Java Stream操作

Java8 Stream操作用的很少，特地写一些例子理解，方便以后使用

/**
 * 功能描述: 书实体类
 * @Author: XinHai.Ma
 * @Date: 2020/9/22 21:19
 */
class Book {
    private long bookId;
    private String bookName;
    private String bookAuthor;
    private String bookDesc;

    public Book() {}

    public Book(long bookId, String bookName, String bookAuthor, String bookDesc) {
        this.bookId = bookId;
        this.bookName = bookName;
        this.bookAuthor = bookAuthor;
        this.bookDesc = bookDesc;
    }

    public long getBookId() {
        return bookId;
    }

    public void setBookId(long bookId) {
        this.bookId = bookId;
    }

    public String getBookName() {
        return bookName;
    }

    public void setBookName(String bookName) {
        this.bookName = bookName;
    }

    public String getBookAuthor() {
        return bookAuthor;
    }

    public void setBookAuthor(String bookAuthor) {
        this.bookAuthor = bookAuthor;
    }

    public String getBookDesc() {
        return bookDesc;
    }

    public void setBookDesc(String bookDesc) {
        this.bookDesc = bookDesc;
    }

    @Override
    public String toString() {
        return "Book{" +
                "bookId=" + bookId +
                ", bookName='" + bookName + '\'' +
                ", bookAuthor='" + bookAuthor + '\'' +
                ", bookDesc='" + bookDesc + '\'' +
                '}';
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Book book = (Book) o;
        return bookId == book.bookId &&
                Objects.equals(bookName, book.bookName) &&
                Objects.equals(bookAuthor, book.bookAuthor) &&
                Objects.equals(bookDesc, book.bookDesc);
    }

    @Override
    public int hashCode() {
        return Objects.hash(bookId, bookName, bookAuthor, bookDesc);
    }
}

 

使用例子：

import java.util.*;
import java.util.stream.Collectors;

public class StreamDemo {

    public static void main(String[] args) {
        List<Book> bookList = initData();

        // map 利用function提取出一个集合
        // 提取id集合
        bookList.stream().map(Book::getBookId).forEach(System.out::println);
        // 提取bookName集合
        bookList.stream().map(Book::getBookName).forEach(System.out::println);

        // 过滤出满足条件的数据
        bookList.stream().filter(book -> {
            if(book.getBookId() > 50) {
                return true;
            }
            return false;
        }).forEach(System.out::println);

        System.out.println("==去除重复元素==");
        // 向bookList集合添加一个重复元素
        bookList.add(new Book(1, "book1", "author1", "desc1"));
        // 去掉重复元素 distinct实现依赖于hashCode和equals方法
        bookList.stream().distinct().forEach(System.out::println);

        System.out.println("==定制排序==");

        // 定制排序 倒序
        bookList.stream().sorted((book1, book2) -> {
            return Integer.parseInt(String.valueOf(book2.getBookId() - book1.getBookId()));
        }).forEach(System.out::println);

        // 匹配
        boolean findResult = bookList.stream().allMatch(book -> book.getBookName().equals("book99"));
        System.out.println("匹配结果: " + findResult);

        findResult = bookList.stream().anyMatch(book -> book.getBookName().equals("book88"));
        System.out.println("匹配结果: " + findResult);

        findResult = bookList.stream().noneMatch(book -> book.getBookName().equals("book77"));
        System.out.println("匹配结果: " + findResult);

        // 查询最大值元素
        Optional<Book> max = bookList.stream().max((book1, book2) -> {
            return Integer.parseInt(String.valueOf(book1.getBookId() - book2.getBookId()));
        });
        System.out.println("max: " + max.get());

        // 查询最小值元素
        Optional<Book> min = bookList.stream().min((book1, book2) -> {
            return Integer.parseInt(String.valueOf(book1.getBookId() - book2.getBookId()));
        });
        System.out.println("min: " + min.get());

        // 筛选出bookName为book1的集合，找出集合第一个元素
        Optional<Book> first = bookList.stream().filter(book -> {
            if(book.getBookName().equals("book1")) {
                return true;
            }
            return false;
        }).findFirst();
        System.out.println("first: " + first.get());

        long total = bookList.stream().count();
        System.out.println("集合大小: " + total);


        // 去除重复元素，将结果转换为Set集合
        Set<Book> set = bookList.stream().distinct().collect(Collectors.toSet());
        // 将List根据BookName为key转化为Map
        Map<String, List<Book>> map = bookList.stream().collect(Collectors.groupingBy(Book::getBookName));
        // 将bookName集合放到一个新集合
        bookList.stream().map(Book::getBookName).collect(Collectors.toList());


        // flatMap 层次结构扁平化 还不是很理解，就知道要这么用
        List<Book> bookList1 = new ArrayList<>();
        bookList1.addAll(bookList);
        List<Book> bookList2 = new ArrayList<>();
        bookList2.addAll(bookList);
        List<Book> bookList3 = new ArrayList<>();
        bookList3.addAll(bookList);
        List<List<Book>> books = new ArrayList<>(3);
        books.add(bookList1);
        books.add(bookList2);
        books.add(bookList3);
        List<Book> bookStream = books.stream().flatMap(List::stream).collect(Collectors.toList());
        bookStream.forEach(System.out::println);
    }

    /**
     * 功能描述: 初始化数据
     * @Param: []
     * @Return: java.util.List<stream.Book>
     * @Author: XinHai.Ma
     * @Date: 2020/9/22 21:18
     */
    public static List<Book> initData() {
        List<Book> bookList = new ArrayList<>(100);
        for (int i=1; i<=100; i++) {
            Book book = new Book(i, "book"+i, "author"+i, "desc"+i);
            bookList.add(book);
        }
        return bookList;
    }

}

 

将List集合整理为Map、Set集合：

// 查询数据
List<RouteLine> lineList = routeLineRepository.findAll();
Map<String, RouteLine> map = lineList.stream().collect(Collectors.toMap(RouteLine::getCode, RouteLine -> RouteLine));
ConcurrentMap<String, String> concurrentMap = lineList.stream().collect(Collectors.toConcurrentMap(RouteLine::getCode, RouteLine::getName));
Set<String> set = lineList.stream().map(RouteLine::getCode).collect(Collectors.toSet());

 

越长大越孤单，哎