How can I use additional checks in the for loop considering Hashmap?

Question

I have created two hashmaps and I want to iterate both of them inside the same for loop.

HashMap<String,Double> hashmapA = new HashMap<>();
HashMap<String,Double> hashmapB = new HashMap<>();

So, if I iterate the elements in hashmapA as follows:

for(String map1:hashmapA.keyset()){
...
}

How can I iterate the values of hashmapB inside the same loop? Actually, I don't want to use the inner loop.

is the question how to iterator two hashmaps in one loop ? @Hami — Ryuzaki L
– Ryuzaki L, Commented Mar 5, 2020 at 21:54

dan1st · Accepted Answer · 2020-03-05 22:03:49Z

1

If you just want to iterate over all keys:

Just create a new HashSet out of the keys of the first Map and add the keys of the second Map:

Collection<Map.Entry<String,Double>> entries=new HashSet<>(hashmapA.entrySet());
keys.addAll(hashmapB.entrySet());
for(Map.Entry<String,Double> entry:entries){
    String key=entry.getKey();
    Double value=entry.getValue();
    ...
}

This can also be done using Java 8 Streams:

for(Map.Entry<String,Double> entry: Stream.concat(hashmapA.entrySet().stream(),hashmapB.entrySet().stream()){
     String key=entry.getKey();
    Double value=entry.getValue();
    ...
}

If you just want the intersection of the maps, you can use:

Collection<String> keys=new HashSet<>(hashmapA.keySet());
keys.retainAll(hashmapB.keySet());
for(String key:keys){
    Double aValue=hashmapA.get(key);
    Double bValue=hashmapB.get(key);
    ...
}

Or (with Streams):

for(String key: hashmapA.entrySet().stream().filter(k->hashmapB.containsKey(k))){
    Double aValue=hashmapA.get(key);
    Double bValue=hashmapB.get(key);
    ...
}

As @bsaverino stated in the comments:

Regarding your latest remark @Hami then just iterating over the keys of hashmapA and using hashmapB.containsKey(...) could have been enough.

The following will also work in your case:

for(String key:hashmapA.keySet()){
    if(hashmapB.containsKey(key){
        Double aVal=hashmapA.get(key);
        Double bVal=hashmapB.get(key);
    }
}

edited Mar 5, 2020 at 22:03

answered Mar 5, 2020 at 21:34

dan1st

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14 Comments

dan1st Over a year ago

I Fixed that :)

dan1st Over a year ago

You can get the values from both sets using retainAll(you'll need to use .keySet() for this)

dan1st Over a year ago

I've added this too.

bsaverino Over a year ago

Regarding your latest remark @Hami then just iterating over the keys of hashmapA and using hashmapB.containsKey(...) could have been enough.

dan1st Over a year ago

You came up with the idea(even if I'm sure that many peaple already came up with the same idea) - You deserve the credit.

|

Ryuzaki L · Accepted Answer · 2020-03-05 21:31:10Z

1

Iterator is one of the best choice either with keyset or entrySet

Iterator hmIterator1 = hashmapA.entrySet().iterator(); 
Iterator hmIterator2 = hashmapB.entrySet().iterator(); 

 while (hmIterator1.hasNext() && hmIterator2.hasNext()) { 

       hmIterator1.next();
       hmIterator2.next();

    }

answered Mar 5, 2020 at 21:31

Ryuzaki L

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2 Comments

bsaverino Over a year ago

A necessary improvement I guess is to put an OR (||) instead of AND (&&) in the while condition and then check inside loop if each iterator .hasNext() is true before calling .next(). And by the way performing some pre-processing on the hashmaps before some iterations would sound better I believe (i.e. perform union and exclude duplicates or intersects).

Ryuzaki L Over a year ago

I would leave that to OP either to use OR and AND condition with the logic inside of loop @bsaverino

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