CODEWITHSUNDEEP
javagenerics
Simon Tenbeitel
Simon Tenbeitel

Reputation: 877

Getter for List of Generic type (passed as parameter)

I have a List containing objects of different subclasses. Now I want to get all of a specific subclass. So I tried to write a generic getter method. My approach is obviously wrong, as I get compile errors because of the T.

public List<T extends Resource> getAllInstancesOfType(T) {
    List<T> resources = new ArrayList<>();
    for (Resource resource : resources) {
        if (resource instanceof T)
            resources.add((T) resource);
    }
    return resources;
}

Upvotes: 0

Views: 525

Answers (3)

Paul Boddington
Paul Boddington

Reputation: 37655

Because generic information is erased, you cannot use instanceof with a type parameter. Instead you need a Class object.

public <T extends Resource> List<T> getAllInstancesOfType(Class<T> clazz) {
    List<T> resources = new ArrayList<>();
    for (Resource resource : otherResources) {
        if (clazz.isInstance(resource))
            resources.add((T) resource);
    }
    return resources;
}

You can use this by passing SomeResourceType.class.

Upvotes: 2

awsome
awsome

Reputation: 2153

Here is a method to which you can pass the list and the Class that you want to get from the list

public <T extends Resource, S extends Resource> List<S> getAllInstancesOfType(List<T> resources, Class<S> clazz) {
        List<S> subResources = new ArrayList<S>();
        for (Resource resource : resources) {
            if (clazz.isAssignableFrom(resource.getClass()))
                subResources.add((S)resource);
        }
        return subResources;
}

And here is how to use this method

  System.out.println(getAllInstancesOfType(Arrays.asList(new Resource(), new Course(), new Course()), Course.class));

Upvotes: 1

Abdelhak
Abdelhak

Reputation: 8387

This is the syntax.

public <T extends Resource> List<T> magicalListGetter(Class<T> type) {
   ...
  return list;
}

or like this same to your code, should give a name of param T in input. 

 public <T extends Resource> List<T> magicalListGetter(T t) {
           ...
        return list;
        }

Upvotes: -1

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