Iterator and remove

Question about the task

Under discussion

Hi guys, I'm a bit confused, when I iterate over a map, I understand that I can remove directly from that map, without a copy of them map.. but with by code, it doesn't seem to work - do I need a copy? or am I doing some other thing wrong ? :-)

Create a Map<String, Integer> and add ten entries that represent (last name, salary) pairs.
Remove from the map all people whose salary is below 500.

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, Integer) elements and contains 10 entries that represent (last name, salary) pairs.
The removeItemFromMap() method must remove from the map all people whose salary is below 500.

package com.codegym.task.task08.task0818; import java.util.HashMap; import java.util.Iterator; import java.util.Map; /* Only for the rich */ public class Solution { public static HashMap<String, Integer> createMap() { HashMap<String, Integer> map = new HashMap<>(); map.put("Tranberg1", 100000); map.put("Tranberg2", 100); map.put("Tranberg3", 200); map.put("Tranberg4", 2000); map.put("Tranberg5", 100); map.put("Tranberg6", 90); map.put("Tranberg7", 80); map.put("Tranberg8", 200); map.put("Tranberg9", 100); map.put("Tranberg10", 3000); return map; } public static void removeItemFromMap(HashMap<String, Integer> map) { Iterator<Map.Entry<String, Integer>> iterator = map.entrySet().iterator(); while (iterator.hasNext()) { Map.Entry<String, Integer> pair = iterator.next(); if (pair.getValue() < 100) { map.remove(pair.getKey()); } } } public static void main(String[] args) { removeItemFromMap(createMap()); } }

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Anitamalina

Level 18 , København, Denmark

18 December 2020, 19:31

to me this is the same, and work the same way.. why would I ever use a iterator then ? if have to make a copy in both solutions ?

import java.util.HashMap;
import java.util.Map;

/*
Only for the rich

*/

public class Solution {
    public static HashMap<String, Integer> createMap() {
        HashMap<String, Integer> map = new HashMap<>();
        map.put("Tranberg1", 100000);
        map.put("Tranberg2", 100);
        map.put("Tranberg3", 200);
        map.put("Tranberg4", 2000);
        map.put("Tranberg5", 100);
        map.put("Tranberg6", 90);
        map.put("Tranberg7", 80);
        map.put("Tranberg8", 200);
        map.put("Tranberg9", 100);
        map.put("Tranberg10", 3000);

        return map;
    }

    public static void removeItemFromMap(HashMap<String, Integer> map) {
        HashMap<String, Integer> copy = new HashMap<String, Integer>(map);

        for (Map.Entry<String, Integer> pair : copy.entrySet()) {
            if (pair.getValue() < 100) {
                map.remove(pair.getKey());
            }
        }
    }

    public static void main(String[] args) {
        removeItemFromMap(createMap());
    }
}

Guadalupe Gagnon

Level 37 , Tampa, United States

18 December 2020, 19:48

Iterator doesn't create a copy, it is a anonymous class of an Interface (that is an advanced description, don't worry about it too much at this point) contained within each Collections class that has access to the underlying data storage. The big plus to this is that it is included with any Collection. So you can make an iterator for a Map, Tree, Set, List. The iterator has all the same methods and functionality no matter the Collection its created from, so you don't need to know anything special about each just to access the data. Making a copy blocks out a whole chunk of memory from the CPU to contain the copy, while Iterator doesn't have to do that as it is just an interface of methods to access the already present data. This means that it is quite a bit more efficient too.

Guadalupe Gagnon

Level 37 , Tampa, United States

18 December 2020, 19:28

Yes, but only using the correct method from the iterator. Example methods using the iterator: iterator.next() iterator.hasNext() iterator.######() << find the proper method to *remove* You just need to replace one line in your code with the correct implementation. P.S. Per task, remove anyone below 500

Anitamalina

Level 18 , København, Denmark

18 December 2020, 19:38

aaaah, got it now! iterator.remove(); Thanks! and no copy map ;)