Java Streams API is a powerful feature introduced in Java 8 that allows developers to perform functional-style operations on collections. It simplifies the code by enabling the use of lambda expressions and functional interfaces. In order to make the most out of Java Streams API, it is important to understand the functional programming concepts that it is built upon. Let’s dive into some of these key concepts.

1. Higher-order functions

Higher-order functions are functions that can accept other functions as parameters or return functions as results. In Java Streams API, operations like map(), filter(), and reduce() are examples of higher-order functions. These operations take functional interfaces, such as Function, Predicate, and BinaryOperator, as arguments.

Using map() to transform elements in a collection is an example of a higher-order function. It takes a Function as an argument, which specifies the transformation to be applied to each element.

List<Integer> numbers = List.of(1, 2, 3, 4, 5);
List<Integer> squaredNumbers = numbers.stream()
                                      .map(x -> x * x)
                                      .collect(Collectors.toList());

2. Lazy evaluation

Java Streams API uses lazy evaluation, which means that the operations on a stream are only executed when a terminal operation is invoked. This allows for more efficient processing of large data sets as only the necessary elements are processed.

For example, in the following code snippet, the filter() operation is not evaluated until the collect() operation is called. Hence, only the even numbers are added to the new list.

List<Integer> numbers = List.of(1, 2, 3, 4, 5);
List<Integer> evenNumbers = numbers.stream()
                                   .filter(x -> x % 2 == 0)
                                   .collect(Collectors.toList());

Conclusion

By understanding the functional programming concepts behind Java Streams API, you can write efficient and concise code. Higher-order functions and lazy evaluation are key concepts that enable you to perform functional-style operations on collections. Embracing these concepts will help you take full advantage of the Java Streams API and write clean, readable, and maintainable code.

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