Home » Java » How to for each the hashmap? [duplicate]

How to for each the hashmap? [duplicate]

Posted by: admin November 2, 2017 Leave a comment

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This question already has an answer here:

Answers:

I know I’m a bit late for that one, but I’ll share what I did too, in case it helps someone else :

HashMap<String, HashMap> selects = new HashMap<String, HashMap>();

for(Map.Entry<String, HashMap> entry : selects.entrySet()) {
    String key = entry.getKey();
    HashMap value = entry.getValue();

    // do what you have to do here
    // In your case, another loop.
}

Questions:
Answers:

Lambda Expression Java 8

In Java 1.8 (Java 8) this has become lot easier by using forEach method from Aggregate operations(Stream operations) that looks similar to iterators from Iterable Interface.

Just copy paste below statement to your code and rename the HashMap variable from hm to your HashMap variable to print out key-value pair.

HashMap<Integer,Integer> hm = new HashMap<Integer, Integer>();
/*
 *     Logic to put the Key,Value pair in your HashMap hm
 */

// Print the key value pair in one line.
hm.forEach((k,v) -> System.out.println("key: "+k+" value:"+v));

Here is an example where a Lambda Expression is used:

    HashMap<Integer,Integer> hm = new HashMap<Integer, Integer>();
    Random rand = new Random(47);
    int i=0;
    while(i<5){
        i++;
        int key = rand.nextInt(20);
        int value = rand.nextInt(50);
        System.out.println("Inserting key: "+key+" Value: "+value);
        Integer imap =hm.put(key,value);
        if( imap == null){
            System.out.println("Inserted");
        }
        else{
            System.out.println("Replaced with "+imap);
        }               
    }

    hm.forEach((k,v) -> System.out.println("key: "+k+" value:"+v));

Output:

Inserting key: 18 Value: 5
Inserted
Inserting key: 13 Value: 11
Inserted
Inserting key: 1 Value: 29
Inserted
Inserting key: 8 Value: 0
Inserted
Inserting key: 2 Value: 7
Inserted
key: 1 value:29
key: 18 value:5
key: 2 value:7
key: 8 value:0
key: 13 value:11

Also one can use Spliterator for the same.

Spliterator sit = hm.entrySet().spliterator();

UPDATE


Including documentation links to Oracle Docs.
For more on Lambda go to this link and must read Aggregate Operations and for Spliterator go to this link.

Questions:
Answers:

Map.values():

HashMap<String, HashMap<SomeInnerKeyType, String>> selects =
    new HashMap<String, HashMap<SomeInnerKeyType, String>>();

...

for(HashMap<SomeInnerKeyType, String> h : selects.values())
{
   ComboBox cb = new ComboBox();
   for(String s : h.values())
   {
      cb.items.add(s);
   }
}

Questions:
Answers:

You can iterate over a HashMap (and many other collections) using an iterator, e.g.:

HashMap<T,U> map = new HashMap<T,U>();

...

Iterator it = map.values().iterator();

while (it.hasNext()) {
    System.out.println(it.next());
}

Questions:
Answers:

I generally do the same as cx42net, but I don’t explicitly create an Entry.

HashMap<String, HashMap> selects = new HashMap<String, HashMap>();
for (String key : selects.keySet())
{
    HashMap<innerKey, String> boxHolder = selects.get(key);
    ComboBox cb = new ComboBox();
    for (InnerKey innerKey : boxHolder.keySet())
    {
        cb.items.add(boxHolder.get(innerKey));
    }
}

This just seems the most intuitive to me, I think I’m prejudiced against iterating over the values of a map.

Questions:
Answers:

Use entrySet, see example here:

Questions:
Answers:

As mentioned already, in Java 8 we have got forEach method that accepts a lambda expression. But we have also got stream APIs.

Iterate over entries (Using forEach and Streams):

sample.forEach((k,v) -> System.out.println(k + "=" + v)); 
sample.entrySet().stream().forEach((entry) -> {
            Object currentKey = entry.getKey();
            Object currentValue = entry.getValue();
            System.out.println(currentKey + "=" + currentValue);
        });

The advantage with streams is they can be parallelized easily and can be useful when we have multiple CPUs at disposal. We simply need to use parallelStream() in place of stream() above. If we want to iterate over keys we can use sample.keySet() and for values sample.values().