JAVA使用下面的方法头编写方法，返回两个数组列表的并集

import java.util.Set;
import java.util.List;
import java.util.HashSet;
import java.util.TreeSet;
import java.util.Iterator;
import java.util.ArrayList;
import java.util.LinkedList;
 
public class GAGA {
    
    public static void main(String args[]) {
        getList();
    }
    
    // 获取两个ArrayList的差集、交集、去重并集(数据量大小不限制)
    private static void getList() {
        List<String> firstArrayList = new ArrayList<String>();
        List<String> secondArrayList = new ArrayList<String>();
        List<String> defectList = new ArrayList<String>();//差集List
        List<String> collectionList = new ArrayList<String>();//交集List
        List<String> unionList = new ArrayList<String>();//去重并集List
        try {
            firstArrayList.add("3");
            firstArrayList.add("5");
            firstArrayList.add("45");
            firstArrayList.add("4");
            firstArrayList.add("3");
 
            secondArrayList.add("33");
            secondArrayList.add("51");
            secondArrayList.add("5");
            secondArrayList.add("4");
            secondArrayList.add("13");
            // 获取差集
            defectList = receiveDefectList(firstArrayList, secondArrayList);
            Iterator<String> defectIterator = defectList.iterator();
            System.out.println("===================差集===================");
            while(defectIterator.hasNext()) {
                System.out.println(defectIterator.next());
            }
            // 获取交集
            collectionList = receiveCollectionList(firstArrayList, secondArrayList);
            Iterator<String> collectionIterator = collectionList.iterator();
            System.out.println("===================交集===================");
            while(collectionIterator.hasNext()) {
                System.out.println(collectionIterator.next());
            }
            // 获取去重并集
            unionList = receiveUnionList(firstArrayList, secondArrayList);
            Iterator<String> unionIterator = unionList.iterator();
            System.out.println("===================去重并集===================");
            while(unionIterator.hasNext()) {
                System.out.println(unionIterator.next());
            }
        }catch(Exception e) {
            e.printStackTrace();
        }
    }
    
    /**
     * @方法描述：获取两个ArrayList的差集
     * @param firstArrayList 第一个ArrayList
     * @param secondArrayList 第二个ArrayList
     * @return resultList 差集ArrayList
     */
    public static List<String> receiveDefectList(List<String> firstArrayList, List<String> secondArrayList) {
        List<String> resultList = new ArrayList<String>();
        LinkedList<String> result = new LinkedList<String>(firstArrayList);// 大集合用linkedlist  
        HashSet<String> othHash = new HashSet<String>(secondArrayList);// 小集合用hashset  
        Iterator<String> iter = result.iterator();// 采用Iterator迭代器进行数据的操作  
        while(iter.hasNext()){  
            if(othHash.contains(iter.next())){  
                iter.remove();            
            }     
        }  
        resultList = new ArrayList<String>(result);
        return resultList;
    }
    
    /**
     * @方法描述：获取两个ArrayList的交集
     * @param firstArrayList 第一个ArrayList
     * @param secondArrayList 第二个ArrayList
     * @return resultList 交集ArrayList
     */
    public static List<String> receiveCollectionList(List<String> firstArrayList, List<String> secondArrayList) {
        List<String> resultList = new ArrayList<String>();
        LinkedList<String> result = new LinkedList<String>(firstArrayList);// 大集合用linkedlist  
        HashSet<String> othHash = new HashSet<String>(secondArrayList);// 小集合用hashset  
        Iterator<String> iter = result.iterator();// 采用Iterator迭代器进行数据的操作  
        while(iter.hasNext()) {
            if(!othHash.contains(iter.next())) {  
                iter.remove();            
            }     
        }
        resultList = new ArrayList<String>(result);
        return resultList;
    }
    
    /**
     * @方法描述：获取两个ArrayList的去重并集
     * @param firstArrayList 第一个ArrayList
     * @param secondArrayList 第二个ArrayList
     * @return resultList 去重并集ArrayList
     */
    public static List<String> receiveUnionList(List<String> firstArrayList, List<String> secondArrayList) {
        List<String> resultList = new ArrayList<String>();
        Set<String> firstSet = new TreeSet<String>(firstArrayList);
        for(String id : secondArrayList) {
            // 当添加不成功的时候 说明firstSet中已经存在该对象
            firstSet.add(id);
        }
        resultList = new ArrayList<String>(firstSet);
        return resultList;
    }
}
 

输出结果如图：