Understanding < ? extends > in Java: A Complete Guide

 

Understanding <? extends> in Java: A Complete Guide

Java generics introduced a way to enforce type safety while allowing flexibility in code. One of the most powerful features of Java generics is the wildcard, represented by a question mark (?). Among the various forms of wildcards, the bounded wildcard <? extends> plays a pivotal role in ensuring flexibility without compromising type safety.

This article dives deep into the concept of <? extends> in Java, its usage, benefits, limitations, and practical examples to solidify your understanding.

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Table of Contents

1. What is <? extends>?
2. The Need for <? extends>
3. Syntax and Declaration
4. Key Use Cases
5. Benefits of Using <? extends>
6. Limitations of <? extends>
7. Comparing <? extends> with Other Wildcards
8. Practical Examples
9. Best Practices
10. Conclusion

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1. What is <? extends>?

The <? extends> wildcard in Java generics represents an upper bounded wildcard. It restricts the type parameter to be of a specific type or any of its subclasses. The syntax <? extends T> ensures that the unknown type is a subclass (or the same class) of T.

For example:

List<? extends Number> numbers;

This declaration means that numbers can hold a list of any type that extends the Number class, such as Integer, Double, or Float.

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2. The Need for <? extends>

Generics in Java are designed to provide type safety and flexibility. However, certain scenarios require working with collections or APIs where the exact type is not known. <? extends> addresses this by allowing a method or a class to accept a range of types while maintaining type safety.

Example: If you have a method designed to work with any collection of numbers, using <? extends Number> enables the method to handle List<Integer>, List<Double>, and more.

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3. Syntax and Declaration

The syntax for <? extends> is straightforward:

Collection<? extends T>

Here:

• T is the upper bound.
• The wildcard ? represents an unknown type.

Example:

public void processNumbers(List<? extends Number> list) {
    for (Number num : list) {
        System.out.println(num);
    }
}

This method can process lists of Integer, Double, or any subclass of Number.

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4. Key Use Cases

4.1 Reading Data from a Collection

<? extends> is ideal when reading elements from a collection. Since the type is bounded, the elements can be safely treated as the upper bound type.

Example:

public double sum(List<? extends Number> numbers) {
    double total = 0;
    for (Number number : numbers) {
        total += number.doubleValue();
    }
    return total;
}

4.2 Generic Method Arguments

It allows generic methods to accept arguments of different subtypes.

Example:

public static void printList(List<? extends Shape> shapes) {
    for (Shape shape : shapes) {
        shape.draw();
    }
}

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5. Benefits of Using <? extends>

• Type Flexibility: Works with a range of types within the specified bound.
• Readability: Makes APIs and methods more generic and reusable.
• Type Safety: Prevents runtime errors by ensuring compile-time checks.

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6. Limitations of <? extends>

• Write Restrictions: You cannot add elements to a collection declared with <? extends> because the exact type is unknown. Example:

List<? extends Number> list = new ArrayList<Integer>();
list.add(10); // Compilation error

• Limited Usage with Superclasses: It's not suitable when you need to modify a collection.

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7. Comparing <? extends> with Other Wildcards

Wildcard	Description	Example Use Case
<?>	Unbounded wildcard; accepts any type.	Printing elements in a generic collection.
<? extends T>	Upper bound; accepts T or its subclasses.	Reading data from a collection.
<? super T>	Lower bound; accepts T or its superclasses.	Adding elements to a collection.

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8. Practical Examples

8.1 Working with a Hierarchy

Suppose you have a class hierarchy:

class Animal {}
class Dog extends Animal {}
class Cat extends Animal {}

You can use <? extends Animal> to process a collection of animals:

public void feedAnimals(List<? extends Animal> animals) {
    for (Animal animal : animals) {
        System.out.println("Feeding " + animal);
    }
}

8.2 Generic Method

public static <T extends Number> double multiply(T value, double factor) {
    return value.doubleValue() * factor;
}

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9. Best Practices

1. Use <? extends> for Read-Only Access: It's a common convention to use <? extends> when the collection is only read.
2. Avoid Adding Elements: Remember that you cannot add elements to a <? extends> collection.
3. Combine with Unbounded Wildcards: Use <?> for maximum flexibility when type bounds are not required.

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10. Conclusion

The <? extends> wildcard is an essential tool in Java generics, offering a fine balance between flexibility and type safety. While it allows handling collections and APIs in a type-safe manner, understanding its limitations is equally important. By mastering <? extends>, you can write more robust and reusable generic code.