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

问题

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;
    }
}

回答1:

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.

回答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();
//-------------^

回答3:

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)

来源：https://stackoverflow.com/questions/44341995/why-type-casting-is-required-while-using-itrerator-object-in-the-while-loop

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java

collections