1.对列表进行分组,构建成一个map对象。
键为用户名称,值为用户对象列表。
Person p1 = new Person("张三", new BigDecimal("10.0")); Person p2 = new Person("王五", new BigDecimal("10.0")); Person p3 = new Person("李四", new BigDecimal("10.0")); Person p4 = new Person("李四", new BigDecimal("10.0")); Person p5 = new Person("张三", new BigDecimal("10.0")); List<Person> list = new ArrayList<>(); list.add(p1); list.add(p2); list.add(p3); list.add(p4); list.add(p5);
这个我们之前写代码可以需要写成如下的方式:
private Map<String,List<Person >> convertToMap(List<Person > list){ Map<String,List<Persion>> map=new HashMap<>(); for(Person p: list){ String name=p.getName(); if(map.containsKey(name){ map.get(name).add(p); } else{ List<Person> list=new ArrayList(); list.add(p); map.put(name,p); } } return map; }
写了一大段代码。
使用steam 就简单了,一行代码解决问题。
Map<String, List<Person>> collect = list.stream().collect(Collectors.groupingBy(person -> person.getName()));
System.out.println(collect);
2.将list 转成 map 对象。
Person p1 = new Person("1","张三", new BigDecimal("10.0")); Person p2 = new Person("2","王五", new BigDecimal("10.0")); Person p3 = new Person("3","李四", new BigDecimal("10.0")); Person p4 = new Person("4","李四", new BigDecimal("10.0")); Person p5 = new Person("5","张三", new BigDecimal("10.0")); List<Person> list = new ArrayList<>(); list.add(p1); list.add(p2); list.add(p3); list.add(p4); list.add(p5);
上面的数据 我们转成 Map<String,Person> 对象。
Map<String, Person> nodeMap = bpmSolUsergroups.stream().collect(Collectors.toMap(p->p.getId(), p -> p));
3.对列表进行过滤
public static void main(String[] args) { List<BpmCheckFile> filesInst =new ArrayList<>(); BpmCheckFile file1=new BpmCheckFile(); file1.setJumpId("1"); BpmCheckFile file11=new BpmCheckFile(); file11.setJumpId("1"); BpmCheckFile file2=new BpmCheckFile(); file2.setJumpId("2"); filesInst.add(file1); filesInst.add(file11); filesInst.add(file2); List<BpmCheckFile> files= filesInst.stream().filter(p->p.getJumpId()=="1").collect(Collectors.toList()); System.err.println(files.size()); }
对列表数据进行过滤。
4.将JSONARRAY 转换成 Map对象
public static void main(String[] args) { String str="[{id:1,name:'ray'},{id:2,name:'zyg'}]"; JSONArray ary=JSONArray.parseArray(str); Map<String, JSONObject> map= ary.stream().collect(Collectors.toMap(i->{JSONObject json=(JSONObject)i; return json.getString("id");} , p->{JSONObject json=(JSONObject)p;return json;})); System.err.println(map); }
5. 对列表 Map 对象进行分组统计
Person p1 = new Person("张三", 1); Person p2 = new Person("王五", 2); Person p3 = new Person("李四", 3); Person p4 = new Person("李四", 4); Person p5 = new Person("张三", 5); Map<String,Object> map=new HashMap(); map.put("name","张三"); map.put("amount",1); Map<String,Object> map1=new HashMap(); map1.put("name","王五"); map1.put("amount",2); Map<String,Object> map2=new HashMap(); map2.put("name","李四"); map2.put("amount",3); Map<String,Object> map3=new HashMap(); map3.put("name","李四"); map3.put("amount",4); Map<String,Object> map4=new HashMap(); map4.put("name","张三"); map4.put("amount",5); List<Map<String,Object>> list = new ArrayList<>(); list.add(map); list.add(map1); list.add(map2); list.add(map3); list.add(map4); Map<String, List<Map<String,Object>>> collect =list.stream().collect(Collectors.groupingBy(item->{ return (String)item.get("name"); }));