How to correct this simple Java code? [How to print all elements of a variable with apprarent type = Object and actual type = int (array)?]

Question

public class HelloWorld{

     public static void main(String []args){
        System.out.println("Hello World");
        String s = new String("abc");
        int [] a = {1,2,3,4};
        System.out.println(s);
        System.out.println(a[1]);
        Object o1 = s;
        Object o2 = a;
        System.out.println(o1);
        System.out.println(o2[2]);

     }
}

I am geeting the following error after compiling and executing the aforementioned code on the website http://www.compileonline.com/compile_java_online.php :

Compiling the source code....
$javac HelloWorld.java 2>&1

HelloWorld.java:12: error: array required, but Object found
        System.out.println(o2[2]);
                             ^
1 error

How do I print each element of the array saved in the object o2?

score 4 · Answer 1 · answered Jul 23 '14 at 05:11

4

You need to cast it to int array first and print the index of it.

sample:

System.out.println(((int[])o2)[2]);

result:

answered Jul 23 '14 at 05:11

Rod_Algonquin

26,074
6
52
63

1

You don't need to "print the index of it" (which your code snippet doesn't anyway). – laune Jul 23 '14 at 05:19

score 2 · Accepted Answer · answered Jul 23 '14 at 05:17

2

In Java, every array is also an Object.

But every non-primitive value is an Object, so clearly not every Object is an array. It could be a String, an Integer, an object of your own class, etc. The compiler doesn't know about this at compile time: it only becomes clear at runtime what the actual type of the Object is.

You can check if your object is an array using instanceof and you can cast it to an array type:

if (o2 instanceof int[]) {
    int[] myarray = (int[])o2;
    System.out.println(myarray[2]);
}

But you need to cast it before you can use it as an array, it's not optional.

answered Jul 23 '14 at 05:17

Erwin Bolwidt

30,799
15
56
79

Your answer states the correct reason why `o2[2]` not allowed and uses `instance of`.(+1) – akash Jul 23 '14 at 05:24
...and it shows how to check for an int[] before the cast (+1) – laune Jul 23 '14 at 05:26

score 0 · Answer 3 · answered Jul 23 '14 at 05:18

Here's the solution use following code:

public class HelloWorld{

 public static void main(String []args){
    System.out.println("Hello World");
    String s = new String("abc");
    int [] a = {1,2,3,4};
    System.out.println(s);
    System.out.println(a[1]);
    Object o1 = s;
    Object o2 = a;
    System.out.println(o1);
    System.out.println(((int[])o2)[2]);

 }

}

Output

Hello World
abc
2
abc
3

Cause: When you assign a array to object, for traversing you need to cast it back to array and than get the values

score 0 · Answer 4 · edited May 23 '17 at 11:51

You can write a wrapper method to print the Object. This method will check the type and print the object accordingly. Like:

public class HelloWorld {

    public static void main(String []args){
        System.out.println("Hello World");
        String s = new String("abc");
        int [] a = {1,2,3,4};
        System.out.println(s);
        System.out.println(a[1]);
        Object o1 = s;
        Object o2 = a;
        printObject(o1);
        printObject(o2);
    }

    private static void printObject(Object object) {
        if (object instanceof String) {
            System.out.println(object);
        } else if (object instanceof int[]) {
            for (int element : (int []) object) {
                System.out.println(element);
            }
        }
        // Add other datatype checks
    }
}

Output will be:

Hello World
abc
2
abc
1
2
3
4

As you want to print all the elements of the object, this method do the same.

You can also check the this link to know about object type verification.

How to correct this simple Java code? [How to print all elements of a variable with apprarent type = Object and actual type = int (array)?]

4 Answers4