Get the type of array element in Java

Question

I have the following method:

public static int arraySum(Object obj) {
}

The method should return the sum of all elements in obj; a precondition for this method is that obj be an Integer array of any dimension, i.e. Integer, Integer[], Integer[][], so on.

In order to write the body of the method arraySum(), I'm using a foreach loop and recursion; however, for the foreach loop I need to know which type the elements of obj are. Is there a way to find out the type (i.e. Integer, Integer[], etc.) of obj?

EDIT: This is for an assignment for my CS course. I don't want to simply ask how to write the method, that's why I'm asking such a specific question.

Is there a specific reason why you're not just using 2 overloads? — Jeroen Vannevel
– Jeroen Vannevel, Commented Oct 19, 2013 at 21:14
if(myarray[I] instanceof type) ....; think this is what you want? — pedromss
– pedromss, Commented Oct 19, 2013 at 21:15
@pedromss obj could have any number of dimensions, meaning lots and lots of if statements — apparatix
– apparatix, Commented Oct 19, 2013 at 21:18
Related to stackoverflow.com/questions/5670972/…. You should have a good reason of wanting to do this. It sounds like a XY problem to me. Maybe you should give us more context on why you want to do this. — Ingo Bürk
– Ingo Bürk, Commented Oct 19, 2013 at 21:19

arshajii · Accepted Answer · 2013-10-19 21:22:10Z

7

I believe it's simpler than you think:

public static int arraySum(Object obj) {
    if (obj.getClass() == Integer.class)
        return ((Integer) obj).intValue();

    int sum = 0;
    for (Object o : (Object[]) obj)
        sum += arraySum(o);

    return sum;
}

Basically we exploit the fact that an Integer array of any dimension is still an Object[].

Object obj = new Integer[][][]{{{1,2,3}},{{4,5,6},{7,8,9}},{{10}}};

System.out.println(arraySum(obj));

answered Oct 19, 2013 at 21:22

arshajii

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3 Comments

A4L Over a year ago

This will cause a ClassCastException if you call it like this arraySum(new Object()). There should be an additional check for an array type obj.getClass().isArray() before the cast in the for loop and of course a null check.

arshajii Over a year ago

@A4L "a precondition for this method is that obj be an Integer array of any dimension" Although the OP can add those checks if he really needs them. It's a trivial addition, and the core of the method will remain the same.

A4L Over a year ago

OP must then make sure to pass only arrays to it. +1 still an elegant solution!

ilalex · Accepted Answer · 2013-10-19 21:23:16Z

0

You can do it with simple instanceof, for example:

if (obj instanceof Integer[][]) 
    System.out.println("2d");
if (obj instanceof Integer[]) 
    System.out.println("1d");
if (obj instanceof Integer) 
    System.out.println("0d");

answered Oct 19, 2013 at 21:23

ilalex

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