Java - How to check the array length via reflection (Field in class)

Question

I have the following class:

public class GeneralClass
{
    public int x;
    public int[] arr = new int[3];
}

And the following logic:

public void loadClass(Class cls) {          
    Field[] fields = cls.getDeclaredFields();
    for (Field field : fields) {            
            if (field.getType().isArray()) {
                // How can I print the array length ?
            }
    }
}

How can I know what is the size (length) of the array ?

user2336315 · Accepted Answer · 2014-11-20T19:43:13.737

8

You can use Array.getLength, however you need to provide a concrete instance so that's why I use cls.newInstance() (it's assuming there is a no-arg constructor)

public static void loadClass(Class<?> cls) throws IllegalArgumentException, IllegalAccessException, InstantiationException {          
        Field[] fields = cls.getDeclaredFields();
        for (Field field : fields) {      
                if (field.getType().isArray()) {
                    int length = Array.getLength(field.get(cls.newInstance()));
                    System.out.println(length);
                }
        }
    }

Note that if the field was static you could get it at a class level, so int length = Array.getLength(field.get(null)); would work.

edited Nov 20 '14 at 19:43

answered Nov 20 '14 at 19:35

user2336315

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Seven answers, and only two understood what the question was asking, and only one provided the solution. Kudos. – David Conrad Nov 20 '14 at 19:40
@DavidConrad Scary, isn't it? – Seelenvirtuose Nov 20 '14 at 19:43

score 2 · Answer 2 · answered Nov 20 '14 at 19:33

2

The array field arr belongs to an instance of the class, hence its length cannot be determined by the Field object obtained via reflection as this is essentially class level metadata.

answered Nov 20 '14 at 19:33

Invisible Arrow

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But we declared the array size in the class ? (it is not enogth) ? – user3668129 Nov 20 '14 at 19:34
@user3668129 Read the answer, especially this part: "[...] belongs to an _instance_ of the class". On an instance level you could read the size of the array (with the reflectional approach), on the class level you cannot. – Seelenvirtuose Nov 20 '14 at 19:38

score 0 · Answer 3 · answered Nov 20 '14 at 19:26

0

Simple

System.out.println(field.length);

answered Nov 20 '14 at 19:26

Gabriel Oliveira

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9

there is no length in Field class – user3668129 Nov 20 '14 at 19:28
OP is asking about a reflectional access. – Seelenvirtuose Nov 20 '14 at 19:36

score 0 · Answer 4 · edited Nov 20 '14 at 19:28

0

the way to find the length of an array is fields.length

edited Nov 20 '14 at 19:28

Anthony Forloney

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answered Nov 20 '14 at 19:28

Jamie Paolino

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Did you try your answer ? there is no "length" attribute in field (field is of type Field) – user3668129 Nov 20 '14 at 19:31
@user3668129 The syntax is `.length` to access the *length* of an array. Given by your code, `fields` is the array containing `Field` objects, so `fields` is what you are interested in. – Anthony Forloney Nov 20 '14 at 19:35
No, `arr` is what Opie is interested in. – David Conrad Nov 20 '14 at 19:38

score -1 · Answer 5 · edited May 23 '17 at 12:01

-1

Array length is fairly easy. It is

arrayName.length

So you can say :

System.out.print(   "length  is " +  arr.length  );

Where is array's length property defined?

edited May 23 '17 at 12:01

Community

1
1

answered Nov 20 '14 at 19:26

Caffeinated

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? I'm getting "Field", so I cant do what you suggest me to do – user3668129 Nov 20 '14 at 19:28

Java - How to check the array length via reflection (Field in class)

5 Answers5