Java实现List转换为Map的方法小结

目录

• 一、List转换为Map
• 1.1、List转化为Map<Integer,String>
• 1.2 、List转化为Map<Integer,Object>
• 1.3、List转化为Map<Integer,List>
• 1.4 List转化为Map<Integer,List>
• 1.5、 List<Map<String,Object>> 转化为Map<String,Object>
• 1.6、List<Map<String,String>> 转化为Map<String,Map<String,String>>
• 1.7、List<Map<String,String>> 转化为Map<String,String>

一、List转换为Map

示例：代码案例

public class People ( private Integer id; private String name; private Integer age private String address; //TODO 构造方法和get和set方法省略 ) 

final static List<People> peopleList = new ArrayList<People>(); // 初始化集合数据 static { People p1 = new People(0001, "张三", 12, "江苏南京"); People p2 = new People(0002, "李四", 14, "上海"); People p3 = new People(0003, "王二", 11, "浙江台州"); People p4 = new People(0004, "李五", 12, "河南郑州"); peopleList.add(p1); peopleList.add(p2); peopleList.add(p3); peopleList.add(p4); }

1.1、List转化为Map<Integer,String>

Map<Integer, String> map = peopleList.stream().collect(Collectors.toMap (People::getId, People::getAddress, (value1, value2) -> value1)); 

1.2 、List转化为Map<Integer,Object>

Map<Integer, People> map = peopleList.stream().collect(Collectors.toMap (People::getId, each -> each, (value1, value2) -> value1)); 

1.3、List转化为Map<Integer,List>

Map<Integer, List<People>> map = peopleList.stream().collect(Collectors .groupingBy(People::getId)); 

1.4 List转化为Map<Integer,List>

Map<Integer, List<String>> map3 = peopleList.stream().collect(Collectors. toMap(People::getId, each -> Collections.singletonList(each.getName()), (value1, value2) -> { List<String> union = new ArrayList<>(value1); union.addAll(value2); return union; })); 

1.5、 List<Map<String,Object>> 转化为Map<String,Object>

final static List<Map<String, Object>> mapStudentList = new ArrayList<>(); public static void main(String[] args) { Map<String, Object> map4 = mapStudentList.stream().collect(Collectors.toMap(each -> Objects.toString(each.get("id"), ""), each -> each.get("student"), (key1, key2) -> key1)); } / * 初始化集合数据 */ static { Student stu1 = new Student("0001", "张三", 12, "江苏南京"); Student stu2 = new Student("0002", "李四", 14, "江苏无锡"); Student stu3 = new Student("0003", "王二", 11, "浙江台州"); Student stu4 = new Student("0004", "李五", 12, "浙江温州"); Map<String, Object> map1 = new HashMap<>(); map1.put("id", "0001"); map1.put("student", stu1); Map<String, Object> map2 = new HashMap<>(); map2.put("id", "0002"); map2.put("student", stu2); Map<String, Object> map3 = new HashMap<>(); map3.put("id", "0003"); map3.put("student", stu3); Map<String, Object> map4 = new HashMap<>(); map4.put("id", "0004"); map4.put("student", stu4); mapStudentList.add(map1); mapStudentList.add(map2); mapStudentList.add(map3); mapStudentList.add(map4); }

1.6、List<Map<String,String>> 转化为Map<String,Map<String,String>>

 final static List<Map<String, String>> listMapList = new ArrayList<>(); public static void main(String[] args) { Map<String, Map<String, String>> map5 = listMapList.stream().collect(Collectors.toMap(each -> each.get("id"), each -> each, (key1, key2) -> key1)); System.out.println("map5 = " + map5); } / * 初始化集合数据 */ static { Map<String, String> map1 = new HashMap<>(); map1.put("id", "0001"); map1.put("name", "张三"); map1.put("age", "12"); map1.put("address", "江苏南京"); Map<String, String> map2 = new HashMap<>(); map2.put("id", "0002"); map2.put("name", "李四"); map2.put("age", "14"); map2.put("address", "江苏无锡"); Map<String, String> map3 = new HashMap<>(); map3.put("id", "0003"); map3.put("name", "王二"); map3.put("age", "11"); map3.put("address", "浙江台州"); Map<String, String> map4 = new HashMap<>(); map4.put("id", "0004"); map4.put("name", "李五"); map4.put("age", "12"); map4.put("address", "浙江温州"); listMapList.add(map1); listMapList.add(map2); listMapList.add(map3); listMapList.add(map4); }

