Java HashMap Removing Key/Value

Question

I'm just looking for an explanation and/or insight as to why its better to iterate over a HashMap.

For instance the code below (in my eyes) does the exact same (or it should). However if I don't iterate over the HashMap the key is not removed.

_adjacentNodes.remove(node);        

Iterator<Map.Entry<String, LinkedList<Node>>> iterator = _adjacentNodes.entrySet().iterator();
while (iterator.hasNext()) {
     Map.Entry<String, LinkedList<Node>> entry = iterator.next();
     if(node.getNodeID().contentEquals(entry.getKey())){
          iterator.remove();
     }
}

What is going on?

This is nice example, how to iterate over EntrySet and remove actual value in HashMap (_adjacentNodes). Much more conveneint is to iterate over KeySet. The condition will be little more complicated (...'node.getNodeID().contentEquals(_adjacentNodes.get(entry.getKey()).getNodeID())'...), but finding the value from the HashMap should be quick. — hariprasad
– hariprasad, Commented Dec 16, 2019 at 10:28

hkn · Accepted Answer · 2011-06-30 07:24:44Z

14

Since your key is a String you should remove String not Node. So try

_adjacentNodes.remove(node.getNodeID());

answered Jun 30, 2011 at 7:24

hkn

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Comments

Mike Kwan · Accepted Answer · 2011-06-30 07:28:33Z

remove() does work as expected. For example given this program:

import java.util.HashMap;


public class HashMapExample {
    public static void main(String[] args) {
        HashMap<String, Integer> map = new HashMap<String, Integer>();

        map.put("a", 1);
        map.put("b", 2);

        System.out.println("Before removal");
        for( String s : map.keySet() ) {
            System.out.println( s );
        }

        System.out.println("\n\nAfter removal");

        map.remove("a");
        for( String s : map.keySet() ) {
            System.out.println( s );
        }
    }
}

This will output the following:

Before removal
b
a


After removal
b

The only thing I can think of which is going wrong is that the node object you are trying to remove at the start is not the same node object as the one you get from the iterator. That is, they have the same 'NodeID' but are different objects. Perhaps it is worth it for you to check the return of remove().

Edit: Ha I didn't spot the String/Object mistake but at least we were going down the right path ; )

hkn just responded stating my key is a String and I was trying to remove the object.

comrad · Accepted Answer · 2011-06-30 07:29:11Z

0

The point here is that if you are iterating over the hashmap and then trying to manipulate it, it will fail because you cannot do that (there is even an exception for that).

So you need to use an iterator to remove an item on the very list you are iterating over.

answered Jun 30, 2011 at 7:29

comrad

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1 Comment

comrad Over a year ago

Your posting is not true. To quote Oracle's Tutorial on collections: Note that Iterator.remove is the only safe way to modify a collection during iteration; the behavior is unspecified if the underlying collection is modified in any other way while the iteration is in progress. See Quoting Source

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