List.stream().map()

In Java, List.stream().map() is a powerful combination of methods used to transform elements in a list using the Stream API. Let's break down what each part does:

List

A List in Java is an ordered collection (also known as a sequence). The List interface provides a way to store elements and access them by their index. It is a part of the Java Collections Framework and can be implemented by various classes like ArrayList, LinkedList, etc.

stream()

The stream() method is a default method of the Collection interface, and since List is a sub-interface of Collection, all List instances can use this method. The stream() method returns a sequential Stream with the elements of the list as its source. A Stream represents a sequence of elements on which one or more operations can be performed.

map()

The map() method is an intermediate operation of the Stream interface. It takes a Function as an argument and applies this function to each element of the stream, producing a new stream with the transformed elements.

Combining List.stream().map()

When you combine List.stream() with .map(), you are essentially creating a stream from a list and then transforming each element of that list using a specified function.

Here is a concrete example to illustrate this:

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class Example {
    public static void main(String[] args) {
        List<String> strings = Arrays.asList("a", "b", "c", "d");

        // Using List.stream().map() to transform elements
        List<String> upperCaseStrings = strings.stream()
                                               .map(String::toUpperCase)
                                               .collect(Collectors.toList());

        System.out.println(upperCaseStrings); // Output: [A, B, C, D]
    }
}

In this example:

1. Arrays.asList("a", "b", "c", "d") creates a list of strings.
2. strings.stream() converts the list into a stream.
3. map(String::toUpperCase) applies the toUpperCase method to each element of the stream, transforming each string to its uppercase equivalent.
4. collect(Collectors.toList()) collects the transformed elements into a new list.

The result is that the original list of lowercase strings is transformed into a list of uppercase strings.

Summary

• List: A collection of elements.
• stream(): Converts the list into a stream.
• map(): Applies a function to each element of the stream, transforming it.
• Example: List.stream().map() is used to create a stream from a list and apply a transformation function to each element, resulting in a new stream of transformed elements. This new stream can then be collected back into a list or another collection if needed.