list数据比对与list对象比对

- 日理万妓 2022-04-10 04:26 300阅读 0赞

第一种方式使用工具类进行比较

package com.hzbank.counter.utils;
    
    import java.util.ArrayList;
    import java.util.HashMap;
    import java.util.List;
    import java.util.Map;
    
    /**
     * lists数据比对
     * @author nick
     */
    public class ListUtils {
    
       /* public static void main(String[] args) {
            List<String> list1 = new ArrayList<String>();
            List<String> list2 = new ArrayList<String>();
            for (int i = 0; i < 30000; i++) {
                list1.add("test" + i);
            }
            for (int i = 0; i < 80000; i++) {
                list2.add("test" + i * 2);
            }
            getDiffrentOne(list1, list2);
            getDiffrentTwo(list1, list2);
            getDiffrentThree(list1, list2);
            getDiffrentFour(list1, list2);
    
        }*/
    
        // 方法1，两层遍历查找，遍历次数为list1.size()*list2.size()，有点蠢
        private static List<String> getDiffrentOne(List<String> list1, List<String> list2) {
            // diff 存放不同的元素
            List<String> diff = new ArrayList<String>();
            // 开始查找的时间，用于计时
            /*long start = System.currentTimeMillis();*/
            for (String str : list1) {
                if (!list2.contains(str)) {
                    diff.add(str);
                }
            }
            // 计时
            /*System.out.println("方法1 耗时：" + (System.currentTimeMillis() - start) + " 毫秒");*/
            return diff;
        }
    
        // 方法2，两层遍历查找，用retainAll()方法查找，也很蠢，方法底层依旧是两层遍历
        private static List<String> getDiffrentTwo(List<String> list1, List<String> list2) {
            /*long start = System.currentTimeMillis();*/
            list1.retainAll(list2);// 返回值是boolean
            /*System.out.println("方法2 耗时：" + (System.currentTimeMillis() - start) + " 毫秒");*/
            return list1;
        }
    
        // 方法3，用Map存放List1和List2的元素作为key，value为其在List1和List2中出现的次数
        // 出现次数为1的即为不同元素，查找次数为list1.size() + list2.size()，较方法1和2，是极大简化
        private static List<String> getDiffrentThree(List<String> list1, List<String> list2) {
            List<String> diff = new ArrayList<String>();
            /*long start = System.currentTimeMillis();*/
            Map<String, Integer> map = new HashMap<String, Integer>(list1.size() + list2.size());
            // 将List1元素放入Map，计数1
            for (String string : list1) {
                map.put(string, 1);
            }
            // 遍历List2，在Map中查找List2的元素，找到则计数+1；未找到则放入map，计数1
            for (String string : list2) {
                Integer count = map.get(string);
                if (count != null) {
                    map.put(string, ++count);// 此处可优化，减少put次数，即为方法4
                    continue;
                }
                map.put(string, 1);
            }
            for (Map.Entry<String, Integer> entry : map.entrySet()) {
                if (entry.getValue() == 1) {
                    diff.add(entry.getKey());
                }
            }
            /*System.out.println("方法3 耗时：" + (System.currentTimeMillis() - start) + " 毫秒");*/
            return diff;
        }
    
        // 优化方法3，减少put次数
        private static List<String> getDiffrentFour(List<String> list1, List<String> list2) {
            List<String> diff = new ArrayList<String>();
           /* long start = System.currentTimeMillis();*/
            Map<String, Integer> map = new HashMap<String, Integer>(list1.size() + list2.size());
            List<String> maxList = list1;
            List<String> minList = list2;
            if (list2.size() > list1.size()) {
                maxList = list2;
                minList = list1;
            }
            for (String string : maxList) {
                map.put(string, 1);
            }
            for (String string : minList) {
                Integer count = map.get(string);
                if (count != null) {
                    map.put(string, ++count);
                    continue;
                }
                map.put(string, 1);
            }
            for (Map.Entry<String, Integer> entry : map.entrySet()) {
                if (entry.getValue() == 1) {
                    diff.add(entry.getKey());
                }
            }
            /*System.out.println("方法3 耗时：" + (System.currentTimeMillis() - start) + " 毫秒");*/
            return diff;
    
        }
    }

第二种使用com.google.common.collect的工具类

<dependency>
                <groupId>com.google.guava</groupId>
                <artifactId>guava</artifactId>
                <version>20.0</version>
            </dependency>

List<String> listA = new ArrayList<String>();
                List<String> listB = new ArrayList<String>();
                for (SessionIdB sessionIdB : sessionIdBll) {
                    listA.add(sessionIdB .getSessionIdB());
                }
                for (SessionIdA sessionIdA : sessionIdAll) {
                    listB.add(sessionIdA .getSessionIdA());
                }
                HashSet<String> setA = new HashSet<String>(listA);
                HashSet<String> setB = new HashSet<String>(listB);
                Sets.SetView<String> difference = Sets.difference(setA, setB);
                List<String> listC=new ArrayList<String>();
                for (String string : difference) {
                    logger.info("-----获取的sessionId--------"+string);
                    listC.add(string);
                }

《参考代码如下》  
set的交集, 并集, 差集

HashSet setA = newHashSet(1, 2, 3, 4, 5);  
    HashSet setB = newHashSet(4, 5, 6, 7, 8);  
       
    SetView union = Sets.union(setA, setB);  
    System.out.println("union:");  
    for (Integer integer : union)  
        System.out.println(integer);           //union:12345867
       
    SetView difference = Sets.difference(setA, setB);  
    System.out.println("difference:");  
    for (Integer integer : difference)  
        System.out.println(integer);        //difference:123
       
    SetView intersection = Sets.intersection(setA, setB);  
    System.out.println("intersection:");  
    for (Integer integer : intersection)  
        System.out.println(integer);  //intersection:45

第三种检查参数是否有值或者为空

方法一(数据量大，效率低)：
    if(list!=null && list.size()>0){
    }
    
    方法二(数据量大，效率高)：
    if(list!=null && !list.isEmpty()){
    }
    
    //use java
    if(list!=null && list.size()>0)
    '''
    if(str!=null && str.length()>0)
    '''
    if(str !=null && !str.isEmpty())
    
    //use guava
    if(!Strings.isNullOrEmpty(str))
    
    //use java
    if (count <= 0) {                                                                                           
        throw new IllegalArgumentException("must be positive: " + count);         
    }    
    
    //use guava
    Preconditions.checkArgument(count > 0, "must be positive: %s", count);