How do I iterate through a Java Linked Hash map?

Question

I searched this question and found answers that used the Map.Entry like here, however the getValue() method returned an Object object instead of the type of object in the map. Like in the example below, I need it to return a User object so I can use a method from that class. When I tried using the while loop below however, it never leaves the loop. I was wondering the correct way to do this.

Map<String, User> users = new LinkedHashMap<String, User>();
users.put(name, user);

while(users.values().iterator().hasNext()){

   currentUser = users.values().iterator().next();
   currentUser.someMethod();
}

Solution 1

You're misusing the Iterator, and you're omitting the Generics specifications.

Iterator<User> it = users.values().iterator();
while (it.hasNext())
{
  User currentUser = it.next();
  currentUser.someMethod();
}

Solution 2

I was wondering the correct way to do this.

You should use the Map.Entry type; you just need to provide type parameters to use with generics:

for (Map.Entry<String,User> entry : users.entrySet()) {
    // entry.getValue() is of type User now
}

Solution 3

The easiest way to iterare is using for. In a LinkedHashMap it would be like this:

   private static LinkedHashMap<Integer, String> dataBase = new LinkedHashMap<Integer, String>();

   for(Integer key : dataBase.keySet()) {
         String ret = dataBase.get(key);
    }

Source

Solution 4

Each time you call .iterator(), it gives you a brand new iterator at the first element.

You need to call .iterator() once and store it in a local variable.

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Comments

[user1875021]

So like Iterator temp = users.values.iterator();?

[user1875021]

Then would I be able to do entry.getValue().someUserMethod();

[user207421]

@user1875021 As he's already told you that entry.getValue() is of type User, the answer to that should be obvious. You could always try it.

[user1875021]

Ah I see. I tried that, but wasn't familiar about Generics so I kept getting an Object. Thanks

[Sergey Kalinichenko]

@user1875021 Absolutely! entry.getValue() is of type User, you can do anything you would like with it.

[user207421]

You don't have to remove anything. You just have to iterate correctly. This code will remove and skip every second element. -1

[findusl]

What is supposed to be the advantage of using entrySet()? He doesn't use the key, so iterating over values() totally get's the job done or?

[Sergey Kalinichenko]

@findusl If OP isn't planning on using the key, of course he should use values(); perhaps it would be slightly more economical in terms of CPU cycles. I was under impression that the code inside the loop wasn't a complete fragment, and wanted to leave OP some flexibility to access the key along with the value.