Introduction:

Generic types in Java provide a powerful mechanism for creating reusable and type-safe code. They allow developers to create classes, interfaces, and methods that can work with different data types while maintaining compile-time type safety.

Java offers two fundamental categories of generic types: bounded and unbounded. In this article, we will understand the concepts of bounded and unbounded generic types, understand their differences, and explore practical examples to demonstrate their usage.

Understanding Bounded Generic Types: Bounded generic types are defined with specific type restrictions. By using bounded type parameters, we can restrict the type arguments that can be used with a generic class, interface, or method. This ensures that only certain types, or their subclasses, are permitted as type arguments.

There are two types of bounded generic types in Java:

• Upper bounded
• Lower bounded

Upper Bounded Generic Types:

An upper bounded generic type restricts the type argument to be a specific type or any of its subclasses. It is denoted using the extends keyword. For example, consider the following generic class:

public class Box<T extends Number> {
    private T value;

    public Box(T value) {
        this.value = value;
    }

    public T getValue() {
        return value;
    }
}

In this example, the type parameter T is an upper bounded generic type, constrained to be a subtype of the Number class. This allows the Box class to work with any type that extends Number, such as Integer, Double, or BigDecimal. It guarantees that only valid numeric types can be used as arguments for the Box class.

Lower Bounded Generic Types:

A lower bounded generic type restricts the type argument to be a specific type or any of its superclasses. It is denoted using the super keyword. For instance, consider the following generic method:

public static <T super Integer> void printList(List<T> list) {
    for (T element : list) {
        System.out.println(element);
    }
}

Here, the type parameter T is a lower bounded generic type, constrained to be a superclass of Integer. This allows the printList method to accept a list containing any type that is either Integer or a superclass of Integer, such as Number or Object.

Unbounded Generic Types:

Unbounded generic types, as the name suggests, do not have any restrictions on the type arguments. They are specified using a simple type parameter without any bound. For example:

public class Box<T> {
    private T value;

    public Box(T value) {
        this.value = value;
    }

    public T getValue() {
        return value;
    }
}

In this case, the type parameter T is an unbounded generic type, allowing any type to be used as an argument for the Box class. It provides maximum flexibility but may lead to potential type compatibility issues if not used carefully.

Conclusion: Bounded and unbounded generic types in Java offer different levels of type restrictions, allowing developers to create flexible and type-safe code. Bounded types ensure that only specific types or their subclasses are used as type arguments, providing compile-time safety. On the other hand, unbounded types offer maximum flexibility but require caution to ensure type compatibility.