Generic Methods vs. Wildcards in Java: When Should I Use Each?

Generic Methods vs. Wildcards: When to Use Which?

Introduction:
Generic methods and wildcards offer flexible type handling in Java, but understanding their distinct use cases can be challenging. This article clarifies the differences and provides guidance on when to employ each approach.

When to Use Wildcards:

Wildcards are suitable when polymorphism (ability to use multiple types) is the primary objective. Specifically, they should be used if:

• The type arguments are used for subtype substitution, allowing different actual types to be passed at runtime.
• No dependency exists between the types of method arguments or return type.

For example, in Collection.containsAll(Collection), the wildcard ? indicates that the collection passed as an argument can be of any type. This allows flexibility in accepting different implementations of Collection.

When to Use Generic Methods:

Generic methods are appropriate when there is a dependency between the types of method arguments and/or return type. This allows enforcing type constraints and expressing complex relationships between types. For instance, in Collections.copy(List, List), the type parameter T ensures that both the destination and source lists share the same base type.

Differences Between Wildcards and Generic Methods:

• Relationship between Types: Wildcards support subtype relationships, while generic methods can enforce specific type dependencies.
• Multiple Bounds: Type parameters can have multiple upper bounds; wildcards can have only one upper or lower bound.
• Single Parameterized Type Argument: Wildcards can be used for methods with only one parameterized type argument, while type parameters can also be used in this situation.

Example: Choosing Between Wildcards and Generic Methods

Consider the following method:

public static <T extends Number> void copy(List<T> dest, List<T> src)

This method enforces that both dest and src lists have the same parameterized type for List. However, if this constraint is not necessary, we could have written it using a wildcard as:

public static void copy(List<? extends Number> dest, List<? extends Number> src)

In the second case, we can pass lists of different parameterized types, but it would no longer be type-safe to copy elements between them.

Conclusion:

Wildcards and generic methods offer different solutions for type handling in Java. Wildcards support polymorphism by allowing flexible subtyping, while generic methods enforce type dependencies and express complex relationships between types. By understanding their distinct features, you can make informed decisions about when to employ each approach.

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