List集合根据对象某个属性去重

import java.util.concurrent.ConcurrentHashMap;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.stream.Collectors;

public class Test {
    public static void main(String[] args) {
        List<User> list = new Lists.newArrayList();
        list.add(new user(1,"张三","上海"));
        list.add(new user(2,"李四","北京"));
        list.add(new user(3,"王五","济南"));
        list.add(new user(4,"李四","北京"));

        // 根据list中的City城市来去重
        newList = list.stream().filter(distinctByKey(User::getCity)).collect(Collectors.toList());
    }
    
    private static <T> Predicate<T> distinctByKey(Function<? super T, Object> keyExtractor) {
        Map<Object, Boolean> seen = new ConcurrentHashMap<>();
        return t -> seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;
    }
}