背景

Java8使用Stream編寫出來的代碼相當簡潔, 有時候需要使用distinct()方法去重, 針對比較復雜的對象的時候, distinct()就沒法工作了.

解決方案

定義一個輔助方法:

private static <T> Predicate<T> distinctByKey(Function<? super T, Object> keyExtractor) {
    Set<Object> seen = ConcurrentHashMap.newKeySet();
    return t -> seen.add(keyExtractor.apply(t));
}

然后通過如下調用:

list.stream().filter(distinctByKey(Book::getName)).forEach(System.out::println);

完整代碼如下

package com.github.stream;

import java.util.ArrayList;
import java.util.List;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;
import java.util.function.Function;
import java.util.function.Predicate;

public class DistinctByProperty {
    public static void main(String[] args) {
        List<Book> list = new ArrayList<>();
        list.add(new Book("Core Java", 200));
        list.add(new Book("Core Java", 300));
        list.add(new Book("Learning Freemarker", 150));
        list.add(new Book("Spring MVC", 200));
        list.add(new Book("Hibernate", 300));

        list.stream().filter(distinctByKey(Book::getName)).forEach(System.out::println);
    }

    private static <T> Predicate<T> distinctByKey(Function<? super T, Object> keyExtractor) {
        Set<Object> seen = ConcurrentHashMap.newKeySet();
        return t -> seen.add(keyExtractor.apply(t));
    }

    static class Book {
        private String name;
        private int price;

        public Book(String name, int price) {
            this.name = name;
            this.price = price;
        }

        public String getName() {
            return name;
        }

        public int getPrice() {
            return price;
        }

        @Override
        public String toString() {
            return "Book{" +
                    "name='" + name + '\'' +
                    ", price=" + price +
                    '}';
        }
    }
}

執(zhí)行結果如下:

Book{name='Core Java', price=200}
Book{name='Learning Freemarker', price=150}
Book{name='Spring MVC', price=200}
Book{name='Hibernate', price=300}