How to check primitive empty array

Question

I'd like to create a method that checks every Object's value is empty.If the input object is null then return true;.If input is type of array, check it's length. Below is my method to implement this logic

public static boolean isEmpty(Object input) {
    if (input == null) {
        return true;
    }
        if (input instanceof Collection) {
        if (((Collection<?>) input).size() == 0) {
            return true;
        }
    }
    if (input instanceof String) {
        if (((String) input).trim().length() == 0) {
            return true;
        }
    }
    if (input instanceof Object[]) {
        if (((Object[]) input).length == 0) {
            return true;
        }
    }
    return false;
}

But the problem is while I testing as like this

int[] a = {};
float[] b = {};
Integer[] c = {};
Float[] d = {};
System.out.println(Validator.isEmpty(a));
System.out.println(Validator.isEmpty(b));
System.out.println(Validator.isEmpty(c));
System.out.println(Validator.isEmpty(d));

I have no idea why a and b are false. Can somebody explain me ?

"I have no idea why a and b are false." Have you checked what you get when you do new float[0] instanceof Object[]? — T.J. Crowder
– T.J. Crowder, Commented Nov 6, 2016 at 13:55
Is there a specific reason for this method? Have you considered using three different isEmpty methods which take different type arguments? — pathfinderelite
– pathfinderelite, Commented Nov 6, 2016 at 13:57
@T.J.Crowder Oops ! I got Incompatible conditional operand types float[] and Object[] — Cataclysm
– Cataclysm, Commented Nov 6, 2016 at 13:57
You can overload the isEmpty method for all primitive array types — OneCricketeer
– OneCricketeer, Commented Nov 6, 2016 at 14:01
By the way, you probably want to check if it's a CharSequence instead of just String. This will catch not only String but also StringBuilder and many other string-like objects. — Klitos Kyriacou
– Klitos Kyriacou, Commented Nov 6, 2016 at 14:07

T.J. Crowder · Accepted Answer · 2016-11-06 14:06:18Z

5

float[] is not instanceof Object[]. If you want to check for all kinds of arrays, you probably want to get the class from the object and check its isArray method. Then you can use Array.getLength to get its length (since, bizarrely, you can't use Class#getField to get the length field):

import java.lang.reflect.*;

class Validator {
    public static void main(String args[]) throws Exception {
        int[] a = {};
        float[] b = {};
        Integer[] c = {};
        Float[] d = {};
        System.out.println(Validator.isEmpty(a));            // true
        System.out.println(Validator.isEmpty(b));            // true
        System.out.println(Validator.isEmpty(c));            // true
        System.out.println(Validator.isEmpty(d));            // true
        System.out.println(Validator.isEmpty(new float[3])); // false (just double-checking)
    }

    public static boolean isEmpty(Object input) {
        if (input == null) {
            return true;
        }
        if (input instanceof String) {
            if (((String) input).trim().length() == 0) {
                return true;
            }
        }
        if (input.getClass().isArray()) {
            return Array.getLength(input) == 0;
        }
        return false;
    }
}

edited Nov 6, 2016 at 14:06

answered Nov 6, 2016 at 13:57

T.J. Crowder

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2 Comments

Cataclysm Over a year ago

Thanks alot for code example. I'm still trouble about how to change my method? before this.

T.J. Crowder Over a year ago

@Cataclysm: Yeah, it turned out to be more of a pain than I was expecting... I wasn't expecting Class#getField to refuse to give me the length field of an array (although it's in the documentation, as it turns out).

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