Why type casting is required while using itrerator object in the while loop?

Question

I am learning java right now. While writing code for traversing ArrayList using Iterator I have to use the class name before using the iterator's object with next() function. Can anybody help me with this?

import java.util.*;

public class arraylistwithuserdefinedclass {
    public static void main(String[] args) {    
        ArrayList<UserId> details=new ArrayList<UserId>();
        UserId a=  new UserId(22,"gurmeet");    
        UserId b=  new UserId(24,"sukhmeet");
        details.add(a);
        details.add(b);
        Iterator itr = details.iterator();
        while(itr.hasNext()) {    
            UserId ui = (UserId) itr.next();
            System.out.println(ui.age +" " + "" + ui.name) ;
        }
    }       
}

class UserId {
    int age;
    String name;
    UserId(int a, String b) {
        age=a;
        name=b;
    }
}

score 4 · Accepted Answer · answered Jun 03 '17 at 08:35

4

Make your Iterator UserId type (specify the type), ie

Iterator<UserId> itr = details.iterator();

Because if you don't specify the type, how will it understand what to return. So for generalized purpose it will return Object type and thats why downcasting is required.

answered Jun 03 '17 at 08:35

Arun Sudhakaran

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what is downcasting? – GURMEET SINGH Jun 03 '17 at 08:46
Are you familiar with narrowing and widening of primitive data types in Java – Arun Sudhakaran Jun 03 '17 at 09:02
no i am not familiar with it – GURMEET SINGH Jun 03 '17 at 10:09
**Casting** https://stackoverflow.com/questions/23414090/what-is-the-difference-between-up-casting-and-down-casting-with-respect-to-class – Arun Sudhakaran Jun 03 '17 at 10:25
**Widening&NArrowing** https://stackoverflow.com/questions/16781649/real-world-application-of-widening-narrowing-conversion – Arun Sudhakaran Jun 03 '17 at 10:28
thanks for helping me – GURMEET SINGH Jun 03 '17 at 11:46

Youcef LAIDANI · Answer 2 · 2017-06-03T09:13:05.297

2

You have to specify the generic type for the Iterator because without this type the Iterator holds type Object :

Iterator itr = details.iterator();// This holds type Object.

for that it is required to cast every Object.

another reference :

it.next() Returns the next object. If a generic list is being accessed, the iterator will return something of the list's type. Pre-generic Java iterators always returned type Object, so a downcast was usually required.

it is not required if you set the type for the Iterator :

Iterator<UserId> itr = details.iterator();
//         ^^--------------------------

So you can use without casting :

while (itr.hasNext()) {
    UserId ui = itr.next();
//-------------^

edited Jun 03 '17 at 09:13

answered Jun 03 '17 at 08:35

Youcef LAIDANI

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if i use it without casting then it shows me an error and i cant compile my code – GURMEET SINGH Jun 03 '17 at 08:39
you have to use `Iterator` like i already mentioned instead of just `Iterator` @GURMEETSINGH – Youcef LAIDANI Jun 03 '17 at 08:40
check my edit @GURMEETSINGH you will understand more now – Youcef LAIDANI Jun 03 '17 at 08:59
thanks for clearing my concept. – GURMEET SINGH Jun 03 '17 at 09:02

ΦXocę 웃 Пepeúpa ツ · Answer 3 · 2017-06-03T08:38:27.583

1

Your iterator is declared raw, that is the reason why you need to cast, do instead declare the iterator as a UserId doing Iterator<UserId>

Iterator<UserId> itr = details.iterator();
    while(itr.hasNext()) {
        ...
        UserId ui = (UserId)

edited Jun 03 '17 at 08:38

answered Jun 03 '17 at 08:36

ΦXocę 웃 Пepeúpa ツ

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yes i am getting this warning ** Iterator is a raw type. References to generic type Iterator should be parameterized** . but what does raw means here. – GURMEET SINGH Jun 03 '17 at 08:38
and do tell me how to remove this problem. – GURMEET SINGH Jun 03 '17 at 08:38
raw means it has no type – ΦXocę 웃 Пepeúpa ツ Jun 03 '17 at 08:38
is right there in the answer... do Iterator instead of Iterator – ΦXocę 웃 Пepeúpa ツ Jun 03 '17 at 08:39
1

raw means you haven't provided a type parameter. Think of them as a dynamic type. – killjoy Jun 03 '17 at 08:39
thanks for your help. – GURMEET SINGH Jun 03 '17 at 08:41

Why type casting is required while using itrerator object in the while loop?

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