Guess the answer and please explain

[Abdul Mohsin]

int[] str={2,3,4,5};

List list=Arrays.asList(str);

System.out.println(list.size());

Regards,

Abdul Mohsin

[Abdul Mohsin]

Hi,

answer is 1

but can anybody explain me the reason .

Regards,

Abdul Mohsin

[John Stone]

because asList(T...a) calls Arrays.ArrayList(E[] array), and in generics only Object subtypes are allowed as type parameters.

Your array, is in fact processed as a single object of type "Object", wrapped to one dimensional array :-).

try this code:

import java.util.*;
 
class Test { 
public static void main(String[] args) { 
Integer[] str={2,3,4,5};
 
List list=Arrays.asList(str);

System.out.println(list.size());
System.out.println(list.getClass().getName());
} 
} 

gives 4

[John Stone]

accessing your array if we use int[]

import java.util.*;
 
class Test { 
public static void main(String[] args) { 
int[] str={2,3,4,5};
 
List list=Arrays.asList(str);

int[] str2 = (int[])list.get(0);
System.out.println( str2.length );
} 
} 

[Abdul Mohsin]

Thanks John,

I got it .

Regards,

Abdul Mohsin

[Barry Gaunt]

Is this:

import java.util.*;
 
class Oh { 
    public static void main(String[] args) { 
 
        List<Integer> list=Arrays.asList(1,2,3,4);
        
        for (int i : list) {
            System.out.println(i);
        }
        System.out.println( list.size() );
    } 
}     

not how it is meant to be used?

[Abdul Mohsin]

Hi John,

I understood that but
Can you explain it a bit more that what happens
when we pass int array and String array to generic vararg method .
How these two are considered differently , int array as one object and String array as normal array.

Regards,

Abdul Mohsin

[Sergio Tridente]

This line won't compile:

Originally posted by Barry Gaunt:
List<Integer> list=Arrays.asList(1,2,3,4);

It'll give: "cannot convert from List<int[]> to List<Integer>"
[ May 21, 2007: Message edited by: Sergio Tridente ]

[John Stone]

Originally posted by Sergio Tridente:
This line won't compile:

It'll give: "cannot convert from List<int[]> to List<Integer>"

[ May 21, 2007: Message edited by: Sergio Tridente ]

Compiles & run fine with java 1.5.

[Sergio Tridente]

Originally posted by John Stone:

Compiles & run fine with java 1.5.

No, it doesn't.

The following code:

import java.util.*;
 
class ArraysTest {
public static void main(String[] args) {
int[] str2={2,3,4,5};
 
List<Integer> list2=Arrays.asList(str2);
System.out.println(list2.size());
System.out.println(list2.getClass().getName());
for(Object i:list2)
System.out.print(i.getClass().getName() + " ");
}
}

is giving me the following error when I try to compile it:

sergio@linux:~> javac ArraysTest.java
ArraysTest.java:7: incompatible types
found : java.util.List<int[]>
required: java.util.List<java.lang.Integer>
List<Integer> list2=Arrays.asList(str2);
^
1 error

Can you try to compile the very same code andpost results? Thank you.

[Barry Gaunt]

For me it compiles and runs producing:
1
2
3
4
4

But I am using the Java 6.0 jdk - perhaps there's a diffence with the Java 5.0 jdk.

Nope - it works under Java 5.0 too.
[ May 21, 2007: Message edited by: Barry Gaunt ]

[Marcos R Oliveira]

Sergio,

Your code doesn�t compile, Barry�s does. In Barry�s code:

  List<Integer> list=Arrays.asList(1,2,3,4);  

each item in the asList() method is autoboxed to an Integer what gives an array of four Integers.

In your code:

  List<Integer> list2=Arrays.asList(str2);  

you are trying to convert List<int[]> to List<Integer>.

Marcos

[Abdul Mohsin]

it will compile and run fine in 1.5

Regards,

Abdul Mohsin

[Barry Gaunt]

Yeah. It would help if he ran the same code as me :roll:

[Sergio Tridente]

Sorry, my mistake. I didn't realise that Barry's code was using varargs.

I tested it and it worked fine. Thanks.

However, now I don't understand why those integers get autoboxed when varargs are involved. Does anyone have an answer? Thank you again.