List的快速除重,多种实现

青旅半醒 2021-10-14 20:27 130阅读 0赞

## 只有代码 ##

package list去重操作;
    
    import com.google.common.collect.Lists;
    import model.Book;
    
    import java.util.*;
    import java.util.concurrent.ConcurrentHashMap;
    import java.util.function.Function;
    import java.util.function.Predicate;
    import java.util.stream.Collectors;
    
    // list去重操作
    
    public class ListRepeat {
    
        public static void main(String[] args) {
    
            List<Book> initList = init(); // 这里先初始化一个有重复的 list
            System.out.println("初始化的list为：");
            initList.forEach(b -> System.out.println(b));
    
    
            System.out.println("distinct1后的的list为：");
            List<Book> books = distinct1(initList);
            books.forEach(b -> System.out.println(b));
    
            System.out.println("distinct2后的的list为：");
            List<Book> books2 = distinct2(initList);
            books2.forEach(b -> System.out.println(b));
    
            System.out.println("distinct21后的的list为：");
            List<Book> books21 = distinct21(initList);
            books21.forEach(b -> System.out.println(b));
    
            System.out.println("distinct3后的的list为：");
            List<Book> books3 = distinct3(initList);
            books3.forEach(b -> System.out.println(b));
    
    
        }
    
    
        public static List<Book> init() {
            Book b1 = new Book().setBookId(1).setPrice(21.3).setLocation("sh").setAuthor("太宰治").setBookName("人间失格");
            Book b2 = new Book().setBookId(1).setPrice(21.36).setLocation("sd").setAuthor("村上").setBookName("当跑步时");
            Book b3 = new Book().setBookId(2).setPrice(13.3).setLocation("ah").setAuthor("黑泽").setBookName("七武士");
            Book b5 = new Book().setBookId(4).setPrice(26.3).setLocation("hb").setAuthor("张嘉译").setBookName("陪你到世界尽头");
            Book b4 = new Book().setBookId(3).setPrice(24.3).setLocation("hn").setAuthor("马克").setBookName("游记");
            Book b6 = new Book().setBookId(5).setPrice(28.3).setLocation("sh").setAuthor("契诃夫").setBookName("taozh");
    
    //        Book b7 = new Book().setBookId(3).setPrice(24.3).setLocation("hn").setAuthor("马克").setBookName(null);
    //        Book b8 = new Book().setBookId(5).setPrice(28.3).setLocation("sh").setAuthor("契诃夫").setBookName(null);
    
            // 需要注意的是，key 为 null 的时候，会出现空指针的异常
    
            Book b9 = new Book().setBookId(3).setPrice(24.3).setLocation("hn").setAuthor("马克").setBookName("  ");
            Book b10 = new Book().setBookId(5).setPrice(28.3).setLocation("sh").setAuthor("契诃夫").setBookName("   ");
    
            return Lists.newArrayList(b1, b2, b3, b4, b5, b6,/*b7,b8,*/b9, b10);
        }
    
        // list去重操作 一
    
        public static List<Book> distinct1(List<Book> list) {
            list = list.stream()
                    .collect(Collectors.collectingAndThen(
                            Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(Book::getBookId))),
                            ArrayList::new));
            return list;
        }
    
        // list去重操作 二
    
        public static List<Book> distinct2(List<Book> list) {
            List<Book> collect = list.stream().filter(distinctByKey(Book::getBookId)).collect(Collectors.toList());
            return collect;
        }
    
        // 除重 2.1
        public static List<Book> distinct21(List<Book> list) {
            List<Book> collect = list.stream().filter(distinctByKey(Book::getBookName)).collect(Collectors.toList());
            return collect;
        }
        public static <T> Predicate<T> distinctByKey(Function<? super T, ?> keyExtractor) {
            Set<Object> seen = ConcurrentHashMap.newKeySet();
            return t -> seen.add(keyExtractor.apply(t));
        }
    
    
        // list去重操作 三
    
        public static List<Book> distinct3(List<Book> list) {
            Collection<List<Book>> values = list.stream().collect(Collectors.groupingBy(p -> p.getBookId())).values();
            List<Book> collect = values.stream().map(l -> l.get(0)).collect(Collectors.toList());
            return collect;
        }
    
    
    }