5.4. Stream Operations

Stream operations are of two types:

1. Intermediate Operations (return a stream):

   • filter(): Filters elements based on a condition. • map(): Transforms elements. • sorted(): Sorts elements. • distinct(): Removes duplicates. • limit(n): Limits output to n elements.

2. Terminal Operations (produce a result or side-effect):

   • forEach(): Performs an action for each element. • collect(): Converts the stream into a collection or result. • reduce(): Reduces stream to a single value. • count(): Counts elements. • anyMatch(), allMatch(), noneMatch(): Matching operations.

Stream operations in Java are divided into two main categories: Intermediate Operations and Terminal Operations.

• Intermediate Operations are operations that return a new stream. This means they can be chained together to form a query. Examples include:

  • filter(): Selects elements from a stream based on a specified condition. For example, filtering a list to include only names that start with 'A'.
  • map(): Transforms each element in the stream. For instance, converting all names to uppercase.
  • sorted(): Orders the elements in natural order or by a specified comparator.
  • distinct(): Removes duplicate elements from the stream.
  • limit(n): Restricts the output to the first 'n' elements.

• Terminal Operations, on the other hand, produce a result or a side effect. They do not return a stream. Examples include:

  • forEach(): Executes a given action for each element in the stream.
  • collect(): Gathers the elements from the stream into a collection.
  • reduce(): Combines elements in the stream into a single value through a specified binary operation (e.g., sum).
  • count(): Counts the number of elements in the stream.
  • anyMatch(), allMatch(), noneMatch(): Used for checking conditions on the stream elements.

Imagine you are organizing a group of friends for a movie night. You first create a list of friends (the stream).

1. You might use filter() to create a subset of friends who said they are available.
2. Then, you can use map() to transform their names to uppercase when sending out the invitations for fun.
3. If you find duplicates in your invite list, you can apply distinct() to ensure everyone gets invited just once.
4. Finally, you might use forEach() to send the invitation to each friend about the movie night.
   In this analogy, your list of friends correspond to a stream, and the various operations correspond to the stream operations you can perform to manipulate the list and notify them.

Example:

List<String> names = Arrays.asList("Alice", "Bob", "Charlie", "David");
names.stream()
    .filter(n -> n.startsWith("C"))
    .map(String::toUpperCase)
    .forEach(System.out::println);

This example illustrates the usage of stream operations in Java.

• First, a list containing names is created using Arrays.asList().
• The stream() method is called on the list, which creates a stream from it.
• filter() is then applied to the stream to keep only those names that start with 'C'. The lambda expression n -> n.startsWith("C") serves as the condition for filtering.
• Next, map() is used to convert the remaining names to uppercase. Using String::toUpperCase is a method reference that makes the code cleaner.
• Finally, forEach() prints each name to the console. This operation goes over all of the names in the stream and prints them out, effectively giving us the uppercase version of 'Charlie'. Overall, this demonstrates a typical use case of stream operations: establishing a pipeline where data flows through several transformations.

Think of the above example as a process of sorting through book titles. You have a list of titles: 'Alice in Wonderland', 'Bob the Builder', 'Charlie and the Chocolate Factory', and 'David Copperfield'.

1. First, you decide to only keep the titles that start with 'C'—like filtering out books you’re not interested in.
2. Then, you want to highlight the favorite title, so you transform 'Charlie and the Chocolate Factory' to make it stand out in uppercase.
3. Finally, you share the selected title with your friends. In this scenario, each of the stream operations corresponds to the steps you took to narrow down and highlight your favorite book title.

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