Java hashMap issue

Hijpo · 30-10-2014 4:44am #1

Hi,

Iv been trying to populate a hashMap and it doesnt seem to work correctly even though in my head it should. Im obviously not understanding hashMap's properly so if someone can explain why this happens it would be great.



Map <String, String> packageList;



public void popMap()
    {  
              
        for(String cl:classList)
        {
                      
            packageList.put(cl , "A");
            packageList.put(cl , "B");
            packageList.put(cl, "C");
            packageList.put(cl , "D");
        }
        
    }

If i was to print that hashMap it displays
LetterA=D
LetterB=D
LetterC=D
LetterD=D

Where as i need it to print
LetterA=A
LetterB=B
LetterC=C
LetterD=D

classList is the name of the Set im populating the Map from and it works. Iterating through the Set with the for loop works and the different values are stored in the Map in the order i would expect. When i print(packageList) it gives me A B C D which is what im looking for.

However the assigned value always outputs D.

duffman85 · 30-10-2014 6:37am

You have 4 calls to put in each iteration. you're overwriting the value with each call so the last call

packageList.put(cl , "D");

is overwriting what was done in

packageList.put(cl , "C");

and the previous 2 calls.

Hijpo · 30-10-2014 11:16am

duffman85 wrote: »
You have 4 calls to put in each iteration. you're overwriting the value with each call so the last call
packageList.put(cl , "D");
is overwriting what was done in
packageList.put(cl , "C");
and the previous 2 calls.

But I thought .put creates a new key/value pair if there are no duplicates?

srsly78 · 30-10-2014 11:25am

It's not creating any new pairs, because you are pushing in the same values each time.

Look at your for loop and fix it. Problem is not with the hash map, it's with what you are feeding into it.

String cl is not changing, you need to iterate over the values of that list.

Aswerty · 30-10-2014 11:35am

Much in line with what others have said.

The loop is doing the following:

Iteration 1:
LetterA = A
LetterA = B
LetterA = C
LetterA = D

Iteration 2:
LetterB = A
LetterB = B
LetterB = C
LetterB = D

and so on for each Letter...

So the last assignment of each iteration is always D. That is why you are getting the result you are getting.

Try doing only one assignment per iteration. Then find how to increase the value "A" to "B" to "C" and so on in each iteration before assigning it as the value for the key.

Hijpo · 30-10-2014 11:35am

srsly78 wrote: »

It's not creating any new pairs, because you are pushing in the same values each time.

Look at your for loop and fix it. Problem is not with the hash map, it's with what you are feeding into it.

String cl is not changing, you need to iterate over the values of that list.

Sorry if I'm going on a bit much but why would cl show to be changing when a print the hash map? Am I using the wrong way to check its contents?

srsly78 · 30-10-2014 11:38am

Remove the for loop and write out your assignments the long way, then you can see exactly what is happening. Also try using breakpoints, so you can inspect exactly what gets put into the map each time.

counterpointaud · 30-10-2014 4:41pm

Hijpo wrote: »

But I thought .put creates a new key/value pair if there are no duplicates?

If the key is the same it's considered a duplicate, and put() will overwrite the previous entry

Hijpo · 30-10-2014 6:07pm

counterpointaud wrote: »

If the key is the same it's considered a duplicate, and put() will overwrite the previous entry

I'm using each iteration of the classList set to assign a value to the hash map.
Following srsly78 advice I used breakpoints to inspect what strings are being inserted and the keys are all correct.

Just need to figure out how to construct the pairings now.

If i populate the value of the hashMap before the loop

packageList.put(null , "A");
packageList.put(null , "B");
packageList.put(null , "C");
packageList.put(null , "D");

Is it possible to populate the keys while retaining the values?

magooly · 30-10-2014 8:46pm

Here is a working example that might get you a little further to where you want to go. Ive used your code where possible.

Please read about how Maps work particularly when .hashcode() and .equals() is called on your key you place in the map.

[HTML]
package tmp;

import java.util.Arrays;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.Set;

public class ListTest {

Set<String> classList = new HashSet<String>(Arrays.asList("A", "B", "C", "D"));

Map<String, String> packageList = new HashMap<String, String>();

public void popMap() {
for (String cl : classList) {
packageList.put("Letter" + cl, cl);
}
}

private void print() {
System.out.print(packageList);
}

public static void main(String args[]) {
ListTest l = new ListTest();
l.popMap();
l.print();
}

}[/HTML]

counterpointaud · 30-10-2014 8:47pm

@OP: Why are you using 4 calls to put() inside the loop? The key is the same for each call for each iteration.

Hijpo · 30-10-2014 9:10pm

counterpointaud wrote: »

@OP: Why are you using 4 calls to put() inside the loop? The key is the same for each call for each iteration.

How I visualised the hash map being populated, going by .put() creating new pairs if the keys are not duplicates, i figured the variable cl holds a different value each time the loop iterates. when it populates the key using cl then I could hard code a value to each new key.

Now that I've read what I was thinking I can understand why it wouldnt work and have slapped my face for it.

@magooly, many thanks for taking the time to help to that level.

Hijpo · 31-10-2014 5:10am

So i ended up with this.

Converted the hashSet containing the class names to an array and created a second array with the package names in it.

Then populated the hashMap using the elements of both arrays like this.

private void createMappings()
    {
        int k = 0;
        int v = 0;
        
   
        do {
           classPackMap.put(classArray[k] , packageList[v]);
              
           k++;
           v++;
           }
           
              while ( v < 4);
            }

It works and im happy i didnt straight up hard code the values in, i was this || close to giving in.

Thanks again for the assistance!

Fate Amenable To Change · 01-11-2014 4:34am

Jut to clarify a little in laymans term the way a hashmap works with the put id the first vlaue will be your key and then the second value will be what you want to store to that key.

So the key could be like "fridaysBreakfeast"

and the value associated could be "breakfeastRoll"

so when you go looking for the value in the hashmap for "fridaysBreakfeast" it will return "breakfeastRoll"
////////////

alternatively you could use the key ""mondaysLunch"
and the value associated with this would "hamSandwich"
so when you go looking for the value in the hashmap for "mondaysLunch" it will return "hamSandwich"

The point being that rather than using numbers to find and store stuff you can sue a combination of letters and numbers.

#################################################
so using put

yourArray.put("mondaysLunch", "hamSandwich");

so everytime you look for mondaysLucbh it returns hamSandwich

Hijpo · 01-11-2014 2:35pm

Fate Amenable To Change wrote: »

Jut to clarify a little in laymans term the way a hashmap works with the put id the first vlaue will be your key and then the second value will be what you want to store to that key.

So the key could be like "fridaysBreakfeast"

and the value associated could be "breakfeastRoll"

so when you go looking for the value in the hashmap for "fridaysBreakfeast" it will return "breakfeastRoll"
////////////

alternatively you could use the key ""mondaysLunch"
and the value associated with this would "hamSandwich"
so when you go looking for the value in the hashmap for "mondaysLunch" it will return "hamSandwich"

The point being that rather than using numbers to find and store stuff you can sue a combination of letters and numbers.

#################################################
so using put

yourArray.put("mondaysLunch", "hamSandwich");

so everytime you look for mondaysLucbh it returns hamSandwich

Thanks for the info. U can follow the idea of creating pairs, I was just struggling with an efficient way of populating the hash map in the first place.

Java hashMap issue

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