Hey there, coding enthusiasts! Are you gearing up for a Java interview, or maybe you just want to level up your Java skills? Well, you've landed in the right place! We're diving deep into the Java Stream API with a bunch of practice questions designed to get you comfortable and confident. The Java Stream API is a powerful tool for processing collections of data in a functional style. Understanding it is crucial, not just for interviews but also for writing cleaner, more efficient, and more readable code. We'll be covering everything from basic stream operations to more complex scenarios, giving you a well-rounded understanding. Ready to get started? Let's jump in and make you a Java Stream API pro!

Getting Started with the Java Stream API: The Fundamentals

Alright, before we get to the juicy practice questions, let's make sure we're all on the same page with the basics. The Java Stream API was introduced in Java 8 and provides a new way to process collections of data. Think of it as a pipeline where you can perform various operations on your data, such as filtering, mapping, sorting, and reducing. One of the main benefits is its ability to handle data in a declarative manner, which means you specify what you want to do rather than how to do it. This often leads to more concise and readable code. The Stream API operates on a sequence of elements, which can come from collections, arrays, or I/O resources. Streams don't store data; they operate on the source and produce a result. A key concept is that streams are lazy; operations are only performed when a terminal operation is called. This lazy evaluation helps in optimizing performance. Streams can be either sequential or parallel. Sequential streams perform operations one after another, while parallel streams can utilize multiple threads to process data concurrently, potentially speeding up the process on multi-core processors. You create a stream using the stream() method on a collection or with the Stream.of() method. The general workflow involves creating a stream, applying intermediate operations (like filter, map, sorted), and then using a terminal operation (like collect, forEach, count) to produce a result. To really grasp the concepts, let's look at some examples and then dive into practice questions. For example, to filter a list of numbers and get only the even ones, you would use .filter(n -> n % 2 == 0). The map operation can transform each element, such as converting a list of strings to uppercase using .map(String::toUpperCase). Sorting is straightforward with .sorted(), and you can collect the results back into a list using .collect(Collectors.toList()). Now, with these concepts in mind, let’s tackle some practice questions!

Core Operations and Methods

The Java Stream API revolves around a set of core operations that allow you to manipulate data effectively. Understanding these methods is crucial for mastering the API. Here's a breakdown:

• filter(Predicate<T> predicate): This is an intermediate operation that allows you to select elements based on a condition. The predicate is a functional interface that represents a boolean-valued function. Essentially, you provide a condition, and the filter method keeps only the elements that satisfy that condition.
• map(Function<T, R> mapper): Another intermediate operation, map, transforms each element of the stream. The mapper is a functional interface that takes an element of type T and returns an element of type R. This allows you to apply a function to each element and change its type or value.
• sorted(): An intermediate operation that sorts the elements of the stream. By default, it sorts elements in their natural order, but you can also provide a Comparator to specify a custom sorting order.
• distinct(): An intermediate operation that ensures that only unique elements are present in the stream. It removes duplicate elements based on their equals() method.
• limit(long maxSize): An intermediate operation that limits the stream to a specified number of elements. It truncates the stream to the first maxSize elements.
• skip(long n): An intermediate operation that skips the first n elements of the stream and returns the remaining elements.
• forEach(Consumer<T> action): A terminal operation that performs an action on each element of the stream. The action is a functional interface that consumes an element and performs an operation.
• collect(Collector<T, A, R> collector): A terminal operation that performs a reduction operation on the elements of the stream. Collectors are used to accumulate elements into a collection, such as a List, Set, or Map, or to perform other reduction operations like calculating the sum or average.
• count(): A terminal operation that returns the number of elements in the stream.
• anyMatch(Predicate<T> predicate): A terminal operation that checks if any element in the stream matches the given predicate. It returns true if at least one element matches; otherwise, it returns false.
• allMatch(Predicate<T> predicate): A terminal operation that checks if all elements in the stream match the given predicate. It returns true if all elements match; otherwise, it returns false.
• noneMatch(Predicate<T> predicate): A terminal operation that checks if no elements in the stream match the given predicate. It returns true if no elements match; otherwise, it returns false.
• reduce(BinaryOperator<T> accumulator): A terminal operation that reduces the elements of the stream to a single value by repeatedly applying a combining operation. This is a very powerful operation that can be used for a wide range of tasks, such as calculating the sum, product, or finding the minimum or maximum value.

Mastering these operations is your key to unlocking the full potential of the Java Stream API. Now, let's put these methods into action with some practice problems.

Practice Questions: Let's Get Coding!

Alright, time to get our hands dirty with some practice questions! These questions are designed to get you comfortable with common stream operations and to test your understanding. Try to solve these problems yourself before looking at the solutions. This is the best way to learn! Remember, the key is to understand why a particular approach works.

Question 1: Filtering Even Numbers

Problem: Given a list of integers, write a Java Stream to filter out only the even numbers and collect them into a new list.

Input: List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);

Expected Output: [2, 4, 6, 8, 10]

Solution: Here's how you can solve this using the Java Stream API. This question is a classic example of using the filter operation.

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

public class EvenNumbers {
    public static void main(String[] args) {
        List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);

        List<Integer> evenNumbers = numbers.stream()
                .filter(n -> n % 2 == 0)
                .collect(Collectors.toList());

        System.out.println(evenNumbers); // Output: [2, 4, 6, 8, 10]
    }
}

Explanation: We create a stream from the numbers list. Then, we use the filter operation with a lambda expression n -> n % 2 == 0 to keep only the even numbers. Finally, we use collect(Collectors.toList()) to gather the filtered elements into a new list.

Question 2: Transforming Strings to Uppercase

Problem: Given a list of strings, convert all strings to uppercase using the Java Stream API.

Input: `List strings = Arrays.asList(