1 implements MyInterface{
    public void doSomething() {
        System.out.println("Behavior # 1");
    }
}

class Sub2 implements MyInterface{
    public void doSomething() {
        System.out.println("Different Behavior");
    }
}

for (MyInteface obj : superClasses) {
    // Will do different things for different implementations
    obj.doSomething();
}

The secret is to define what are the common aspects of the different implementations and be able to squeeze them all into the same signature.

Upvotes: 2

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