Wildcards vs. Generic Methods in Java: When to Use Which?

Wildcards vs. Generic Methods: Understanding Usage Guidelines

Comparison of type parameters and wildcards in Java generics can be confusing. Here's a detailed clarification:

Use Wildcards for Polymorphism

According to the Oracle documentation, wildcards should be used when the type argument is solely intended for polymorphism, allowing different actual argument types at invocation. Examples include:

interface Collection<E> {
    boolean containsAll(Collection<?> c);
    boolean addAll(Collection<? extends E> c);
}

Here, wildcards are used to express that the type argument is irrelevant to the method's functionality. The containsAll method can accept any type of collection, and the addAll method can add any subtype of the collection's type.

Use Generic Methods for Type Relationships

Generic methods, on the other hand, should be used when there is a specific relationship between the types of the method arguments or the return type. For instance:

class Collections {
    public static <T> void copy(List<T> dest, List<? extends T> src) {
    ...
}

Here, the generic method ensures that the destination and source lists have the same parameterized type, making it safe to copy elements between them.

Example Difference

The following two method declarations are different:

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

// Using type parameters
public static <T extends Number> void copy(List<T> dest, List<T> src)

The wildcard version allows passing lists of different subtypes of Number, while the type parameter version guarantees that both lists are of the same specific subtype.

Other Differences

Besides the above guidelines, there are additional differences:

• Wildcards can be used with multiple parameterized types or with no parameters at all.
• Type parameters allow multiple bounds, while wildcards support both upper and lower bounds.

Conclusion

Understanding the usage of wildcards and generic methods is crucial for effective use of Java generics. Wildcards provide flexibility for polymorphism, while generic methods allow enforcing relationships between types. By carefully applying these guidelines, developers can leverage generics to their full potential and enhance their code's flexibility and type-safety.

The above is the detailed content of Wildcards vs. Generic Methods in Java: When to Use Which?. For more information, please follow other related articles on the PHP Chinese website!