6.6. Wildcards in Generics

Wildcards add flexibility to generic code.

Wildcards in generics allow developers to work with a range of types without specifying an exact type. This means you can create methods or classes that can accept different types of data, making the code more versatile and easier to maintain.

Think of a wildcard as a flexible box that can hold different colored balls. Instead of having to create a specific box for each color (like specifying a type), a wildcard box can hold any color, allowing you to use it for various situations.

1. Unbounded Wildcard: <?>

• Accepts any type.

public void printList(List<?> list) {
    for (Object obj : list) {
        System.out.println(obj);
    }
}

An unbounded wildcard, represented as <?>, means that the method can accept a list of any type. This is useful when you don't really care about the specific type being passed, allowing for greater code flexibility. In the provided example, printList can print the contents of any list, regardless of what type of objects it contains.

Imagine a universal remote control. This remote can operate any television brand without needing specific settings for each one. Similarly, the unbounded wildcard allows your code to work with any type of list without needing to define what’s in it.

2. Upper Bounded Wildcard: <? extends Type>

• Accepts Type or its subclasses.

public double sum(List<? extends Number> list) {
    double total = 0;
    for (Number n : list) {
        total += n.doubleValue();
    }
    return total;
}

The upper bounded wildcard, denoted by <? extends Type>, restricts the types to a specified class and its subclasses. In the example, the sum method can accept a list of any objects that are of type Number or extend it (like Integer, Double, etc.). This lets you perform operations that make sense for the base type (like summation), while still being able to work with any specific types that fit the criteria.

Imagine a membership card that allows access to a group of related stores: a bookstore, a consulting service, and a library. You can access all these places using the card, but you're only allowed to enter those related establishments, similar to how the upper bounded wildcard allows access only to a specific type and its children.

3. Lower Bounded Wildcard: <? super Type>

• Accepts Type or its superclasses.

public void addIntegers(List<? super Integer> list) {
    list.add(1);
    list.add(2);
}

The lower bounded wildcard, shown as <? super Type>, allows for specifying types that are superclasses of the given type. This means that you can add elements of the specified type (like Integer) or any of its subclasses. In the example, the addIntegers method can safely add integers to any list that can accept integers or their superclasses (like Number or Object).

Think of a family tree where you can add family members to a common ancestor. If a grandparent can receive children or grandchildren, that's like the lower bounded wildcard: you can add specific types as long as they conform to the overall family type.

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