Java Generics Wildcards: What's the Difference Between `List

Understanding Java Generics (Wildcards)

Wildcards in Java Generics

Java generics provide type safety through parameterized types and wildcards. Wildcards are placeholders that represent unknown types and enhance flexibility in collections.

Difference between List and List

• List (Upper Bound Wildcard): Indicates that the wildcard can hold any subclass or the specified type T. It allows you to read from the list but not write to it.
• List (Lower Bound Wildcard): Specifies that the wildcard can hold any superclass or the specified type T. It allows you to add elements to the list but not read from it.

Bounded vs. Unbounded Wildcards

• Bounded Wildcards: Restrict the type of elements that can be in a collection using an upper bound (? extends T) or a lower bound (? super T).
• Unbounded Wildcards: Represent any type, similar to .

Example:

List<? super Animal> animals = new ArrayList<Dog>();  // Upper Bounded Wildcard
animals.add(new Dog()); // Allowed
Dog animal = animals.get(0); // Not allowed

List<? extends Animal> dogs = new ArrayList<Dog>(); // Lower Bounded Wildcard
dogs.add(new Animal()); // Not allowed
Animal dog = dogs.get(0);  // Allowed

Conclusion:

Wildcards in Java generics provide flexibility in dealing with collections of unknown types. Upper bounded wildcards specify subclasses, while lower bounded wildcards specify superclasses. Unbounded wildcards represent any type. Understanding these concepts is essential for effective use of generics in Java programming.

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