Problem in ArrayList class(java.util.*)

[Cosmos]

i have been reading ArrayList class when i was stuck with some concepts....

i read in Herbert Schildt that we cant pass primitive data type in the argument list of "add" method of List interface....but when i tried to pass 1 as the argument there was no errors...this is my code-

import java.util.*;

public class ArrayListToArray {
    public static void main(String[] args) {
        ArrayList a1=new ArrayList();
        int sum=0;
        a1.add(1);
        a1.add(2);
        a1.add(3);
        System.out.println("Contents of a1:"+a1);
        Object arr[]=a1.toArray();
        for(int i=0;i<arr.length;i++){
           sum=sum+((Integer)arr[i]).intValue();
                  
        }
        System.out.println(sum);
    }
    
}

can you please explain where am i wrong?

one more thing when i tried this-

sum=sum+(int)arr[i];
it didnt work....it was saying that required:int and found:java.lang.Object...

please explain both.....thanks in advance....

[sunde887]

From what I've heard by those much smarter than I, Herbert Schildt books are bad. That being said, the advice is incorrect as given by the author. The list can have ints added to it. Thanks to auto-boxing, it will automatically convert the literal number '1', to an Integer object, which allows 1,2,3,etc to be added to a list. The problem with trying to add them is that your array(arr), contains an array of objects. The '+' operator takes 2 numerals(or strings), and performs the correct operation, but you are attempting to add objects(which causes the error).

You should use generics when using Lists to specify the type that will be inserted and removed in a much simpler fashion.

List<Integer> numList = new ArrayList<Integer>();

You simply add <...> to each class name, where ... stands for the class. For all the primitives there exists an equivalent class which is the same name, started with a capital(int=Integer, double=Double, float=Float, etc).

In your snippet try casting the arr[1] to an Integer

sum = sum+(Integer)arr[1];

And see how it reacts.

[pbrockway2]

i read in Herbert Schildt that we cant pass primitive data type in the argument list of "add" method of List interface....but when i tried to pass 1 as the argument there was no errors

Schildt is wrong and the compiler is right.

What happens is this: you use the primitive int value 1 as an argument and it gets converted to an Integer instance with the value 1. It is a reference to this Integer instance that gets added to the list. This is called "boxing".

sum=sum+(int)arr[i];
it didnt work....it was saying that required:int and found:java.lang.Object...

You declared arr as an array of Object instances. arr[i] is therefore an Object and the compiler is quite rightly complaining. I would have expected it to say something about + being undefined for int and Object, but, in any case, you can't cast an Object into an int. What you have to do is what you have done: cast arr[i] to Integer and then get the value of that Integer instance.

Note that you can also rely on the opposite of the "boxing" process above. That is, the object (Integer)arr[i] will be automatically "unboxed" to provide the int value. Like this

sum = sum + (Integer)arr[i];

[Norm]

The place where "boxing" fails is if you add an int , you can't remove it the same way:
anArrayList.add(234); // this will be boxed into an Integer.
anArrayList.remove(234); // this will not be boxed, it will look for element 234
So boxing is tricky and requires the programmer to know when it is happening and when not.

[Tolls]

How old is the Schildt book?
If it's pre 1.5 then it's understandable.