Simplify the method that returns collection statistics

Question

There is a method that must be reduced to two for loops. One for loading data, and the second for searching in this map the elements of the second list. Any thoughts on how to do this?

package ru.job4j.store; import java.util.HashMap; import java.util.List; import java.util.Map; public class Store { /** * Метод считает кол-во изменений в коллекции. * <p> * Метод работает следующим образом: * Если кол-во ключей из прошлой коллекции больше чем у новой растет счетчик добавления элементов. * Если кол-во ключей из прошлой коллекции меньше чем у новой растет счетчик удаления элементов. * Если ключи не эквивалентные то растет счетчик изменений. * * @param previous - прежняя коллекция. * @param current - новоя коллекция. * @return карту с счетчиками. */ public HashMap<String, Integer> isChange(List<User> previous, List<User> current) { int newAdd = 0; int newChange = 0; int newDelete = 0; Map<Integer, String> mapOfPrevious = new HashMap<>(); Map<Integer, String> mapOfCurrent = new HashMap<>(); HashMap<String, Integer> result = new HashMap<>(); for (User indexPrevious : previous) { mapOfPrevious.put(indexPrevious.getId(), indexPrevious.getName()); } for (User indexCurrent : current) { mapOfCurrent.put(indexCurrent.getId(), indexCurrent.getName()); } for (Integer key : mapOfPrevious.keySet()) { if (!mapOfCurrent.containsKey(key)) { newDelete++; } else if (!mapOfCurrent.get(key).equals(mapOfPrevious.get(key))) { newChange++; } } for (Integer key : mapOfCurrent.keySet()) { if (!mapOfPrevious.containsKey(key)) { newAdd++; } } result.put("Amount new add: ", newAdd); result.put("Amount new change: ", newChange); result.put("Amount new delete: ", newDelete); return result; } static class User { private int id; private String name; public User(int id, String name) { this.id = id; this.name = name; } public int getId() { return this.id; } public String getName() { return this.name; } } }

Answer 1 · 2018-08-28T17:23:19

First, take the number of old and data that came to update:

 int beforeSize = previous.size(); int afterSize = current.size();

Add all users at once to one list:

 List<User> allUsers = new ArrayList<>(); allUsers.addAll(previous); allUsers.addAll(current);

And scatter in the map. When duplicating a key, its value changes; this can be used to get a card with already changed data:

 Map<Integer, String> usersMap = new HashMap<>(); for (User user : allUsers) { usersMap.put(user.getId(), user.getName()); }

Next we get the size of this card:

 int newListSize = usersMap.size();

Now we have three numbers: how many pairs came as “new”, how many were and how many became. Based on these data, you can get the number of new, the number of changed and the number of deleted.

 int newAdd = Math.max(0, newListSize - beforeSize); int newChange = Math.max(0, afterSize - newAdd); int newDelete = Math.max(0, newListSize - newAdd - newChange);

The author has given the full code why it’s not clear why you give in chunks, therefore if the author doesn’t understand, the next questions will follow, where, why.
Therefore, you need to give the full code by changing the author's code.
@And, so that the author does not just copy it himself and read and understand.
If you have not noticed, I will explain the code in parts, otherwise it would be idiocy

Alexandar Vysotskiy Alexandar Vysotskiy 32 four · Accepted Answer · 2018-10-11T14:10:37

The issue was resolved as follows:

 package ru.job4j.store;

import java.util.HashMap; import java.util.List;

public class Store {

 /** * Метод считает кол-во изменений в коллекции. * * @param previous - прежняя коллекция. * @param current - новоя коллекция. * @return карту с счетчиками. */ public HashMap<String, Integer> isChange(List<User> previous, List<User> current) { int deleteCount = 0; int changeCount = 0; int beforeSize = previous.size(); HashMap<Integer, String> allUsers = new HashMap<>(); for (User index : current) { allUsers.put(index.getId(), index.getName()); } for (User index : previous) { if (!allUsers.containsKey(index.getId())) { deleteCount++; } else if (!allUsers.get(index.getId()).equals(index.getName())) { changeCount++; } allUsers.put(index.getId(), index.getName()); } int newListSize = allUsers.size(); HashMap<String, Integer> result = new HashMap<>(); result.put("Amount new add: ", newListSize - beforeSize); result.put("Amount new change: ", changeCount); result.put("Amount new delete: ", deleteCount); return result; } static class User { private int id; private String name; public User(int id, String name) { this.id = id; this.name = name; } public int getId() { return this.id; } public String getName() { return this.name; } }

}

Here you can see the test for it:

https://github.com/AlexandarVysotskiy/AVysotskiy/commit/1479aac7301a200f5611c81e002d342f44359e36

Simplify the method that returns collection statistics

2 answers 2

More articles: