Bounded Types and Wildcards (? extends T, ? super T)

Bounded types and wildcards in Java generics are powerful concepts that enable more flexible and reusable code. They provide fine-grained control over type parameters in generic classes, methods, and interfaces. In this article, we will explore the concepts of bounded types and wildcards (? extends T, ? super T) and provide examples to demonstrate their usage in advanced Java.

Step-by-Step Guide

Step 1: Understanding Bounded Type Parameters

In Java, you can restrict the types that can be used as type parameters. This restriction is called a bounded type parameter. Bounded types are specified using the extends keyword for upper bounds, meaning the type parameter must be a subclass of a given type.

Example: Bounded type parameter with an upper bound

            // Bounded type parameter example
            class Box<T extends Number> {
                private T value;

                public void setValue(T value) {
                    this.value = value;
                }

                public T getValue() {
                    return value;
                }
            }

            public class BoundedTypeExample {
                public static void main(String[] args) {
                    Box<Integer> integerBox = new Box<>();
                    integerBox.setValue(10);
                    System.out.println("Integer Value: " + integerBox.getValue());

                    Box<Double> doubleBox = new Box<>();
                    doubleBox.setValue(10.5);
                    System.out.println("Double Value: " + doubleBox.getValue());

                    // The following will cause a compile-time error
                    // Box<String> stringBox = new Box<>(); // Error: String is not a subclass of Number
                }
            }
        

Step 2: Using Wildcards with Generics

Wildcards in Java generics are used to represent an unknown type. The most common wildcard types are ? extends T (upper bounded wildcard) and ? super T (lower bounded wildcard). These wildcards allow you to work with generics without knowing the exact type.

Step 3: Using ? extends T (Upper Bounded Wildcard)

The ? extends T wildcard is used when you want to allow a method or class to accept types that are either of type T or a subtype of T. This is useful when you want to read data from a structure without modifying it.

Example: Using ? extends T to read elements of a list

            import java.util.*;

            public class UpperBoundedWildcardExample {

                // Method that accepts a list of elements that are instances of Number or its subclasses
                public static void printNumbers(List<? extends Number> list) {
                    for (Number num : list) {
                        System.out.println(num);
                    }
                }

                public static void main(String[] args) {
                    List<Integer> intList = Arrays.asList(1, 2, 3, 4);
                    List<Double> doubleList = Arrays.asList(1.1, 2.2, 3.3);

                    printNumbers(intList);   // Valid: Integer is a subclass of Number
                    printNumbers(doubleList); // Valid: Double is a subclass of Number
                }
            }
        

Step 4: Using ? super T (Lower Bounded Wildcard)

The ? super T wildcard is used when you want to allow a method or class to accept types that are either of type T or a supertype of T. This is useful when you want to add elements to a structure but don't know the exact type, as it guarantees that the structure can accept T or any superclass.

Example: Using ? super T to add elements to a list

            import java.util.*;

            public class LowerBoundedWildcardExample {

                // Method that accepts a list of elements that are instances of Number or its superclasses
                public static void addNumbers(List<? super Integer> list) {
                    list.add(10);  // Valid: Integer is a subclass of Number
                    list.add(20);  // Valid: Integer is a subclass of Number
                }

                public static void main(String[] args) {
                    List<Number> numberList = new ArrayList<>();
                    addNumbers(numberList);  // Valid: Number is a superclass of Integer

                    List<Object> objectList = new ArrayList<>();
                    addNumbers(objectList);  // Valid: Object is a superclass of Integer

                    // The following will cause a compile-time error
                    // List<Double> doubleList = new ArrayList<>();
                    // addNumbers(doubleList); // Error: Double is not a superclass of Integer
                }
            }
        

Step 5: Combining Bounded Wildcards

You can also combine ? extends T and ? super T in more complex scenarios. For example, you might have a method that reads from one list (using ? extends T) and adds to another (using ? super T).

Example: Combining ? extends T and ? super T

            import java.util.*;

            public class CombinedWildcardExample {

                // Method to copy elements from one list to another
                public static void copyList(List<? extends Number> source, List<? super Number> destination) {
                    for (Number num : source) {
                        destination.add(num); // Add elements to the destination list
                    }
                }

                public static void main(String[] args) {
                    List<Integer> intList = Arrays.asList(1, 2, 3);
                    List<Number> numberList = new ArrayList<>();

                    copyList(intList, numberList); // Valid: Integer is a subclass of Number

                    for (Number num : numberList) {
                        System.out.println(num); // Output: 1, 2, 3
                    }
                }
            }
        

Step 6: Best Practices for Using Wildcards and Bounded Types

• Use ? extends T when you need to read from a structure, but don't want to modify it.
• Use ? super T when you need to add elements to a structure, but don't want to restrict the supertype.
• Always ensure the wildcard type is compatible with the expected operations (reading or writing) on the data.
• Be cautious when combining multiple wildcard types to ensure the code remains readable and maintainable.

Conclusion

Bounded types and wildcards (? extends T, ? super T) in Java generics provide a way to write flexible, reusable, and type-safe code. By understanding and using these concepts, you can write methods and classes that work with a wide variety of types while ensuring type safety. Wildcards and bounded types are crucial for writing generic code that is both efficient and maintainable.