Iterating through a HashMap in Java

Question

I am trying to create a Cholorpleth map using Processing and geoJSON. I am having difficulty cycling through my HashMap to check the property that will dictate the intensity of the polygons color.

Currently this is my code:

void draw() {
  map.draw();
  for (int i = 0; i < countyMarkers.size(); i++) {
   Marker county = countyMarkers.get(i);
   String pop = countyData.get("pop").toString();
   println(i + "pop: " + pop);   
}

What I would like to do is loop through my HashMap countyData and get the population of each entry. Use an if statement to compare it to a value and then assign it the color based on the statement that is true.

My HashMap can be seen here:

{name=Carlow, pop=54,612}
{name=Cavan, pop=73,183}
{name=Clare, pop=117,196}
{name=Cork, pop=519,032}
{name=Donegal, pop=161,137}
{name=Dublin, pop=1,273,069}
{name=Galway, pop=250,541}
{name=Kerry, pop=145,502}
{name=Kildare, pop=210,312}
{name=Kilkenny, pop=95,419}
{name=Laois, pop=80,559}
{name=Letrim, pop=31,796}
{name=Limerick, pop=191,809}
{name=Longford, pop=39,000}
{name=Louth, pop=122,897}
{name=Mayo, pop=130,638}
{name=Meath, pop=184,135}
{name=Monaghan, pop=60,483}
{name=Offaly, pop=76,687}
{name=Roscommon, pop=64,065}
{name=Sligo, pop=65,393}
{name=Tipperary, pop=158,754}
{name=Waterford, pop=113,795}
{name=Westmeath, pop=86,164}
{name=Wexford, pop=145,320}
{name=Wicklow, pop=136,640}

I am relatively new to working with this type of data so I am at a loss as to where I am going wrong. I think my logic is correct I am just not sure how to implement it. I'd appreciate any help on the issue.

Currently this line -> println(i + "pop: " + pop);

Outputs the last entry of the hashmap over and over.

This is the output I am getting

0pop: 136,640
1pop: 136,640
2pop: 136,640
3pop: 136,640
4pop: 136,640
5pop: 136,640
6pop: 136,640
7pop: 136,640
8pop: 136,640
9pop: 136,640
10pop: 136,640
11pop: 136,640
12pop: 136,640
13pop: 136,640
14pop: 136,640
15pop: 136,640
16pop: 136,640
17pop: 136,640
18pop: 136,640
19pop: 136,640
20pop: 136,640
21pop: 136,640
22pop: 136,640
23pop: 136,640
24pop: 136,640
25pop: 136,640

I populate my HashMap using this method.

void getCountyDetails(List<Marker>m) {  
  countyData = new HashMap();
  for (Marker county: countyMarkers) {
    countyData.putAll(county.getProperties());
    println(countyData);
  }
}

Modified code here

    Set<String> keySet = countyData.keySet();
  for(String pop : keySet){
     String value = countyData.get(pop).toString();
     println("population:" + value);
   }

Now It is outputting:

population:Wicklow
population:136,640
population:Wicklow
population:136,640
population:Wicklow
population:136,640
population:Wicklow
population:136,640
population:Wicklow
population:136,640
population:Wicklow
population:136,640
population:Wicklow
population:136,640
population:Wicklow
population:136,640
population:Wicklow
population:136,640

It still does not seem to be iterating through each entry in the HashMap

New way Of populating HashMap

void getCountyDetails(List<Marker>m) {  
  countyData = new HashMap();
  int i = 0;
  for (Marker county: countyMarkers) {
    countyData.put(i,county.getProperties());
    i++;
  }
}

This might sound stupid but I don't think I specified the keys, I just used the `putAll()` method to add the data to it and when I printed it that was the output in the console window! — Javacadabra, Feb 06 '14 at 20:27

Svetlin Zarev · Accepted Answer · 2014-02-06T20:45:33.210

2

From your code we can see the you are using countyData.get("pop") - i.e you have hardcoded the key, so it will always return one and the same thing - the value associated with this key.

PS: You can iterate over all the elements of the map, by using the keys form the keySet:

Set<KeyClass> keySet = myMap.keySet();
for(KeyClass key : keySet){
     ValueClass value = myMap.get(key);
     //do stuff
 }

Also you should note that the putAll method will overwrite entries with the same key:

Copies all of the mappings from the specified map to this map (optional operation). The effect of this call is equivalent to that of calling put(k, v) on this map once for each mapping from key k to value v in the specified map

edited Feb 06 '14 at 20:45

answered Feb 06 '14 at 20:35

Svetlin Zarev

14,713
4
53
82

I've added the method where I populate my `HashMap` – Javacadabra Feb 06 '14 at 20:37
You are correctly populating your map. See my `PS`. And you should not use hardcoded value in the `get()` method – Svetlin Zarev Feb 06 '14 at 20:39
I implemented your solution and I have edited my post showing the new output. It is still focusing on the last element of the map for some reason. – Javacadabra Feb 06 '14 at 20:44
Just noticed that there!!! That must be what is happening. – Javacadabra Feb 06 '14 at 20:46
My internet connection stopped for a while :) it should be updated now – Svetlin Zarev Feb 06 '14 at 20:47
You can add a `System.out.println(yourKeyHere)` in the loop, to see if that;s the case. – Svetlin Zarev Feb 06 '14 at 20:49
I just checked and that seems to be what the issue is. I have included my adjustment in my code above. It turns out I was overwriting every entry to the Map when I was populating it. I resolved this by using a key that increments when a new Item is added. – Javacadabra Feb 06 '14 at 20:50

Girish · Answer 2 · 2014-02-06T20:38:20.897

1

 for (Map.Entry<String, Integer> countyMarkers: map.entrySet())
    {
        System.out.println(countyMarkers.getKey() + " " + countyMarkers.getValue());
    }

here I have use entrySet() method, which returns you a Set view of the mappings contained in this map, and then through these mapping you can get the key and value using getKey() and getValue() method

edited Feb 06 '14 at 20:38

answered Feb 06 '14 at 20:30

Girish

1,717
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@Javacadabra have you tried it, my friend – Girish Feb 06 '14 at 20:57

score 0 · Answer 3 · answered Feb 06 '14 at 20:28

0

The problem is:

String pop = countyData.get("pop").toString();

you hard code the key to get "pop"

you probably want:

 String pop = countyData.get(county).toString();

answered Feb 06 '14 at 20:28

dkatzel

31,188
3
63
67

Iterating through a HashMap in Java

3 Answers3