How collections with Objects reacts?

Question

my program contains an object and i want to understand how a collections that contains objects is helping me if i can`t use the methods ?

My code that i used :

ClassMain c = new ClassMain();
Map<String, ClassMain> s = new HashMap<>();
s.put("S", c);
Iterator it = s.keySet().iterator();

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

ClassMain :

public static void main(String[] args) {
}

public void print(){
    System.out.println("Printing");
}

@user3385390 a collection is something used to store a bunch of things, is that what you are asking? — , Mar 05 '14 at 20:52
@RC yes , but i meant to HashMap is it the same ?, why HashMap contains objects ? — user3385390, Mar 05 '14 at 20:54
@user3385390: `Map` is an interface, `HashMap` is an implementing class. — Chris Dargis, Mar 05 '14 at 20:55
I would advise some reading: http://docs.oracle.com/javase/tutorial/collections/ — , Mar 05 '14 at 20:56
@RC Yes , i have learned from that site but it too complicated and full of useless information, do you have something else ? — user3385390, Mar 05 '14 at 21:00
No, this is a complicated subject. Anything less complicated than the Oracle tutorials would barely scratch the surface. The information there is all pertinent and relevant. Perhaps you'd like to ask some more specific questions about the parts you don't understand. — Dawood ibn Kareem, Mar 05 '14 at 21:02

score 2 · Answer 1 · answered Mar 05 '14 at 20:50

Iterator is a generic type. But you use it as a raw type. So you lose the type information it should have.

The code should be:

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

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

Or simpler:

for (String key : s.keySet()) {
    System.out.println(key);
}

score 0 · Answer 2 · edited May 23 '17 at 10:31

This is a raw type and you don't have to use it:

Iterator it = ... ;

keySet returns a parameterized Set<K> which you can use here, as do all of the Collections methods that return another kind of Collection:

Iterator<String> = s.keySet().iterator();

This will let you use the objects returned by next as the actual type you've supplied to the Map:

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

score 0 · Answer 3 · answered Mar 05 '14 at 20:50

Collections are used for, well storing collections of things. You can pull out the objects in a Map like so:

ClassMain c = new ClassMain();
Map<String, ClassMain> s = new HashMap<String, ClassMain>();
s.put("S", c);

for (String key : s.keySet())
{
    ClassMain c = s.get(key);
    c.print();
}

How collections with Objects reacts?

3 Answers3