Stream API in Java 8: How to filter elements in a collection using filter() method

Stream API in Java 8: How to use the filter() method to filter elements in a collection

Introduction:
Java 8 introduces the Stream API, which provides a more concise way for us to process collections , a more efficient way. Stream is a new abstraction layer provided by Java 8, which allows us to process various data in a functional programming way. In the Stream API, the filter() method is one of the very useful methods, which can be used to filter elements in the collection. In this article, we will take a deeper look at the filter() method and explore its usage and examples.

1. What is the filter() method
The filter() method is an intermediate operation method in the Stream API. It receives a Predicate functional interface as a parameter, which is used to filter the elements in the collection. The Predicate interface defines a test(T t) method, which is used to conditionally judge the given parameters and return a Boolean value. The filter() method passes each element in the collection as a parameter to the test() method of Predicate. If true is returned, the element is retained, otherwise it is discarded.

2. How to use the filter() method
To use the filter() method, you first need to create a collection and then convert it to a Stream. Then call the filter() method and use the Predicate functional interface as parameter. The filter() method will return a new Stream containing elements that meet the criteria. Finally, process this new Stream by using the terminal operation method.

The following is a sample code showing the use of the filter() method:

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

public class FilterExample {
    public static void main(String[] args) {
        List<Integer> numbers = new ArrayList<>();
        numbers.add(1);
        numbers.add(2);
        numbers.add(3);
        numbers.add(4);
        numbers.add(5);

        // 使用filter()方法筛选偶数
        List<Integer> evenNumbers = numbers.stream()
                .filter(n -> n % 2 == 0)
                .collect(Collectors.toList());

        System.out.println("偶数列表：" + evenNumbers);
    }
}

In the above example, we first create a List object and add some numbers. Then we convert this List object to a Stream object and use the filter() method to filter out the even numbers. In the parameters of the filter() method, we use the Lambda expression n -> n % 2 == 0 to determine whether the number is even. Finally, we use the collect() method to convert the filtered results into a List object and print it out.

The output result is: "Even list: [2, 4]"

3. Application scenarios of the filter() method
The filter() method has many applications in actual development Scenes. Some common scenarios are listed below:

1. Filter out elements that meet a certain condition. For example, filter out all students older than 18 years old from a student list.

List<Student> adults = students.stream()
        .filter(s -> s.getAge() > 18)
        .collect(Collectors.toList());

2. Filter out a range of elements. For example, filter out all numbers greater than or equal to 10 and less than or equal to 20 from a list of numbers.

List<Integer> range = numbers.stream()
        .filter(n -> n >= 10 && n <= 20)
        .collect(Collectors.toList());

3. Filter out non-empty elements. For example, filter out all non-empty strings from a list of strings.

List<String> nonEmptyStrings = strings.stream()
        .filter(s -> s != null && !s.isEmpty())
        .collect(Collectors.toList());

Summary: The
filter() method is a very commonly used method in the Stream API, which can help us efficiently filter elements in the collection. By using the filter() method, we can process data in a concise and elegant way. I hope this article will help you understand the use of the filter() method and can be used flexibly in actual projects.

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