Welcome to our lesson on Monads in Java! As we continue with our Advanced Functional Programming Techniques course, this lesson will delve into the concept of monads. You'll learn how to leverage monads to manage various programming concerns, such as handling side effects, chaining operations, and enhancing the readability of functional code. This lesson will equip you with practical insights and hands-on examples to help you write more effective and modular Java code.

In this lesson, you will:

• Grasp the concept of monads in functional programming.
• Learn how to implement monads in Java.
• Explore how monads can be used to chain operations seamlessly.
• Discover practical applications of monads in everyday coding tasks.

Monads might seem abstract initially, but they are powerful tools that help manage side effects, enable operation chaining, and maintain clean, readable code. In Java, a common example of a monad is Optional, which is used to handle the absence of values without relying on null checks.

Let's explore how Optional can be used as a monad in Java:

Java
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1import java.util.Optional;
2
3public class Main {
4    public static void main(String[] args) {
5        Optional<Integer> number = Optional.of(10);
6        Optional<Integer> result = number.map(x -> x * 2).filter(x -> x > 15);
7        System.out.println(result.orElse(0));  // Outputs 20
8    }
9}

Here are the key parts of the above code:

1. Creating an Optional Instance: We start by creating an Optional that wraps the value 10. This ensures we have a non-null value, providing a safe starting point for subsequent operations.

2. Mapping the Value: The map operation is applied to double the value within the Optional. This method returns a new Optional containing the transformed value, demonstrating the core principle of monads—applying a function to the wrapped value.

3. Filtering the Value: Next, we use the filter method to keep the value only if it meets a specific condition, in this case, being greater than 15. If the condition isn't met, the result becomes Optional.empty().

4. Handling the Result: Finally, orElse is used to handle the optional result, providing a default value of 0 if the Optional is empty. Since the value 20 passes the filter, it is printed as the output.

To access the underlying value inside an Optional, you can use several methods:

1. orElse(T other): Returns the value if present, otherwise returns other. For example:

   Java
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   1int value = result.orElse(0);  // Default value is 0 if Optional is empty

2. orElseGet(Supplier<? extends T> other): Returns the value if present, otherwise invokes the Supplier and returns its result.

   Java
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   1int value = result.orElseGet(() -> someDefaultMethod());

3. orElseThrow(Supplier<? extends X> exceptionSupplier): Returns the value if present, otherwise throws an exception created by the provided Supplier.

   Java
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   1int value = result.orElseThrow(() -> new NoSuchElementException("Value not present"));

4. get(): Returns the value if present; otherwise throws NoSuchElementException. This method should be used cautiously, as it can throw an exception if the Optional is empty.

   Java
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   1if(result.isPresent()) {
   2    int value = result.get();
   3}

Each method serves a different use case, allowing you to choose the one that best fits your specific scenario and error-handling strategy. In the next course, we'll explore additional error handling techniques to further enhance your code's robustness and reliability.

Monads are crucial for the following reasons:

• Managing Side Effects: Monads provide a structured way to handle side effects, making your code more predictable and easier to manage.
• Chaining Operations: Monads allow you to chain operations in a fluent, readable manner, simplifying complex transformations and workflows.
• Avoiding Nulls: Using Optional as a monad helps avoid the common pitfalls of null values, resulting in safer and more reliable code.

Monads are powerful tools that can significantly enhance your functional programming skills. Ready to dive into the practice section? Let’s get started!