Trouble understanding how to manipulate HashMaps

Question about the task

Under discussion

I've been having trouble really comprehending hashmaps from the start. I don't want to just copy code, that isn't how to learn. I've spent much longer on this than I would like, and no matter how many articles I've read on SO or in the JAVADOCS I'm just having a super hard time understanding the Iterator stuff, how to compare the values, and just generally basic Map manipulation. If anyone can explain some stuff, or point me to a well ELI5 article on how to do different stuff in hashmaps I would be very grateful.

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; /* We don't need repeats */ public class Solution { public static HashMap<String, String> createMap() { HashMap<String, String> nameMap = new HashMap<>(); nameMap.put("Cat", "Sylvester"); nameMap.put("Stallone", "Sylvester"); nameMap.put("Pig", "Porky"); nameMap.put("Clinton", "George"); nameMap.put("Bush", "George"); nameMap.put("Sandler", "Adam"); nameMap.put("Savage", "Adam"); nameMap.put("Einstien", "Alberto"); nameMap.put("Leon", "PonceDe"); nameMap.put("Bucks", "Star"); return nameMap; } public static void removeFirstNameDuplicates(Map<String, String> map) { for (Map.Entry<String, String> pair : createMap().entrySet() ) { String s = pair.getValue(); removeItemFromMapByValue(createMap(), s); } } 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

25 May 2019, 00:52

useful

Hi Connor, What kind of trouble are you having? Looking at your code it looks like your first problem is that you are not using the createMap method properly. This method should be called in the main method, stored to a variable, then that variable should be passed to the removeFirstNameDuplicates method, so:

public static void main(String[] args) {
   HashMap mapVariable = createMap();  // step 1
   removeItemFromMapByValue(mapVariable); // step 2
}

Now you are ready to tackle the method according to the task, but here are some tips: 1) Maps are sets of Key-Value pairings. The keys can not have any duplicates and if you try to add a key that is already in the map it will be over-written (both key and value). 2) The only safe way to remove items from collections while iterating is through an iterator. However in this task Codegym restrict that functionality by requiring you to use their own method, removeItemFromMapByValue. 3) Though you are restricted, there is a HUUUGE clue on how to solve this in that the removeItemFromMapByValue actually removes items from the map passed to it successfully without the use of an iterator. If you can figure out what is done in that method you can copy it in your method 4) Make sure you are only removing values that are actually duplicated in the map. Here is a brief map of steps to solve this task (within the method): A) copy the remove functionality from the method already provided B) modify it to say If(value is a duplicate) pass value to removeItemByValue() B.1) a value will be a duplicate if it is within the map more than once. Make a counter, take a value, check that value against all the other values and increase the counter if a match is found. If the counter reaches 2 then it is a duplicate.

Connor

Level 8 , Boston, United States

28 May 2019, 18:14

Hey Guadalupe! I've just been having trouble with maps ever since I started them. Maps and Iterators, I have been reading articles about them the past like week but then when it comes time to code my brain just like, forgets everything I've been reading. I managed to get through it all, but I know I need more practice to understand it.

Guadalupe Gagnon

Level 37 , Tampa, United States

28 May 2019, 18:22

Just wait until you get to multithreading if you want to talk about complicated =)

Guadalupe Gagnon

Level 37 , Tampa, United States

28 May 2019, 18:28

I had trouble with maps too. What are they even good for? Why not just use a Set or a List? Practice makes perfect though and I have used them to solve a lot of tasks, especially ones counting things. One task has you taking in a string and then counting each letter of the alphabet in it. A map was perfect where the key was each letter a-z, and the value was the count of each. It was easy, I just took each character from the string, used it as a key to get the value stored for that character, then increased the count by 1. By the end i had a full account of each letter. That was the first task that really got me understanding maps.