How to use generics function with List parameter

Question

There are things that have weight, so here's their interface, for example:

public interface Banana {
    public Double getWeight();
}

    // This function is a static function in a public tool class.
    @Nullable
    public static Banana pickItemByTheirWeight(List<Banana> originalVideos) {
        // ...
        return randomOneByWeight;
    }

now I want to create things that have weight, so:

class Video implements Banana {
   @Override
   public Double getWeight() {
       //....
       return weight;
   }
}

now, when I want to pick a video by weight, for example:

List<Video> videos = new ArrayList<>();
// ...
Video randomOne = pickItemByTheirWeight(videos);

I get a compile error. so what is the correct way to do it?

---

Hey guys, it's not that easy.

I tried

public static <T extends Banana> T pickOneByTheirWeight(List<T> domains) {
    // ...
}

public abstract class Banana {
   public abstract Double getWeight();
}

And it's still not working, I cannot call the function without cast the class type.

* can someone tell my why Java choose not to let it just work ???*

---

After reading some basic knowledge to Java Generics, here comes a solution:

public static <T extends Banana> T pickOneByTheirWeight(List<? extends Banana> domains) {
        if (domains.isEmpty()) {
            return null;
        }       
        // calculate total weight
        Double totalWeight = domains.stream().map(x -> {
                Double weight = x.getWeight();
                return weight == null ? 0.0 : weight;
            }).reduce((first, second) -> {
            Double firstWeight = first != null ? first : 0.0;
            Double secondWeight = second != null ? second : 0.0;
            return firstWeight + secondWeight;
        }).get();
        // random process
        final Double randomSeed = Math.random() * totalWeight;
        double currentWeight = 0.0;
        for (Banana v: domains) {
            Double weight = v.getWeight();
            weight = weight == null ? 0.0 : weight;
            currentWeight += weight;
            if (currentWeight >= randomSeed) {
                return (T) v;
            }
        }
        // it'll not reach here indeed.
        logger.warn("pickDomainByTheirWeight cannot pick an element from list by their weights.");
        return null;
    }

But declear the function like this, we can simply call:

List<Video> videos = new ArrayList<>();
// ...
Video p = Utility.pickOneByTheirWeight(videos);

no more cast outside, however, still a cast inside the function. Any better idea?

Answer 1

Because List<Banana> is not a List<Video>. Change method signature to:

@Nullable
public static Banana pickItemByTheirWeight(List<Video> originalVideos) {
    // ...
    return randomOneByWeight;
}

Or better - using common superclass:

@Nullable
public static Banana pickItemByTheirWeight(List<? extends Banana> originalVideos) {
    // ...
    return randomOneByWeight;
}

Answer 2

You get the error because List<Video> is not List<Banana>. Have a look at this SO question.You could make use of generics and change your method to :

@Nullable
public static <T extends Banana> T pickItemByTheirWeight(List<T> bananas) {
    // ... pick something that implements banana from the list and return it
    return randomOneByWeight;
}

This is when you want to return subtype of Banana from your method. If you do not want to do it, you can get rid of this error by changing your method to :

@Nullable
public static Banana pickItemByTheirWeight(List<? extends Banana> bananas) {
    // ...
    return randomOneByWeight;
}

and since it always returns an Banana use it like :

Banana randomOne = pickItemByTheirWeight(videos);

Answer 3

The pickItemByTheirWeight method expects a list of Banana as a parameter and you give it a list of Video. As a Video is a Banana, you must declare a list of Banana to store your video into.

List<Banana> videos = new ArrayList<>();
// ...
// The method returns a Banana, not a Video
Banana randomOne = pickItemByTheirWeight(videos);
// But you may explicitly cast the Banana as a Video (even if you are not sure the Banana you got from this call is really a Video
Video randomTwo = (Video) pickItemByTheirWeight(videos);

Answer 4

there are 2 concept of Generics here:

1. Using Wildcards with Generics for List parameters: since it's already using generics, and you have to use wildcard with generics.

2. generics function template, it's simple basic.

combining 1 + 2, we get the answer:

public static <T extends Banana> T pickOneByTheirWeight(List<? extends Banana> domains) {
   // ...
}

see my original post edits for more information.