Not Sure Why This is Failing

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Could anyone help me with understanding why the 4th requirement is failing? I think I'm checking for it and should be good.

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.*; import java.util.Map; /* 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("lol", "dood"); map.put("lola", "dood"); map.put("lolb", "dood"); map.put("lolc", "doocd"); map.put("lold", "doo8888d"); map.put("lole", "dosdod"); map.put("lolf", "doasfdod"); map.put("lolg", "daooda"); map.put("lolt", "dooad"); map.put("lolh", "dosod"); return map; } public static void removeFirstNameDuplicates(Map<String, String> map) { //write your code here for (Map.Entry<String, String> pair : map.entrySet()) { String fName = pair.getValue(); if (Collections.frequency(map.values(), fName) > 1) { removeItemFromMapByValue(map, fName); } } } 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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Chengyu Fu

Level 20 , New York City, United States

27 May 2020, 20:05

Just redo the iteration from the beginning after deleting something. And if the map is unchanged, end the loop.

Thomas

Level 41 , Bayreuth, Germany

27 May 2020, 17:01

Deleting Collection entries in a for loop instead of using an Iterator (this probably will throw an exception)?

Angelo Spampinato

Level 18 , Boston, United States

27 May 2020, 17:09

I'm not getting an exception, the program is running

Thomas

Level 41 , Bayreuth, Germany

27 May 2020, 18:08

Don't believe, test it. How can your code throw a runtime exception if it hasn't been called at all?

Guadalupe Gagnon

Level 37 , Tampa, United States

27 May 2020, 18:15

Thomas is correct. Removing an item, or items, from a map that is being looped through with a foreach loop (as is done in this code) and then allowing the loop to continue to execute is an illegal operation. It will throw a ConcurrentModificationException on the very next loop cycle when an item is removed. Just add these two lines to the main method, run the program, and observe the fireworks:

Map<String, String> test = createMap();
removeFirstNameDuplicates(test);

Guadalupe Gagnon

Level 37 , Tampa, United States

27 May 2020, 18:18

It is important to understand that the exception is thrown only at the start of the next loop cycle. Therefore if you break the loop before that point when an item is removed then no exception will be thrown.

Angelo Spampinato

Level 18 , Boston, United States

27 May 2020, 19:25

Now I'm using an Iterator in both places and still getting a concurrent error:

public static void removeFirstNameDuplicates(Map<String, String> map) {
        //write your code here

        for(Iterator<Map.Entry<String, String>> iter = map.entrySet().iterator(); iter.hasNext(); ) {
            Map.Entry<String, String> entry = iter.next();
            String fName = entry.getValue();
            if (Collections.frequency(map.values(), fName) > 1) {
                removeItemFromMapByValue(map, fName);
            }
        }
    }

    public static void removeItemFromMapByValue(Map<String, String> map, String value) {
        HashMap<String, String> copy = new HashMap<String, String>(map);

        for(Iterator<Map.Entry<String, String>> it = map.entrySet().iterator(); it.hasNext(); ) {
            Map.Entry<String, String> entry = it.next();
            if (entry.getValue().equals(value))
                it.remove();
        }

Guadalupe Gagnon

Level 37 , Tampa, United States

27 May 2020, 20:14

useful

First rule of Codegym is to never change the starting code provided in the task unless specified. Changing the code inside the removeItemFromMapByValue() method is going to fail the task. I recommend resetting the task at this point. By using an iterator you can safely remove items from a list ONLY if the iterator's method remove() is being used. In the code shared above this is not done inside the removeFirstNameDuplicates() method. When line 8 is reached and an item is removed, the loop here will throw a ConcurrentModificationException like before. To solve this task, after resetting it, I recommend that you study the code provided inside the removeItemFromMapByValue(). This code successfully removes items from a provided list in a safe manner. Understand what it does and you can use that knowledge to pass this task. You are on the correct path. Using Collections.Frequency like you are is probably how I would solve this task today. The only thing you need to fix is removing items safely from a collection.

Angelo Spampinato

Level 18 , Boston, United States

28 May 2020, 03:57

Ah, finally got it by making a copy in removeFirstNameDuplicates(), thank you Guadalupe!