I tried way too long, why is dublicating the map and checking one while removing the other not working?

Question about the task

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1. i know that you can't remove stuff from the map you are iterating 2. i tried stuff from people that posted with the alterations they said it worked for them. Not for me tho 3. so i even tried instead of coping and .remove from the map, to create a new map and .put all the objects that are not supposed to be deleted ( should be the same, right?). than coping it to the map: map = m2. But this is not altering the map variable in main. 4. the previous thing works if i change the removeAll... from a VOID to a HASHMAP with a return value, so when i call it in the main: map = removeAllSummerPeople(map); it works totally fine and the map is altered. But of course the conditions do not allow this. --> So, why is it not working in the first place, that i read the copy of the map and remove from the original? -->And why does the [3.] not alter the map variable in main? Is the varible in main suppose to be 'final' or 'public' or smt?? I ran into the problem before (void not changing the main variable) but i don´t know how I managed last time... I do NOT only want to know what works, but also WHY this does not. thanks for the answers Here is the code that works for me but is not valid:

</package com.codegym.task.task08.task0816;

import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.HashMap;
import java.util.Locale;
import java.util.Map;


public class Solution {
    public static HashMap<String, Date> createMap() throws ParseException {
        DateFormat df = new SimpleDateFormat("MMMMM d yyyy", Locale.ENGLISH);
        HashMap<String, Date> map = new HashMap<String, Date>();

        map.put("Stallone", df.parse("JUNE 1 1980"));
        map.put("a", df.parse("JUNE 1 1980"));
        map.put("b",df.parse("JUNE 1 1980"));
        map.put("c",df.parse("SEPTEMBER 1 1980"));
        map.put("d",df.parse("JUNE 1 1980"));
        map.put("e",df.parse("MARCH 1 1980"));
        map.put("f",df.parse("JUNE 1 1980"));
        map.put("g",df.parse("JUNE 1 1980"));
        map.put("h",df.parse("JUNE 1 1980"));
        map.put("i",df.parse("DECEMBER 1 1980"));

        return map;
    }



        public static HashMap<String, Date> removeAllSummerPeople(HashMap<String, Date> map) {
            //write your code here

            HashMap<String, Date> m2 = new HashMap<>();


            for(Map.Entry<String, Date> entry : map.entrySet())
            {
                int d = entry.getValue().getMonth();

                String s = entry.getKey();

                Date da = entry.getValue();

                if(d <5 || d > 7)
                    //System.out.println("d");
                    m2.put(s, da);

            }

            return m2;



    }

    public static void main(String[] args) throws ParseException {
       HashMap<String, Date> map = createMap();

        for(Map.Entry<String, Date> entry : map.entrySet())
            System.out.println(entry.getKey() +" : "+ (entry.getValue().getMonth()+1));


         map = removeAllSummerPeople(map);


        for(Map.Entry<String, Date> entry : map.entrySet())
            System.out.println(entry.getKey() +" : "+ (entry.getValue().getMonth()+1));



    }
}
>

Create a Map<String, Date> and add ten entries that represent (last name, birth date) pairs.
Remove from the map all people born in the summer.
Hint: At CodeGym, summer lasts from June 1 to August 31.

Requirements:

The program should not display text on the screen.
The program should not read values from the keyboard.
The createMap() method must create and return a HashMap that has (String, Date) elements and contains 10 entries.
The removeAllSummerPeople() method should remove from the map all people born in the summer.

package com.codegym.task.task08.task0816; import java.text.DateFormat; import java.text.ParseException; import java.text.SimpleDateFormat; import java.util.Date; import java.util.HashMap; import java.util.Locale; import java.util.Map; /* Kind Emma and the summer holidays Create a Map<String, Date> and add ten entries that represent (last name, birth date) pairs. Remove from the map all people born in the summer. Hint: At CodeGym, summer lasts from June 1 to August 31. Requirements: 1. The program should not display text on the screen. 2. The program should not read values from the keyboard. 3. The createMap() method must create and return a HashMap that has (String, Date) elements and contains 10 entries. 4. The removeAllSummerPeople() method should remove from the map all people born in the summer. */ public class Solution { public static HashMap<String, Date> createMap() throws ParseException { DateFormat df = new SimpleDateFormat("MMMMM d yyyy", Locale.ENGLISH); HashMap<String, Date> map = new HashMap<String, Date>(); map.put("Stallone", df.parse("JUNE 1 1980")); map.put("a", df.parse("JUNE 1 1980")); map.put("b",df.parse("JUNE 1 1980")); map.put("c",df.parse("JUNE 1 1980")); map.put("d",df.parse("JUNE 1 1980")); map.put("e",df.parse("JUNE 1 1980")); map.put("f",df.parse("JUNE 1 1980")); map.put("g",df.parse("JUNE 1 1980")); map.put("h",df.parse("JUNE 1 1980")); map.put("i",df.parse("DECEMBER 1 1980")); //write your code here return map; } public static void removeAllSummerPeople(HashMap<String, Date> map) { //write your code here HashMap<String, Date> m2 = map; for(Map.Entry<String, Date> entry : m2.entrySet()) { int d = entry.getValue().getMonth(); String s = entry.getKey(); Date da = entry.getValue(); if(d > 4 && d <8) //System.out.println("d"); map.remove(s); } } public static void main(String[] args) throws ParseException { /*HashMap<String, Date> map = createMap(); for(Map.Entry<String, Date> entry : map.entrySet()) System.out.println(entry.getKey() +" : "+ (entry.getValue().getMonth()+1)); removeAllSummerPeople(map); for(Map.Entry<String, Date> entry : map.entrySet()) System.out.println(entry.getKey() +" : "+ (entry.getValue().getMonth()+1));*/ } }

