交集处理null,empty的情况

GetSpecificField.java

package org.caucoder.mylist;

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

/**
 * list 转set并操作
 */
public class GetSpecificField {
    public static void main(String[] args) {
        List<Entity> enList = new ArrayList<>(Arrays.asList(new Entity("huang","zhuangzhuang"),
                    new Entity("Q10","Viking")));

        List<Entity> cnList = new ArrayList<>(Arrays.asList(new Entity("Zhang","qiang"),
                new Entity("Q10","Viking")));
        Set<String> fnameEnSet = enList.stream().map(Entity::getFname).collect(Collectors.toSet());
        Set<String> fnameCnSet = cnList.stream().map(Entity::getFname).collect(Collectors.toSet());
        System.out.println("fnameEnList: "+fnameEnSet);
        System.out.println("fnameCnList: "+fnameCnSet);
        System.out.println("=============================");
        Set<String> commonSet = new HashSet<>(fnameCnSet);
        commonSet.retainAll(fnameEnSet);
        System.out.println("交集： "+commonSet);
        System.out.println("fnameEnList: "+fnameEnSet);
        System.out.println("fnameCnList: "+fnameCnSet);
        System.out.println("=============================");
        fnameCnSet.removeAll(commonSet);
        fnameEnSet.removeAll(commonSet);
        System.out.println("fnameEnList 除去交集: "+fnameEnSet);
        System.out.println("fnameCnList 除去交集: "+fnameCnSet);
        System.out.println("=============================");
        System.out.println("如果set为empty");
        Set<String> tmp = new HashSet<>();
        fnameCnSet.removeAll(tmp);
        fnameEnSet.removeAll(tmp);
        System.out.println("fnameEnList 除去交集empty: "+fnameEnSet);
        System.out.println("fnameCnList 除去交集empty: "+fnameCnSet);
        System.out.println("如果set为null");
        fnameCnSet.removeAll(null);
        fnameEnSet.removeAll(null);
        System.out.println("fnameEnList 除去交集null: "+fnameEnSet);
        System.out.println("fnameCnList 除去交集null: "+fnameCnSet);
    }
}
/**
 fnameEnList: [Q10, huang]
 fnameCnList: [Q10, Zhang]
 =============================
 交集： [Q10]
 fnameEnList: [Q10, huang]
 fnameCnList: [Q10, Zhang]
 =============================
 fnameEnList 除去交集: [huang]
 fnameCnList 除去交集: [Zhang]
 =============================
 如果set为empty
 fnameEnList 除去交集empty: [huang]
 fnameCnList 除去交集empty: [Zhang]
 如果set为null
 Exception in thread "main" java.lang.NullPointerException
 at java.util.Objects.requireNonNull(Objects.java:203)
 at java.util.AbstractSet.removeAll(AbstractSet.java:169)
 at org.caucoder.mylist.GetSpecificField.main(GetSpecificField.java:39)
 */