What is a Stream in Java?

A Stream in Java is a sequence of elements supporting sequential and parallel aggregate operations. It is not a data structure, but a view of data that allows for functional-style operations on collections of elements.

Main characteristics:

• Does not modify the source: A Stream does not modify the collection or array it was created from.
• Lazily evaluated: Intermediate operations like filter, map are not executed immediately. Execution begins only when a terminal operation such as forEach, collect, or sum is invoked.
• Can be used only once: After a terminal operation, the Stream is considered closed and cannot be reused.

Main types of operations:

• Intermediate: Return a new Stream. Examples: filter(), map(), sorted(), distinct(), limit(), skip().
• Terminal: Return a result or a side-effect and end the Stream pipeline. Examples: forEach(), collect(), count(), sum(), average(), reduce(), min(), max(), anyMatch(), allMatch(), noneMatch().

Example usage:

// Creating a Stream from a list of strings
List<String> names = Arrays.asList("Alice", "Bob", "Charlie", "David");

// Intermediate operation: filter names starting with 'A'
Stream<String> filteredNamesStream = names.stream()
                                          .filter(name -> name.startsWith("A"));

// Intermediate operation: convert all names to uppercase
Stream<String> upperCaseNamesStream = filteredNamesStream
                                              .map(String::toUpperCase);

// Terminal operation: print the filtered and transformed names
upperCaseNamesStream.forEach(System.out::println);

// Chained version:
names.stream()
     .filter(name -> name.startsWith("A")) // Intermediate
     .map(String::toUpperCase) // Intermediate
     .forEach(System.out::println); // Terminal

Using the Stream API simplifies working with collections, making code more readable and efficient, especially when parallelized.