1.7、List<Map<String,String>> 转化为Map<String,String>

 final static List<Map<String, String>> listmapstringlist = new ArrayList<>(); public static void main(String[] args) { Map<String, String> map6 = listmapstringlist.stream().collect(Collectors.toMap(each -> each.get("id"), each -> each.get("name"), (key1, key2) -> key1)); } / * 初始化集合数据 */ static { Map<String, String> map1 = new HashMap<>(); map1.put("id", "0001"); map1.put("name", "张三"); map1.put("age", "12"); map1.put("address", "江苏南京"); Map<String, String> map2 = new HashMap<>(); map2.put("id", "0002"); map2.put("name", "李四"); map2.put("age", "14"); map2.put("address", "江苏无锡"); Map<String, String> map3 = new HashMap<>(); map3.put("id", "0003"); map3.put("name", "王二"); map3.put("age", "11"); map3.put("address", "浙江台州"); Map<String, String> map4 = new HashMap<>(); map4.put("id", "0004"); map4.put("name", "李五"); map4.put("age", "12"); map4.put("address", "浙江温州"); listmapstringlist.add(map1); listmapstringlist.add(map2); listmapstringlist.add(map3); listmapstringlist.add(map4); }

 list.add("c"); list.add("a"); list.add("b"); list.add("d"); List<User> userList = new ArrayList<>(); userList.add(new User(new Long(1),"李世民","123lsm@163.com","男","110")); userList.add(new User(new Long(2),"宇文成都","123ywcd@163.com","男","111")); userList.add(new User(new Long(3),"裴元庆","123pyq@163.com","男","112")); userList.add(new User(new Long(4),"程咬金","123cyj@163.com","男","113")); userList.add(new User(new Long(5),"李元霸","123lyb@163.com","男","114")); userList.add(new User(new Long(6),"潇美娘","123xmn@163.com","女","11q")); userList.add(new User(new Long(6),"潇美娘","123xmn@163.com","女","11q")); for (int i = 0; i < list.size(); i++) { System.out.println("第一种遍历list方法" + list.get(i)); } for (String s : list) { System.out.println("第二种遍历list方法:" + s); } Iterator<String> iterator = list.iterator(); while (iterator.hasNext()) { System.out.println("第三种遍历list方法:" + iterator.next()); } list.forEach(new Consumer<String>() { @Override public void accept(String s) { System.out.printf("第四种遍历list方法:" + s + "\n"); } }); List<User> userList1 = new ArrayList<>(); User user1 = new User(); userList.forEach(user -> { user1.setId(user.getId()); user1.setUserName(user.getUserName()); user1.setSex(user.getSex()); user1.setPhone(user.getPhone()); user1.seteMail(user.geteMail()); userList1.add(user1); System.out.println("第五种遍历方法#:"+userList1); }); List<User> mm = userList.stream().filter(user -> user.getSex().equals("男")).collect(Collectors.toList()); System.out.println("第六种:过滤掉不想要的列表,留下想要的列表#:"); mm.forEach(System.out::println); List<String> gg = userList.stream().map(User::getSex).distinct().collect(Collectors.toList()); System.out.println("=========gg对某个字段去重！"); gg.forEach(System.out::println); List<String> g = userList.stream().map(user -> user.getUserName()).distinct().collect(Collectors.toList()); System.out.println("=========g对某个字段去重！"); g.forEach(System.out::println); List<User> users = userList.stream().distinct().collect(Collectors.toList()); System.out.println("去除完全重复的数据!"); users.forEach(System.out::println); ArrayList<User> arrayList = userList.stream().collect(Collectors.collectingAndThen(Collectors .toCollection(() -> new TreeSet<>(Comparator.comparing(user -> user.getSex()))), ArrayList::new)); System.out.println("arrayList根据指定的字段去除重复的数据"); arrayList.forEach(System.out::println); List<Map<String,String>> mapList = new ArrayList<>(); Map<String,String> mapL = new