Can I define an array of size 0?

Yes, you can define an array of size 0. As per Java documentation, array has public final field length, which contains the number of components of the array. length may be positive or zero.

Example

int[] arr1 = {};

int[] arr2 = new int[0];

App.java

package com.sample.app;

public class App {
	public static void main(String args[]) {
		int[] arr1 = {};
		int[] arr2 = new int[0];

		System.out.println("arr1 length : " + arr1.length);
		System.out.println("arr2 length : " + arr2.length);

	}
}

Run App.java, you will see below messages in console.

arr1 length : 0

arr2 length : 0

Reference

https://docs.oracle.com/javase/specs/jls/se7/html/jls-10.html#jls-10.7

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Is length property of array final?

Yes, length property of array is final. As per Java documentation, array has public final field length, which contains the number of components of the array. length may be positive or zero.

App.java

package com.sample.app;

public class App {
 public static void main(String args[]) {
  int[] arr1 = {};
  int[] arr2 = new int[0];

  int[] arr3 = new int[3];
  arr3[0] = 1;

  System.out.println("arr1 length : " + arr1.length);
  System.out.println("arr2 length : " + arr2.length);
  System.out.println("arr3 length : " + arr3.length);
  
 }
}

Run App.java, you will see below messages in console.

arr1 length : 0
arr2 length : 0
arr3 length : 3

Reference

https://docs.oracle.com/javase/specs/jls/se7/html/jls-10.html#jls-10.7

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