Assumed Knowledge

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Learning Outcomes
  • Understand why and how to perform null checks on arrays.

Author: Gaurav Gupta

The first thing one must understand is that the “contents” of an array is a reference to the memory location where the items are stored. The name of the array is the reference while the actual collection of items is the instance.

This has been emphasized under Compound Data.

Second - things will be much much easier if you draw memory diagrams (as shown in Compound Data.

Let’s dig in!

What happens when you declare an integer variable and print it?

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int a;
println(a); //compilation error

It will give you a compilation error: The local variable a may not have beenin itialized. To pacify Processing (or Java), you must initialize it to some value.

Similarly, with an array, you must initialize it with a value. The special value null means the array (or in general, the object) has been declared but is not ready to refer to an instance.

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int[] data;
println(data); //compilation error

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int[] data = null;
println(data); //will display null

Since a null array (or, in general, object) does not refer to an instance yet, it doesn’t have any length attribute.

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int[] data = null;
println(data.length); //NullPointerException

And obviously, you cannot access its items either:

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int[] data = null;
println(data[0]); //NullPointerException

Lesson 1: Before we access the length attribute of an array, we must ensure it’s not null.

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//UNSAFE:
for(int i=0; i < data.length; i++) {
	//access item at index i
}

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//SAFE:
if(data != null) {
	for(int i=0; i < data.length; i++) {
		//access item at index i
	}
}

Similarly, for a two-dimensional array, either the 2d array itself can be null or one or more of its subarrays can be null. Following are two versions of a program that store the sum of all items in a variable total, the first one unsafe, and the second one safe.

Version 1 (Unsafe)

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int total = 0;
for(int i=0; i < twoDimArray.length; i++) {
	for(int k=0; k < twoDimArray[i].length; k++) {
		total+=twoDimArray[i][k];
	}
}

Some of the examples for which the above code will generate run-time errors are:

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int[] taxi = null;
int[] cab = { {10, 70}, null, {20, 90} };

Version 2 (Safe)

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int total = 0;
if(twoDimArray != null) {
	for(int i=0; i < twoDimArray.length; i++) {
		if(twoDimArray[i] != null) {
			for(int k=0; k < twoDimArray[i].length; k++) {
				total+=twoDimArray[i][k];
			}
		}
	}
}

This code will work for all possible two-dimensional sub-arrays such as:

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int[] data = {10, 70, 20, 90};
int[] taxi = null;
int[] cab = { {10, 70}, null, {20, 90} };
int[] mixedBag = { {10, 70, 20}, null, {90}, null, {} };

Moral of the story:

  1. Always check an array is NOT null before accessing its length, and,
  2. Always check the index is within bounds before accessing an item at that index (our loop expressions are doing that in the above examples).