Demo1:id归类到同一组map集合
小demo:
需求:将user id归类到同一组map集合中
import java.util.ArrayList; import java.util.HashMap; import java.util.List; import java.util.Map; public class Test { public static void main(String[] args) { List<User> userList = new ArrayList<>(); User user1 = new User(); user1.setId(1L); user1.setAge("12"); User user2 = new User(); user2.setId(2L); user2.setAge("13"); User user3 = new User(); user3.setId(2L); user3.setAge("15"); userList.add(user1); userList.add(user2); userList.add(user3); Map<Long, List<User>> maps = new HashMap<Long, List<User>>(); if( null != userList && !userList.isEmpty() ) { for ( User user:userList) { if( maps.get(user.getId()) == null ) { List<User> list = new ArrayList<User>(); list.add(user); maps.put(user.getId(),list); } else { List<User> list = maps.get(user.getId()); list.add(user); maps.put(user.getId(),list); } } } for(Map.Entry<Long, List<User>> entry:maps.entrySet()) { System.out.println("id:"+entry.getKey()+";value:"+entry.getValue().size()+":::::"+entry.getValue()); } } } class User { private Long id; private String age; public Long getId() { return id; } public void setId(Long id) { this.id = id; } public String getAge() { return age; } public void setAge(String age) { this.age = age; } @Override public String toString() { return "User{" + "id=" + id + ", age='" + age + '\'' + '}'; } } class B { private List<String> a ; private List<String> b ; public List<String> getA() { return a; } public void setA(List<String> a) { this.a = a; } public List<String> getB() { return b; } public void setB(List<String> b) { this.b = b; } }
读书犯困,因为那是梦开始的地方;