Java Regex Matcher encountering NullPointerException, when handed null input

Question

I am trying to use the java regex matcher-class and I'm encountering a NullPointerException in my code:

String input = null;
System.out.println("\nEnter your username: ");
Matcher matcher = VALID_USERNAME_REGEX.matcher(input); //EXCEPTION IS HERE
if (matcher.find()) {
    username = input;
} else {
    input = null;
    System.out.println("\nInvalid input, please try again!");
}

Stack Trace: (line: 173 is in the above code where the comment "//EXCEPTION IS HERE" is located)

Exception in thread "main" java.lang.NullPointerException
    at java.util.regex.Matcher.getTextLength(Unknown Source)
    at java.util.regex.Matcher.reset(Unknown Source)
    at java.util.regex.Matcher.<init>(Unknown Source)
    at java.util.regex.Pattern.matcher(Unknown Source)
    at org.console.Interface.createAccount(Interface.java:173)
    at org.console.Interface.login(Interface.java:78)
    at org.application.Application.run(Application.java:31)
    at org.application.Application.main(Application.java:37)

The value of the input should be what the user enters in the console. The regex should match the input to validate that it meets the regex requirements.

seems like `input` is `null` , try to print `input` before `matcher` — Pavneet_Singh, Dec 22 '16 at 15:37
@Brenton unless you left out some code `input` is NOT what the user entered. — Henry, Dec 22 '16 at 15:39
Instead of adding the solution to the question, can you remove it from the question and self answer this question? Answers don't really belong in the question. — Emily Mabrey, Dec 22 '16 at 15:47
Note to the author, the duplicate closure doesn't mean the question was bad in any way. It covers an undocumented NullPointerException in a commonly used API. It just means the question is answerable through an equivalent/more general questions answers — HopefullyHelpful, Dec 22 '16 at 16:14

score 12 · Accepted Answer · answered Dec 22 '16 at 15:43

It's not explicitly stated in the javadoc of Pattern#matcher, but it will not work with a null input (it creates a new Matcher instance which will reset itself upon initialization, leading to the observed exception). You'll have to explicitly test for != null beforehand.

Side note: Although undocumented, this has already been declined to change:

The current API doc does not explicitly or implicitly mention whether or not a NPE will/should be thrown. But it is difficult to define a good semantics for a "null" in Matcher as a legal input text, should it be semantically equivalent to empty string ""? Obviously not. Then, what the match/find result should be expected when match null against boundary pattern like "^" and "$" (probably no match/no find for all patterns), and who will be benefited from such a semantics? The described use scenario is reasonable but should be easily achieved by use an empty string "" instead. Dont see a strong justification to "fix" the current behavior. Instead, updating the package spec to describe the NPE behavior might be a better choice.

score 2 · Answer 2 · edited Dec 22 '16 at 16:48

2

I added input = in.nextLine(); above the matcher line.

String input = null;
System.out.println("\nEnter your username: ");
input = in.nextLine();
Matcher matcher = VALID_USERNAME_REGEX.matcher(input); //EXCEPTION IS HERE
if (matcher.find()) {
    username = input;
} else {
    input = null;
    System.out.println("\nInvalid input, please try again!");
}

edited Dec 22 '16 at 16:48

TylerH

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answered Dec 22 '16 at 15:51

Brenton

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Java Regex Matcher encountering NullPointerException, when handed null input

2 Answers2