Bounded type parameters are a powerful feature in Java generics that allow you to restrict the types that can be used as type arguments in a parameterized type. This concept enhances type safety and provides more flexibility when working with generic classes and methods.

Understanding Bounded Type Parameters

In Java, you can use bounded type parameters to specify that a generic type must be a subtype of a particular class or implement certain interfaces. This is achieved using the extends keyword for upper bounds and the super keyword for lower bounds.

Upper Bounded Type Parameters

Upper bounds restrict a type parameter to be a subtype of a specific type. The syntax is:

<T extends UpperBoundType>

Here's an example of a method using an upper bounded type parameter:

public static <T extends Number> double sum(T[] array) {
    double total = 0;
    for (T element : array) {
        total += element.doubleValue();
    }
    return total;
}

In this example, T is restricted to be a subtype of Number, allowing the method to work with any numeric type.

Lower Bounded Type Parameters

Lower bounds specify that a type parameter must be a supertype of a particular type. The syntax is:

<T super LowerBoundType>

Here's an example demonstrating a lower bounded type parameter:

public static void addNumbers(List<? super Integer> list) {
    for (int i = 1; i <= 5; i++) {
        list.add(i);
    }
}

This method can accept a List of Integer or any supertype of Integer, such as Number or Object.

Benefits of Bounded Type Parameters

Improved type safety by restricting the types that can be used
Enhanced code reusability across different but related types
Ability to invoke methods of the bounding type within generic code
Better integration with the Java type system and existing class hierarchies

Multiple Bounds

Java also supports specifying multiple bounds for a type parameter. This is useful when you need a type to satisfy multiple conditions:

<T extends ClassA & InterfaceB & InterfaceC>

In this case, T must be a subtype of ClassA and implement both InterfaceB and InterfaceC.

Best Practices

Use bounded type parameters when you need to invoke methods defined in the bound
Prefer upper bounds over lower bounds for better readability and usability
Combine bounded type parameters with Java Wildcard Generics for maximum flexibility
Be cautious with multiple bounds, as they can lead to complex type hierarchies

Bounded type parameters are closely related to other generic concepts in Java. To deepen your understanding, explore Java Generic Methods and Java Generic Classes.

Conclusion

Bounded type parameters are a crucial feature in Java generics, offering a balance between flexibility and type safety. By mastering this concept, you can write more robust and reusable code that leverages the full power of Java's type system.