Converting int array to collections

Question

I'm learning Java step by step. Now I know up to arrays, and next is Collections.

As I'm learning based on projects, I noticed that most of programmers choose to use Collections rather than plain arrays to eliminate the loops and to make use of the predefined functions (like sort and much more). So, first I'm trying to convert an int[] into a Collection. But I get an error in the foreach loop...

import java.util.Arrays;
import java.util.List;
import java.util.Scanner;

public class Coll {

    public static void main(String[] args) {
        Coll obj = new Coll();
        obj.op();
    }

    void op() {
        System.out.println("To convert Array to Collections");
        Scanner getinp = new Scanner(System.in);
        System.out.println("Enter the size of array : ");
        int siz = getinp.nextInt();
        int[] arry = new int[siz];
        System.out.println("Enter the values of elements");
        for (int i = 0; i < arry.length; i++) {
            System.out.println("arry[" + i + "] : ");
            arry[i] = getinp.nextInt();
        }

        List arrycoll = Arrays.asList(arry);
        for (String elem : arrycoll) {
            System.out.println(elem);
        }
    }
}

The error is in the following line:

for(String elem : arrycoll)

Pointing to arrycoll, it states:

required `String` but found `Object`

If I define the arry as String and store Strings, it wouldn't be a error, but for int it's not working..

Use *generics*.. `List`. Also, you are inserting an Integer and trying to get a String back.. — TheLostMind, Jul 02 '14 at 06:08
You can actually use the *foreach* syntax with plain arrays as well. And you can sort your arrays using `Arrays#sort(int[])` just as you'd use `Collections.sort(List)`. — ccjmne, Jul 02 '14 at 06:21

Elliott Frisch · Accepted Answer · 2014-07-02T06:20:56.190

0

Given this message

it states required String but found object

You are getting two important pieces of information. One, you are using String for elements of your int[]. And, two you have a raw type (so your Integer elements are being treated as Object elements). Don't use Raw Types! I'm pretty sure you wanted something like

List<Integer> arrycoll = Arrays.asList(arry);
for(Integer elem:arrycoll) { // <-- Integer, not String.
  System.out.println(elem);
}

Edit

Of course to work with a collection, you'll need an object. int has the Integer wrapper type, so you'll need to change

int[] arry=new int[siz];

to

Integer[] arry=new Integer[siz];

edited Jul 02 '14 at 06:20

answered Jul 02 '14 at 06:10

Elliott Frisch

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Now you will get this compiler error at: `Arrays.asList(arry)`. Type mismatch: cannot convert from List to List – kai Jul 02 '14 at 06:16
thank you.. changing it to Integer arry from int arry works out and after making it as list, Collections.reverse(arrycoll) works fine.. But what is the diff b/w use of Integer and int.. From the start I thought integer is expansion and int is keyword for defining datatype in program.. @Elliott Frisch – user562 Jul 02 '14 at 06:47
@krnaveen14 No. `int` is a [primitive type](http://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html). `Integer` is a wrapper class. The primitive types are a performance optimization. Also, the `Collection`(s) require Object instances. – Elliott Frisch Jul 02 '14 at 06:51
The main difference is that Integers can be used as subclasses of Object, for example be stored in a list or map as generic parameters. The "Integer" *class* also provides additional methods for the int. While most of the time this is hidden due to auto-boxing, you can explicitly cast between int to Integer. Also, a major key difference is that Integer can have the value *null*. – EpicPandaForce Jul 02 '14 at 08:52

Dean Leitersdorf · Answer 2 · 2014-07-02T06:36:41.060

0

No problem!

Change List to List which will now staticky type the Objects in the list to integers (Badically, they are all treated as Integers)
Replace String with Integer, as now all the objects in the list are of type Integer (not String or anything else)

edited Jul 02 '14 at 06:36

answered Jul 02 '14 at 06:11

Dean Leitersdorf

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Ritesh Gune · Answer 3 · 2014-07-02T06:36:22.890

0

required String but found Object

In your for each loop, you are using String as type for element whereas arrycoll is list of raw List references whose type is Object.

List arrycoll

You've declared list of raw List references. You have to tell the compiler that they'll be List<Integer> values:

List<Integer> arrycoll

and

use Integer instead of String for elem
Replace int[] arry = new int[siz]; with Integer[] arry = new Integer[siz];

Hence code will become:

        Integer[]  arry = new Integer[siz];

        List<Integer> arrycoll = Arrays.asList(arry);
        for (Integer elem : arrycoll) {
            System.out.println(elem);
        }

edited Jul 02 '14 at 06:36

answered Jul 02 '14 at 06:30

Ritesh Gune

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That wont work because `Arrays.asList(arry);` still return a List of this type: `List` – kai Jul 02 '14 at 06:32
yup. this works out.. thank you.. But I cant make yours as accepted becuase Elliott posted the answer first.. – user562 Jul 02 '14 at 06:56
Not a problem, Welcome. – Ritesh Gune Jul 02 '14 at 07:04

Wundwin Born · Answer 4 · 2014-07-02T08:49:02.193

0

For print purpose only, simply try like this; casting as int[]

List arrycoll = Arrays.asList(arry);
for (Object elem : arrycoll) {

    int[] iArr1 = (int[]) elem;     
    for (int i : iArr1) {
        System.out.println(i);
    }
}

edited Jul 02 '14 at 08:49

answered Jul 02 '14 at 06:33

Wundwin Born

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Converting int array to collections

4 Answers4