There is this question that I am stuck on:

The answer is Version 3 but I am unsure why. I tried testing it with a test string assigned null, heres version 2 I used:

String str = null;
if (str != null || !str.equals("")){
    System.out.println(str);
}

And it was giving me a null pointer exception for all versions except version 3 which I am really confused about.

CodePudding user response：

If you do any operation on a null String, it gives null pointer exception. ( Here you are comparing it with !str.equals(""))

Another point to note is : in || condition , if first condition is true, the second wont be evaluated.

whereas in && , if the first condition is false , the second condition is not evaluated.

For this reason, Both of these are called short circuit operators.

CodePudding user response：

The conditions in your if-statements have to be checked. So, if you have str != null || !str.equals("") in your if-statement, then you will check str != null and then check !str.equals("") if the first statement is false. Let's say that the string is null. The first condition is false and the second condition tries to check !str.equals(""). Then you get a NullPointerException, because you can't compare null to a string.

However, if you use str != null && !str.equals(""), then str != null will be checked first and the second statement !str.equals("") will only be checked if the first one is true. Meaning, the code will never get to !str.equals("") if the string is null. So, you never get the NullPointerException.

You can't use str.equals("") because null doesn't have an equals method. By using == you check the reference of your string. Everything that is null has the same reference, so if your string is null, then it will point to the null reference, and == can be used to check that refernce.