NullPointerException

If you try to perform an operation on a null reference, such as checking the .length() of a String, your code will crash.

void main() {
    String thing = null;
    // NullPointerException
    System.out.println(thing.length());
}

When this happens the error that Java shows you will be a "NullPointerException". This will look something like the following.

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "String.length()" because "thing" is null
	at Main.main(Main.java:4)

Because this is easy to make happen by mistake, it is worth familiarizing yourself with the format of it.

This way you can laser focus on the part that says something like "Cannot invoke "String.length()" because "thing" is null" and know that the issue is with some variable named thing that you are trying to call .length() on.