When to use wildcards in Java Generics?

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The big difference between

public <T extends Animal> void takeThing(ArrayList<T> list)

and

public void takeThing(ArrayList<? extends Animal> list)

is that in the former method you can refer to “T” within the method as the concrete class that was given. In the second method you cannot do this.

Here a more complex example to illustrate this:

// here i can return the concrete type that was passed in
public <T extends Animal> Map<T, String> getNamesMap(ArrayList<T> list) {
    Map<T, String> names = new HashMap<T, String>();
    for (T animal : list) {
        names.put(animal, animal.getName()); // I assume there is a getName() method
    }
    return names;
}

// here i have to use general Animal
public Map<Animal, String> getNamesMap(ArrayList<? extends Animal> list) {
    Map<Animal, String> names = new HashMap<Animal, String>();
    for (Animal animal : list) {
        names.put(animal, animal.getName()); // I assume there is a getName() method
    }
    return names;
}

With the first method if you pass in an List of Cats you get a Map with Cat as key. The second method would always return a Map with general Animal key.

By the way this is not valid java syntax:

public <? extends Animal> void takeThing(ArrayList<?> list)

Using this form of generic method declaration you have to use a valid java identifier and not “?”.

Edit:

The form “? extends Type” only applies to variable or parameter type declaration. Within a generic method declration it has to be “Identifier extends Type” as you are able to refer to the “Identifier” from within your method.

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