Java Reflection - Get size of array object

Question

I was wondering if any knows hows to get the size of an array object using reflection?

I have a Vehicles component containing an array object of type Car.

Vehicles.java

public class Vehicles{

    private Car[] cars;

    // Getter and Setters
}

Car.java

public class Car{

    private String type;
    private String make;
    private String model;

    // Getter and Setters
}

I was wondering how I would be able to get the size of the cars array within the vehicles component using Java Reflection?

I current have the following:

final Field[] fields = vehicles.getClass().getDeclaredFields();

if(fields.length != 0){
    for(Field field : fields){
        if(field.getType().isArray()){
            System.out.println("Array of: " + field.getType());
            System.out.println(" Length: " + Array.getLength(field.getType()));
        }
    }
}

which results in the following error:

java.lang.IllegalArgumentException: Argument is not an array
    at java.lang.reflect.Array.getLength(Native Method)

Any ideas?

Guillaume · Answer 1 · 2013-04-09T16:26:20.817

16

The method Array.getLength(array) expects an array instance. In you code sample you are calling it on the array type for the field. It won't work as an array field can accept arrays of any lengths!

The correct code is:

Array.getLength(field.get(vehicles))

or simpler

Array.getLength(vehicles.cars);

or simplest

vehicles.cars.length

Take care of a null vehicles.cars value though.

edited Apr 09 '13 at 16:26

answered Apr 09 '13 at 16:19

Guillaume

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3

Well, you might as well say cars.length :-) – Gyro Gearless Apr 09 '13 at 16:23
1

You are right! The original code was so complicated for such a simple thing that I get messed up and kept the reflection... – Guillaume Apr 09 '13 at 16:27

Gyro Gearless · Answer 2 · 2013-04-09T16:36:58.020

4

I suppose you have to pass the array object itself to Array.getLength() so try

Array.getLength(field.get(vehicles))

edited Apr 09 '13 at 16:36

answered Apr 09 '13 at 16:22

Gyro Gearless

5,131
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14

`field.get()` on an object which would have that field. – Sotirios Delimanolis Apr 09 '13 at 16:25
Sure, have to pass the vehicles object! – Gyro Gearless Apr 09 '13 at 16:37

score 1 · Answer 3 · answered Apr 09 '13 at 16:21

1

try

System.out.println(" Length: " + Array.getLength(field.get(vehicles)));

answered Apr 09 '13 at 16:21

Evgeniy Dorofeev

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Java Reflection - Get size of array object

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