HashMap<>(); Map<String,String> mapL2 = new HashMap<>(); mapL.put("a","李世民"); mapL.put("b","李元霸"); mapL.put("c","秦叔宝"); mapL2.put("a","李世民"); mapList.add(mapL); mapList.add(mapL2); ArrayList<Map<String, String>> mapArrayList = mapList.stream().collect(Collectors.collectingAndThen(Collectors .toCollection(() -> new TreeSet<>(Comparator.comparing(map -> map.get("a")))), ArrayList::new)); System.out.println("根据list集合中某个对象里的字段值去重:"+mapArrayList); /* * findAny和findFirst都是获取第一条信息,如果未找到则返回null * */ User orElse = userList.stream().filter(user -> user.getUserName().equals("潇美娘")).findAny().orElse(null); System.out.println("获取存在潇美娘的第一条用户信息orElse:"+orElse); User anElse = userList.stream().filter(user -> user.getUserName().equals("罗成")).findFirst().orElse(null); System.out.println("获取不存在罗成的第一条用户信息anElse:"+anElse); System.out.println("下面介绍下map=========欢迎来到map家族！"); Map<String, Integer> map = new HashMap<>(); map.put("e", 2); map.put("h", 1); map.put("g", 9); map.put("f", 6); for (Map.Entry<String, Integer> m : map.entrySet()) { System.out.println("第一种map集合遍历:" + "KEY:" + m.getKey() + "\t" + "VALUE:" + m.getValue()); } for (String key : map.keySet()) { System.out.println("第二种map集合遍历:" + "KEY:" + key + "\t" + "VALUE:" + map.get(key)); } Iterator<Map.Entry<String, Integer>> iterator1 = map.entrySet().iterator(); while (iterator1.hasNext()) { Map.Entry<String, Integer> entry = iterator1.next(); System.out.println("第三种map集合遍历:" + "KEY:" + entry.getKey() + "\t" + "VALUE:" + entry.getValue()); } map.forEach(new BiConsumer<String, Integer>() { @Override public void accept(String s, Integer s2) { System.out.println("第四种map集合遍历:" + "KEY:" + s + "\t" + "VALUE:" + s2); } }); map.entrySet().forEach(entry -> System.out.println("第五种map集合遍历:" + "KEY:" + entry.getKey() + "\t" + "VALUE:" + entry.getValue())); map.values().forEach(s -> { System.out.println("直接获取map中的所有VALUE" + s); }); map.forEach((k, v) -> { System.out.println("第六种最好的方法map集合遍历:" + "KEY:" + k + "\t" + "VALUE:" + v); }); HashMap<String,Integer> maps = map.entrySet().stream().sorted((o1, o2) -> o1.getValue().compareTo(o2.getValue())) .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (o1, o2) -> o1, LinkedHashMap::new)); maps.forEach((k,v)->System.out.println("对maps集合中value数据升序排序:"+"k:"+k+"\t"+"v:"+v)); HashMap<String,Integer> mapsS = map.entrySet().stream().sorted((o1, o2) -> o2.getValue().compareTo(o1.getValue())) .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (o1, o2) -> o2, LinkedHashMap::new)); mapsS.forEach((k,v)->System.out.println("对mapsS集合中value数据降序排序:"+"k:"+k+"\t"+"v:"+v)); HashMap<String,Integer> map0 = map.entrySet().stream().sorted((Map.Entry.comparingByValue())) .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (o1, o2) -> o1, LinkedHashMap::new)); map0.forEach((k,v)->System.out.println("对map0集合中value数据升序排序:" + "k:"+k+"\t"+"v:"+v)); HashMap<String,Integer> map1 = map.entrySet().stream().sorted(Map.Entry.<String,Integer>comparingByKey().reversed()) .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (o1, o2) -> o1, LinkedHashMap::new)); map1.forEach((k,v)-> System.out.println("对map1集合中key降序排序:" + "k:"+k+"\t"+"v:"+v)); HashMap<String,Integer> map2 = map.entrySet().stream().sorted(Map.Entry.<String,Integer>comparingByValue().reversed()) .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (o1, o2) -> o1, LinkedHashMap::new)); map2.forEach((k,v)-> System.out.println("对map集合中value降序排序:" + "k:"+k+"\t"+"v:"+v)); } 

