how to iterates a hashmap, does it need to cast the keySet() in Java?

Question

I have a question about iterate a hashmap:

Map<String, Integer> map = new HashMap<String, Integer>();

I want to iterate this hashmap and printout out the key and value pair using the following code:

Iterator iterator = portMap.keySet().iterator();

but it indicates me to cast the portMap.keySet().iterator(); to (Iterator)portMap.keySet().iterator();, but the iterator() return as a iterator already, why I need to cast it? And I checked some other code, there is no cast for the return value of iterator(), what is the reason, and how to fix it?

Is there any errors? I guess your IDE would just be showing a warning (yellow underline in eclipse), that's because whats returned could be Iterator and you are assigning it to Iterator — Dhana Krishnasamy, Feb 05 '14 at 14:12

assylias · Answer 1 · 2014-02-05T14:17:43.090

5

If you want to print the keys AND the values, you should use entrySet instead:

for (Map.Entry<String, Integer> e : map.entrySet()) {
    System.out.println(e.getKey() + " = " + e.getValue());
}

As for your casting issue, it may be because you are importing a different Iterator class in your imports. With the correct imports, this compiles fine without casting:

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;

//.....

Map<String, Integer> map = new HashMap<String, Integer>();
Iterator<String> iterator = map.keySet().iterator();

edited Feb 05 '14 at 14:17

answered Feb 05 '14 at 14:09

assylias

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why iterator should be inported then? – ratzip Feb 05 '14 at 14:12
@ratzip i think you mean "what" and not "why" – diazazar Feb 05 '14 at 14:13

score 0 · Answer 2 · answered Feb 05 '14 at 14:29

You can use the enhanced loop in the following way:

for (Entry entry :  map.entrySet()) {
    System.out.format("%s , %s\n", entry.getKey(), entry.getValue());
}

so like this you loop over all entries of the map. In the import section you should have the following import of the Entry

import java.util.Map.Entry;

hence the complete imports section would look like this:

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

import java.util.Map.Entry;

If you want to go over the keys and extract the corresponding value for each of them you can go the following way:

import java.util.HashMap;
import java.util.Map;
// ...
for (String s : map.keySet()) {
   System.out.format("%s , %s\n", s, map.get(s));
}

If you want to use an iterator and loop over the keys and extracting from the map based on the key:

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
//...

Iterator<String> it = map.keySet().iterator();

while (it.hasNext()){
   String currentKey = it.next();
   System.out.format("%s , %s\n", currentKey), map.get(currentKey));
}

I hope these help you.

how to iterates a hashmap, does it need to cast the keySet() in Java?

2 Answers2