Code Example: String comparison with .equals(args[0])

I'm learning Java with the help of a book and don't understand the following example/statement:

For args[0].equals("Schrödinger") you need to make sure the args[0] is not null, whereas "Schrödinger".equals(args[0]) doesn't need this kind of check because "Schrödinger" can't be null.

Code:

if("Schrödinger".equals(args[0])) {
	System.out.println("Hallo");
}

However I do get an error if there is no parameter in args, which also seems logical as you need to grab the value at args[0] at some point for the comparison, even if the string "Schrödinger" isn't null. I'm confused, is the example from the book wrong or am I not understanding something?

The code provided in the book only handles args[0] being null, not args having a length of 0. Assuming your program is supposed to pass some command-line arguments to it, and you do not pass any argument while executing it, the length of the following array, args will remain 0.

public static void main(String[] args)

As a result, an attempt to access an element from the first position, which is specified with the index, 0 will result in ArrayIndexOutOfBoundsException. Since the index of an array starts with 0, the maximum index can go up to the size of the array minus 1.

You can avoid the problem you have faced by checking the size of the array first before accessing an element within the maximum index e.g.

if(args.length >= 1 && "Schrödinger".equals(args[0])) {
   //...
}

Similarly, if you want to access an element up the index, 1, you should check if the size is 2 or more e.g.

if(args.length >= 2 && "Schrödinger".equals(args[1])) {
   //...
}

By the way, a condition like "Schrödinger".equals(args[0]) is known as Yoda conditions.

Your assumption is correct. For the main method If the array is not provided then you can not access the first element of the array. It will throw an ArrayIndexOutOfBoundsException since there are no elements.

>For args[0].equals("Schrödinger") you need to make sure the args[0] is not null.

True, since you're calling .equals("Schrödinger") on the object, which is expected to be at index 0 in the args array.

>whereas "Schrödinger".equals(args[0]) doesn't need this kind of check because "Schrödinger" can't be null.

True, as "Schrödinger" is a String literal, which by definition is an Object, so you explicitly have it, and there's no even hypothetical chance it'll be null.

> However I do get an error if there is no parameter in args, which also seems logical as you need to grab the value at args[0] at some point for the comparison, even if the string "Schrödinger" isn't null.

True, but to be precise here, error doesn't happen at the comparison stage, because of you can't compare something to nothing, rather, your error happens here, because you're not passing anything, hence the args array is empty, and args[0] will throw the ArrayIndexOutOfBoundsException exception.

I hope I made each and every step clear for you.

Running any method for a null object is not allowed, and you will get an exception. You have to check if args[0] is null (or of length 0) before you do anything with it.

For User defined args array

Case 1: args[0].equals("Schrödinger")<br/>
When args[0] is null(no value is passed), you cannot call equals method on
null hence NullPointerException is thrown
<br/>
Case 2: "Schrödinger".equals(args[0])<br/>
In this case despite args[0] being null equals is called on string literal and
no exception is thrown + the condition evaluates to false. Hence this approach
is the recommended one or use Optionals.

For args of public static void main

In case of args array of public static void main it will throw ArrayIndexOutOfBoundException. If JVM finds no command line arguments its initializes the args as String[] args = new String[0] i.e of length 0. Hence while accessing it throws exception