@Data public class TestDemo { //获取第pageNum页数据 public static List<?> partition(ArrayList<?> list, int pageSize, int pageNum) { //将List按照pageSize拆分成多个List List<? extends List<?>> partition = Lists.partition(list, pageSize); //总页数 int pages = partition.size(); pageNum = pageNum <= 0 ? 0 : (pageNum <= (pages - 1) ? pageNum-1 : (pages - 1)); return partition.get(pageNum); } public static void main(String[] args) { List<TestDemo> list = new ArrayList<>(); list.add(new TestDemo(1L,11L,100L)); list.add(new TestDemo(2L,11L,600L)); list.add(new TestDemo(3L,33L,333L)); list.add(new TestDemo(4L,33L,null)); list.add(new TestDemo(5L,44L,null)); //（一、转Map）如果有重复的key,则保留key1,舍弃key2 Map<Long, TestDemo> map = list.stream().collect(Collectors.toMap(TestDemo::getId, item -> item, (key1, key2) -> key1)); //（二、排序） // Comparator.comparing(类::属性一).reversed(); 得到排序结果后再排序 //Comparator.comparing(类::属性一,Comparator.reverseOrder());是直接进行排序 //升序、空值排在最前面、reversed反转排序 List<TestDemo> sortList = list.stream().sorted( Comparator.comparing(TestDemo::getTotal, Comparator.nullsFirst(Long::compareTo)).reversed() ).collect(Collectors.toList()); //属性一升序，属性二降序 sortList = list.stream().sorted( Comparator.comparing(TestDemo::getId).thenComparing(TestDemo::getGroup, Comparator.reverseOrder()) ).collect(Collectors.toList()); //（三、分组） Map<Long, List<TestDemo>> groupMapList = list.stream().collect(Collectors.groupingBy(TestDemo::getGroup)); Map<Long, TestDemo> groupMapObject = list.stream().collect( //取分组中的一条数据 Collectors.groupingBy(TestDemo::getGroup, Collectors.collectingAndThen(Collectors.toList(), item -> item.get(0))) ); //分组求和得到新集合 List<TestDemo> groupSumList = new ArrayList<>(); list.parallelStream().collect(Collectors.groupingBy(TestDemo::getGroup, Collectors.toList())) .forEach((id, groupList) -> { groupList.stream().reduce( (a, b) -> new TestDemo(a.getId(), a.getGroup(), a.getTotal(), b.getTotal()) ).ifPresent(groupSumList::add); }); //（四、去重） List<String> stringList = new ArrayList<String>() {{ add("A"); add("A"); add("B"); add("B"); add("C"); }}; stringList = stringList.stream().distinct().collect(Collectors.toList()); //对象属性去重 List<TestDemo> distinctList = list.stream().collect( Collectors.collectingAndThen(Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(TestDemo::getGroup))), ArrayList::new)); //（五、提取） List<Long> groupNumList = list.stream() .map(TestDemo::getGroup) //流转化为Long .distinct() //去重 .collect(Collectors.toList()); //（六、过滤） List<TestDemo> filterList = list.stream().filter(item -> item.getTotal() != null && item.getTotal() < 200).collect(Collectors.toList()); List<TestDemo> filtersList = list.stream() .filter(item -> {//多条件过滤 if(item.getTotal() == null) { return false; } if (item.getTotal() > 200 && item.getTotal() < 400) { return true; } return false; }).collect(Collectors.toList()); //（七、取值） // 平均数 Double avg1 = list.stream().filter(item -> item.getTotal() != null).mapToLong(TestDemo::getTotal).average().orElse(0);//为空的不参与计算 Double avg2 = list.stream().filter(item -> { if(item.getTotal() == null) { //为空的参与计算 item.setTotal(0L);//避免计算时报空指针错误 } return true; }).collect(Collectors.averagingLong(TestDemo::getTotal)); // 最大值 Long max = list.stream().mapToLong(TestDemo::getTotal).max().orElse(0); // 最小值 Long min = list.stream().mapToLong(TestDemo::getTotal).min().orElse(0); //求和 Long sum = list.stream().mapToLong(TestDemo::getTotal).sum(); } private Long id; private Long group; private Long total; public TestDemo(){} public TestDemo(Long id, Long group, Long total) { this.id = id; this.group = group; this.total = total; } public TestDemo(Long id, Long group, Long aTotal, Long bTotal) { this.id = id; this.group = group; this.total = (aTotal==null ? 0 : aTotal) + (bTotal==null ? 0 : bTotal); } }

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