Module 1 - Optionals

Learning Objectives

• Understand the purpose and benefits of using Optionals in Java
  Optionals provide a type-level solution to represent the presence or absence of a value, helping to avoid NullPointerExceptions and making code more readable and expressive.
• Implement a method that returns an empty Optional when it has no result
  When a method cannot produce a meaningful result, returning Optional.empty() is safer than returning null.

  public Optional<User> findUserById(String id) {
    // If user not found
    return Optional.empty();
  }
• Implement a method that wraps its return value in an Optional
  Learn to use Optional.of() and Optional.ofNullable() to create Optional instances that may contain values.

  public Optional<Product> findProduct(String sku) {
    Product product = database.lookup(sku);
    return Optional.ofNullable(product);
  }
• Implement code to access the wrapped value of an Optional
  Discover safe ways to extract values from Optionals using methods like get(), orElse(), orElseGet(), and orElseThrow().

  Optional<String> optional = getOptionalValue();
  
  // Safe access with default value
  String value = optional.orElse("default");
  
  // With supplier for lazy evaluation
  String computed = optional.orElseGet(() -> computeDefault());
  
  // Throwing custom exception if empty
  String required = optional.orElseThrow(() ->
    new IllegalStateException("Value must be present"));
  
• Explain the dangers of having a method return null
  Returning null requires callers to remember null checks, leads to NullPointerExceptions, and creates ambiguity about whether null is a valid return value or indicates an error condition.
• Outline how to use the filter, map, and flatMap methods of the Optional class
  These methods allow functional-style operations on Optional values, enabling powerful transformations and conditionals.

  Optional<User> user = findUser(id);
  
  // Filter: keeps value only if predicate is true
  Optional<User> activeUser = user.filter(u -> u.isActive());
  
  // Map: transforms the value if present
  Optional<String> username = user.map(User::getUsername);
  
  // FlatMap: for when the mapper returns an Optional
  Optional<Address> address = user.flatMap(User::getAddress);
  
• Outline when Optionals shouldn't be used
  Optionals aren't suitable for class fields, method parameters, or collections of optional elements. They should primarily be used as method return types when a value might be absent.