6.6 - Wildcards in Generics
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Unbounded Wildcards
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Today, we're going to learn about wildcards in generics. What do you think happens when we don't restrict the type, like using a `<?>` wildcard?
Does it mean we can use any type without restrictions?
Exactly! An unbounded wildcard allows you to accept any type. For instance, if we have a method that prints lists, it can handle `List<?>`. Why do you think this flexibility is useful?
I guess it makes our code reusable for different types of data?
Right! Reusability is key. Let’s consider a method you might use to print elements of a list. Can you think of how we could define such a method?
"Like this?
Upper Bounded Wildcards
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Now, let’s move on to upper bounded wildcards. What do you think is the significance of using `<? extends Type>`?
It restricts the type to a specific class or its subclasses, right?
Exactly! This is useful, for example, when you need to sum numbers without worrying about the specific number subclasses. Can someone describe how you'd write a method that uses this feature?
"Something like:
Lower Bounded Wildcards
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Finally, let’s discuss lower bounded wildcards. What do you think using `<? super Type>` allows us to do?
It restricts the type to the specified class or its superclasses?
Correct! This is especially useful for inserting elements into a collection. Can you think of an example where this would be beneficial?
"If I wanted to add integers to a list, I'd use something like:
Introduction & Overview
Read summaries of the section's main ideas at different levels of detail.
Quick Overview
Standard
This section describes three types of wildcards in Java generics: unbounded, upper bounded, and lower bounded. Each type allows for specific behaviors and constraints when working with generic code, making programs more adaptable while preserving compile-time type safety.
Detailed
Wildcards in Generics
Wildcards in generics add flexibility to your code in Java by allowing you to create methods and classes that can work with various types without specifying them explicitly. There are three main types of wildcards:
- Unbounded Wildcard (
<?>): This wildcard can accept any type. For example, a method that processes a list of unknown objects can be declared usingList<?>. This is useful when you do not need to know about the specific type of the object being used.
- Upper Bounded Wildcard (
<? extends Type>): This wildcard means that you can accept a specific type and its subclasses. It's beneficial when working with a network of class hierarchies. For instance:
- Lower Bounded Wildcard (
<? super Type>): This wildcard allows you to accept a specified type and its superclasses. This is commonly used for writing to a collection.
Each type of wildcard has its uses, and understanding these is crucial for writing flexible and type-safe generic code.
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Introduction to Wildcards
Chapter 1 of 4
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Chapter Content
Wildcards add flexibility to generic code.
Detailed Explanation
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.
Examples & Analogies
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.
Unbounded Wildcard
Chapter 2 of 4
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Chapter Content
- Unbounded Wildcard: <?>
- Accepts any type.
public void printList(List list) {
for (Object obj : list) {
System.out.println(obj);
}
}
Detailed Explanation
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.
Examples & Analogies
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.
Upper Bounded Wildcard
Chapter 3 of 4
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Chapter Content
- Upper Bounded Wildcard: <? extends Type>
- Accepts Type or its subclasses.
public double sum(List list) {
double total = 0;
for (Number n : list) {
total += n.doubleValue();
}
return total;
}
Detailed Explanation
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.
Examples & Analogies
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.
Lower Bounded Wildcard
Chapter 4 of 4
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Chapter Content
- Lower Bounded Wildcard: <? super Type>
- Accepts Type or its superclasses.
public void addIntegers(List list) {
list.add(1);
list.add(2);
}
Detailed Explanation
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).
Examples & Analogies
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.
Key Concepts
-
Unbounded Wildcard: Allows any type in a generic context.
-
Upper Bounded Wildcard: Limits to a specific type and its subclasses for type safety.
-
Lower Bounded Wildcard: Accepts a specific type and its superclasses to facilitate insertion.
Examples & Applications
Using an unbounded wildcard to print a list: public void printList(List<?> list) { ... }.
Summing numbers with an upper bounded wildcard: public double sum(List<? extends Number> list) { ... }.
Adding integers to any superclass list with a lower bounded wildcard: public void addIntegers(List<? super Integer> list) { ... }.
Memory Aids
Interactive tools to help you remember key concepts
Rhymes
Wildcards are wild and free, for any type they’ll let you be.
Stories
Imagine a zoo with various animals. Unbounded wildcards are like a keeper curating all species, while upper bounded wildcards are zookeepers overseeing just cats, and lower bounded wildcards are caretakers managing all types of mammals.
Memory Tools
Use the acronym UUL: Unbounded - anything goes, Upper Bounded - subclasses flow, Lower Bounded - super class grow!
Acronyms
Remember the acronym 'WUL' - Wildcards (for various) Usability in Lists.
Flash Cards
Glossary
- Unbounded Wildcard
A wildcard that allows any type; denoted as
<?>.
- Upper Bounded Wildcard
A wildcard that allows the specified type and its subclasses; denoted as
<? extends Type>.
- Lower Bounded Wildcard
A wildcard that allows the specified type and its superclasses; denoted as
<? super Type>.
Reference links
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