The relationship between Java generics and metaprogramming

Generics and metaprogramming are powerful tools in Java for creating flexible and reusable code: Generics allow the use of parameterized types, enhance type safety, and eliminate the need for primitive types. Metaprogramming manipulates code through reflection so that it can determine generic information and implement abstractions at runtime. In practice, generics and metaprogramming can be combined to create generic filter methods without the need to create specific code for each situation.

The relationship between generics and metaprogramming in Java

Generics and metaprogramming are powerful tools in Java that can help Developers build more flexible and reusable code.

Generics

Generics allow developers to create classes, interfaces, and methods using parameterized types that can be used with any type of data. This eliminates the need for primitive types (such as Object) and enhances type safety.

For example, a generic List:

public class CustomList<T> {
    private List<T> items;
    
    public void addItem(T item) {
        items.add(item);
    }
}

This List can be used to store any type of object without specifying a specific type.

Metaprogramming

Metaprogramming refers to the ability to manipulate code or a program at runtime. Reflection in Java allows developers to inspect and modify information about classes, methods, and fields.

For example, we can use reflection to get the type parameters of the CustomList class:

Class<CustomList<String>> listClass = CustomList.class;
TypeVariable<?>[] typeParams = listClass.getTypeParameters();
System.out.println(typeParams[0].getName()); // 输出 "T"

Relationship

Generics and metaprogramming are closely related, because generics Type information is available in metaprogramming. Developers can achieve higher levels of abstraction by leveraging reflection to dynamically determine generic parameters.

For example, we can use reflection to create a CustomList instance whose type parameter is a specific type:

CustomList<String> stringList =
        (CustomList<String>) listClass.getDeclaredConstructor().newInstance();

Practical case

Now, let’s show a use Practical examples of generics and metaprogramming. Suppose we have an interface that defines a filter method that filters a collection and returns a new collection:

public interface Filter<T> {
    boolean test(T item);
}

We can use generics and metaprogramming to create a generic filter Method that can filter any collection using any filter:

public static <T> List<T> filter(List<T> items, Filter<T> filter) {
    List<T> filteredItems = new ArrayList<>();
    for (T item : items) {
        if (filter.test(item)) {
            filteredItems.add(item);
        }
    }
    return filteredItems;
}

Now we can use this method to filter different types of collections and filters:

List<Integer> numbers = filter(Arrays.asList(1, 2, 3, 4, 5),
        item -> item % 2 == 0);

List<String> strings = filter(Arrays.asList("apple", "banana", "cherry"),
        item -> item.startsWith("b"));

By using generics and metaprogramming, we achieved a general solution that can filter in various situations without having to create specific code for each situation.

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