Iterate through a Map by key

Question

I got the following error by trying to iterate through a HashMap by a key value:

Can only iterate over an array or an instance of java.lang.Iterable

I found some answers but those use .entrySet() and I need to iterate using a key value. This is my code so far:

for (ProtocolNode pendingNode : this.getPendingNodes().get(ExtractNumber(jid))){
  // Do stuff (ExractNumber is a String)
}

And the function:

public HashMap<String, ProtocolNode> getPendingNodes()
{
   return this.pending_nodes;
}

Thanks.

If you've got 1 key value only, then there is no need for iterating... Please elaborate and give more details on your issue. — dotvav, Nov 30 '15 at 09:22
Could you describe your problem a bit more? Do you want to loop over a single key or all keys? For the first one, if your value doesn´t represent something you can iterate over, then you´ll have the specific value you want yet, otherwise just use `this.getPendingNodes().entrySet()`. — SomeJavaGuy, Nov 30 '15 at 09:22
@KevinEsche I want to iterate through the full HashMap and only return the matches as ProtocolNode pendingNode — gi097, Nov 30 '15 at 09:24
@JohnDoe if you want to iterate over a `Map` to get matches then you are in some way using the map the wrong way. Your key value should notify you about matches and you shouldn´t be in need to iterate over the map again. — SomeJavaGuy, Nov 30 '15 at 09:27

score 0 · Answer 1 · edited May 23 '17 at 12:01

0

If you look at the method get in the javadoc.

https://docs.oracle.com/javase/7/docs/api/java/util/HashMap.html

You will see that it returns a value, you can't iterate over a value. You are just accessing one item, the item associated with the 'jid'.

If you take a look at this answer: Iterate through a HashMap you can see how to iterate over a hashmap. And you will just need to use 'getPendingNodes()' to get the HashMap.

edited May 23 '17 at 12:01

Community

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1

answered Nov 30 '15 at 09:24

Dylan Meeus

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this is only partionally true, If the `Map` would be defined as `Map>` then you could use the get method to iterate over the list value. – SomeJavaGuy Nov 30 '15 at 09:25
@JohnDoe take a look at the link. and Kevin: yeah that is true :-) – Dylan Meeus Nov 30 '15 at 09:27

score 0 · Accepted Answer · answered Nov 30 '15 at 09:29

0

to iterate over a map you need to keep a Collection instance. Therefore change your map as below:

public HashMap<String, List<ProtocolNode>> getPendingNodes()
{
   return this.pending_nodes;
}

or to fix your own code use below method:

Map<String, ProtocolNode> testMap = new HashMap<String, ProtocolNode>();

        for( Entry<String, ProtocolNode> entry : testMap.entrySet() ){
            if( entry.getKey().equals( "MYKEY" ) ){
                System.out.println( entry.getValue() );
            }
        }

You have to use entryset, there's no other way.

answered Nov 30 '15 at 09:29

Sercan Ozdemir

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using `testMap.containsKey()` as a condition would be a lot more performant instead of looping over the whole `Map`. – SomeJavaGuy Nov 30 '15 at 09:34
If he wants to reach the value he must use entryset, otherwise you are correct @KevinEsche. – Sercan Ozdemir Nov 30 '15 at 09:35
If it helps, please accept as an answer for further reference @JohnDoe – Sercan Ozdemir Nov 30 '15 at 09:35
@SercanOzdemir I know, I had to wait before I could accept – gi097 Nov 30 '15 at 11:21

Iterate through a Map by key

2 Answers2