CodeGym/Help with Java Tasks/what's wrong with my iterator?

Level 17

Mansfield

28.03.2021
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14comments

what's wrong with my iterator?

Question about the task

We don't need repeats

Java Syntax, Level 8, Lesson 8

Under discussion

it says that package map2 not found even though I'm pointing to map2 Map when i try to create iterator

Create a Map<String, String> and add ten entries that represent (last name, first name) pairs.
Remove people with the same first name.

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, String) elements and contains 10 entries.
The removeFirstNameDuplicates() method must remove from the map all people who have the same first name.
The removeFirstNameDuplicates() method must call the removeItemFromMapByValue() method.

package com.codegym.task.task08.task0817; import java.util.HashMap; import java.util.Map; import java.util.Iterator; import java.util.Collections; /* We don't need repeats */ public class Solution { public static HashMap<String, String> createMap() { //write your code here HashMap<String, String> map = new HashMap<String, String>(); map.put("Jorie","Peralta"); map.put("Hailey","Williams"); map.put("Jorie","Cruz"); map.put("Honey","Smith"); map.put("Lexi","Role"); map.put("Josh","Ching"); map.put("Max","Mendoza"); map.put("Josh","Sta ana"); map.put("Michael","Garcia"); map.put("Jorie","Mitchel"); return map; } public static void removeFirstNameDuplicates(Map<String, String> map) { //write your code here HashMap<String, String> map2 = new HashMap<String, String>(); for(Map.Entry<String, String> pair : map.entrySet()){ String name = pair.getKey(); String lname = pair.getValue(); map2.put(lname, name); } Iterator<String> it = new map2.iterator(); while(it.hasNext()){ removeItemFromMapByValue(map2,it.next().getValue()); } } public static void removeItemFromMapByValue(Map<String, String> map, String value) { HashMap<String, String> copy = new HashMap<String, String>(map); for (Map.Entry<String, String> pair : copy.entrySet()) { if (pair.getValue().equals(value)) map.remove(pair.getKey()); } } public static void main(String[] args) { } }

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Guadalupe Gagnon

Level 37 , Tampa, United States

29 March 2021, 16:46

No one has mentioned this below, but the task says to: "Create a Map<String, String> and add ten entries that represent (last name, first name) pairs." Make sure that your map is correct, last name as the the key and first name as the value. When you fix that you do not, and should not, swap the last name and first name in the removeFirstNameDuplicates() method. Now, when you fix the above, you will need to next remove all items in the list that share a first name with another item. In the code above, if everything worked, it would be removing all the items from the list because there is no mechanism for checking if the name is a duplicate. Be sure that you are only removing the ones that fit the task requirements and not all the items. Example: if you had the just the first names (last names don't matter): {"Larry", "Olive", "Larry"} Then you should pass "Larry" to the removeItemFromMapByValue() method because that name appears more than one time in your items.

John

Level 17 , Mansfield, Philippines

30 March 2021, 02:59

i think i get it now but how can i compare a value of the map to its next value using iteration?

Guadalupe Gagnon

Level 37 , Tampa, United States

30 March 2021, 13:13

That is a problem that you need to solve. One way is maybe keep a second collections that store the names that the code has already seen. For each name in the Map, check if it is already in the second collection first. If it is not already in the collection then add it to that collection. If it is already in the collection then that would mean that your code has already seen the name and it should be removed. This is not the only though, so any way that you can think of (and subsequently implement) that works is acceptable.

Gellert Varga

Level 23 , Szekesfehervar, Hungary

28 March 2021, 20:06

- The "new" keyword is not required in line 42. - In line 42, it is not possible to run iterator() method on the map2 object, because the HashMap class does not have iterator() method. See all its methods: https://docs.oracle.com/javase/8/docs/api/java/util/HashMap.html (But ArrayList and HashSet have this method. You can see the lists of all their methods: https://docs.oracle.com/javase/8/docs/api/java/util/HashSet.html https://docs.oracle.com/javase/8/docs/api/java/util/ArrayList.html ) - In line 42, you can use the iterator() method to create an Iterator object as follows: the values() method must be run on the Hashmap object. This will return you a Collection that contains all the "values" of the HashMap. You can now run the iterator() method on this:

map2.values().iterator();

Accordingly, in line 44, you will not need the getValue() method.

Gellert Varga

Level 23 , Szekesfehervar, Hungary

28 March 2021, 20:20

But even after that, the program doesn't work well... On line 44, carefully consider which HashMap object you want to pass to the removeItemFromMapByValue() method!... Think about which one should you delete unnecessary items from?

John

Level 17 , Mansfield, Philippines

29 March 2021, 06:32

John

Level 17 , Mansfield, Philippines

29 March 2021, 06:32

still dont work :(

Nouser

Level 36 , Germany

29 March 2021, 08:31

@Gellert Using values you iterate over a Collection but there also is the possibility to iterate over a set view of the map itself. key and value pairs are not saved directly in the Map class but as an inner class that holds the key- value pair. Here you also find the entrySet method that returns a set of all Map.Entry objects inside the map. And on that set you of course can invoke iterator.

Iterator<Map.Entry<String, String>> it = map.entrySet().iterator();

or (as John already did)

for (Map.Entry<String, String> kv : map.entrySet()) { ...

@John Just consider starting the same way as I told you last time. Write something in your main method to test your code. First reate a map

Map<String, String> map = createMap();

call the method you want to test

removeFirstNameDuplicates(map);

output the map

for(Map.Entry<String, String> kv : map.entrySet())
    System.out.println(kv.getKey() + " <===> " + kv.getValue());

(Code may contain glitches as I typed it here and haven't tested it) As you now know how to iterate over the map, you should see that some exception gets thrown (ConcurrentModificationException). Gellert gave you a hint. And I name it more clearly again. You can not remove elements from a map (here map2) you currently are iterating over (again here map2). Lastly you should add to your temp map just that names you want to delete, that are doubles.

John

Level 17 , Mansfield, Philippines

29 March 2021, 09:47

@Nouser thanks for the big help, (i`ll try to solve this again later) but for now can i ask you question? are you already a java developer? and can you be my java guru? i really like to succeed in java programming :)

John

Level 17 , Mansfield, Philippines

29 March 2021, 12:37

but im not removing elements in a map2 instead im calling tha removeFromVal method and passing the next value of map2 in parameter and the map Map

Nouser

Level 36 , Germany

29 March 2021, 15:52

in line 52 you're invoking the remove method on map and that's an argument (map2) you pass to the removeItemFromMapByValue while iterating over map2. In the end you're looping over map2 and removing elements from that map which is not allowed.

John

Level 17 , Mansfield, Philippines

29 March 2021, 16:54

but still dont work even if i change it to (map)

Nouser

Level 36 , Germany

29 March 2021, 17:57

Looking into my crystal ball... hmm, doesn't work atm. You should post some code. Without it's hard to say what you're doing wrong.

John

Level 17 , Mansfield, Philippines

30 March 2021, 02:01

sadly i cant copy paste on my phone, im almost about to click the solution tab coz im havubg a hardtime solving this