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Guadalupe Gagnon

Level 37 , Tampa, United States

27 August 2020, 15:57

solution

The task says that you are to: #1 create a map and add ten entries to it in the createMap() method #2 remove from that map all entries born in the 3 specified months in the removeAllSummerPeople() method In the code that you shared in the description you are very close to solving this, however the second point of removing from the ORIGINAL map all the items is the point not being reached. In the code from the attachment line 50 is not making a new copy of the map but instead setting the value of the new variable to point to the object that map points to. This is called aliasing in code and it is EXTREMELY important to understand it so that you use it properly. Think of an object as a kite and a variable as the handle for the kite. It would be attached by a string. By making a new variable and setting its value to another object, all you are doing is adding a new handle and string to the same kite. So if both "map" and "m2" are handles they would be attached to the same kite in this code. You can see this if you test run this code:

public static void main(String[] args) throws ParseException {
   ArrayList<String> handle1; // create a handle
   ArrayList<String> handle2; // create a second handle
   handle1 = new ArrayList<>(); // attaches a kite to the handle
   handle2 = handle1; // attaches handle to the same kite that handle1 is attached to
   handle1.add("testVal1");
   handle1.add("testVal2");
   handle1.add("testVal3"); // adds some values

   // outputs values in the second object (none were added)
   System.out.println("Values in second object: " + handle2);
   handle2.remove(1); // remove from second object

   // outputs values in the first object (should be 3??)
   System.out.println("Values in first object: " + handle1);
}

MoJo

Level 23 , Cairns, Australia

Expert

27 August 2020, 16:13

Wow, thanks for showing this. Is it true, that this does not apply to primitives like ints? i. e.:

<int i = 234;
        int a = i;

        System.out.println(i +" " + a);

        i = i-2;

        System.out.println(i + " " + a);>

output: 234 234 232 234

Guadalupe Gagnon

Level 37 , Tampa, United States

27 August 2020, 16:48

useful

Correct: It does not apply to primitives. Also there are classes that restrict access to the internal data and return new objects when that data is changed (gets rid of old kite and replaces it with new kite). While you shouldn't spend too much time mastering this concept yet, this becomes a very important topic in advanced coding where multiple variables, running on separate threads, could be all pointing to the same Object in memory. Imagine a school bus full of children where each student had a steering wheel for the bus.

MoJo

Level 23 , Cairns, Australia

Expert

27 August 2020, 19:22

Ok, got it! Thanks for helping me (again ^^ ). C u around

Misiu

Level 41 , Gdansk, Poland

27 August 2020, 15:35

useful

Old code, line 50: HashMap<String, Date> m2 = map; "m2" is the same map as "map", the same reference. You have to create new "m2" map.

MoJo

Level 23 , Cairns, Australia

Expert

27 August 2020, 15:58

Thank your for your response. I tried initiating m2 and than coping map to it by for() function.

</ashMap<String, Date> m2 = new HashMap<>();

            for(Map.Entry<String, Date> entry : map.entrySet()){
                m2.put(entry.getKey(),entry.getValue());
            }>

 
This worked. What i also tried and what I am confused about why it didn´t work is creating a new m2, putting all the objects except summer-born-people in it and than setting the 'map' reference to it, like so: </...
if(d <5 || d > 7)

                    m2.put(s, da);

            }

            map = m2;
...
>


Do you got any clues why this doesn´t work by any chance?

MoJo

Level 23 , Cairns, Australia

Expert

27 August 2020, 16:25

I see, I´ve just found out that this is the proper way to copy maps: HashMap<String, String> copy = new HashMap<String, String>(map);