[java] How to for each the hashmap?

I have this field:

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

For each Hash<String, HashMap> I need to create a ComboBox, whose items are the value (which happens to be a HashMap itself) of HashMap <String, **HashMap**>.

By way of (non-functioning) demonstration:

for (int i=0; i < selects.size(); i++) {
    HashMap h = selects[i].getValue();
    ComboBox cb = new ComboBox();

    for (int y=0; y < h.size(); i++) {
        cb.items.add(h[y].getValue);
    }
}

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());
}

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.

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);
   }
}

Use entrySet,

/**
 *Output: 
D: 99.22
A: 3434.34
C: 1378.0
B: 123.22
E: -19.08

B's new balance: 1123.22
 */

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

public class MainClass {
  public static void main(String args[]) {

    HashMap<String, Double> hm = new HashMap<String, Double>();

    hm.put("A", new Double(3434.34));
    hm.put("B", new Double(123.22));
    hm.put("C", new Double(1378.00));
    hm.put("D", new Double(99.22));
    hm.put("E", new Double(-19.08));

    Set<Map.Entry<String, Double>> set = hm.entrySet();

    for (Map.Entry<String, Double> me : set) {
      System.out.print(me.getKey() + ": ");
      System.out.println(me.getValue());
    }

    System.out.println();

    double balance = hm.get("B");
    hm.put("B", balance + 1000);

    System.out.println("B's new balance: " + hm.get("B"));
  }
}

see complete example here:

• http://www.java2s.com/Code/JavaAPI/java.util/HashMapentrySet.htm

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.

Streams Java 8

Along with forEach method that accepts a lambda expression we have also got stream APIs, in Java 8.

Iterate over entries (Using forEach and Streams):

sample.forEach((k,v) -> System.out.println(k + "=" + v)); 
sample.entrySet().stream().forEachOrdered((entry) -> {
            Object currentKey = entry.getKey();
            Object currentValue = entry.getValue();
            System.out.println(currentKey + "=" + currentValue);
        });
sample.entrySet().parallelStream().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. With parallel streams it makes more sense to use forEach as forEachOrdered would make no difference in performance. If we want to iterate over keys we can use sample.keySet() and for values sample.values().

Why forEachOrdered and not forEach with streams ?

Streams also provide forEach method but the behaviour of forEach is explicitly nondeterministic where as the forEachOrdered performs an action for each element of this stream, in the encounter order of the stream if the stream has a defined encounter order. So forEach does not guarantee that the order would be kept. Also check this for more.