将User对象集合通过城市city加到Map中

刺骨的言语ヽ痛彻心扉 2022-05-19 04:15 86阅读 0赞

有一个User对象集合，需要把集合中的对象将城市相同的放到一个集合中，使用Map，为Map<String,List<User>>。

代码如下：

package test.liuhao.testmap;
    
    import java.util.ArrayList;
    import java.util.HashMap;
    import java.util.List;
    import java.util.Map;
    import java.util.Map.Entry;
    
    /**
     *   将User对象集合按城市加入Map中
     */
    public class TestMap {
    	public static class User {
    		Integer id;
    		String username;
    		String city;
    	}
    
    	public static void main(String[] args) {
    		List<User> userList = new ArrayList<User>();
    		User user1 = new User();
    
    		user1.id = 001;
    		user1.username = "Lily";
    		user1.city = "北京";
    		userList.add(user1);
    
    		User user2 = new User();
    		user2.id = 002;
    		user2.username = "sophie";
    		user2.city = "上海";
    		userList.add(user2);
    
    		User user3 = new User();
    		user3.id = 003;
    		user3.username = "Bob";
    		user3.city = "深圳";
    		userList.add(user3);
    
    		User user4 = new User();
    		user4.id = 004;
    		user4.username = "John";
    		user4.city = "广州";
    		userList.add(user4);
    
    		User user5 = new User();
    		user5.id = 005;
    		user5.username = "Krystal";
    		user5.city = "上海";
    		userList.add(user5);
    
    		User user6 = new User();
    		user6.id = 006;
    		user6.username = "Lucy";
    		user6.city = "北京";
    		userList.add(user6);
    
    		User user7 = new User();
    		user7.id = 007;
    		user7.username = "iKun";
    		user7.city = "北京";
    		userList.add(user7);
    
    		Map<String, List<User>> userMap = new HashMap<String, List<User>>();
    		userMap = listToMap(userList);
    
    		for (Entry<String, List<User>> entry : userMap.entrySet()) {
    			System.out.print(entry.getKey() + ":");
    			for (User user : entry.getValue()) {
    				System.out.print("{" + user.id + "," + user.username + "," + user.city + "};");
    			}
    			System.out.println();
    		}
    	}
    
    	private static Map<String, List<User>> listToMap(List<User> userList) {
    		Map<String, List<User>> userMap = new HashMap<String, List<User>>();
    		List<User> tempUsers = new ArrayList<User>();
    
    		for (User user : userList) {
    			String key = user.city;
    			if (userMap.containsKey(key)) { //若key已在userMap中，则获取该key的集合列表，并添加数据
    				tempUsers = userMap.get(key);
    				tempUsers.add(user);
    			} else {//若key不在userMap中，则new一个新的集合列表添加数据
    				tempUsers = new ArrayList<User>();
    				tempUsers.add(user);
    			}
    			userMap.put(key, tempUsers);
    		}
    		return userMap;
    	}
    }

运行结果如下：

上海:{2,sophie,上海};{5,Krystal,上海};
    广州:{4,John,广州};
    北京:{1,Lily,北京};{6,Lucy,北京};{7,iKun,北京};
    深圳:{3,Bob,深圳};