为什么NumberFormat.format会抛出NullPointerException？

Question

I've just found a very strange NullPointerException . First, I create a NumberFormat like this (note that the default Locale would be Germany, I don't know if that helps):

NumberFormat angleFormat = NumberFormat.getNumberInstance(Locale.UK);
angleFormat.setMaximumFractionDigits(5);
angleFormat.setMinimumFractionDigits(0);

Then, I tried to format a double with it. This is done with a new Thread created by a Lambda, while angleFormat is declared at the method containing the Lambda. The code where the Exception is thrown looks like this:

con.println("D" + moveId + (state.isEnemyInSightOf(e) ? "+" : "-")
        + angleFormat.format(e.getAngle()) // line 123 - error is here
        + (state.isMissileInSightOf(e) ? "+" : "-")
        + angleFormat.format(e.getSight())
        + (e.getLastShot() >= 10 || e.getLastShot() <= -1 ? "+" : "-")
        + angleFormat.format(e.getLives()));

e.getAngle() returns a double , so it can't return null. However, I get this Exception:

Exception in thread "Thread-1" java.lang.NullPointerException
    at java.text.DecimalFormat.fastDoubleFormat(Unknown Source)
    at java.text.DecimalFormat.fastFormat(Unknown Source)
    at java.text.NumberFormat.format(Unknown Source)
    at server.game.Simulator.lambda$0(Simulator.java:123)
    at server.game.Simulator$$Lambda$3/23162747.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)

I'm sure that e isn't null because of the Exception's stacktrace, it would a) be thrown one line earlier and b) not at java.text.DecimalFormat.fastDoubleFormat

Why is there a NullPointerException beeing thrown sometimes , and sometimes it works without problems? And what does that mean? The error seems to be reproducable, but not very often.

Answer 1

From the comments: I create the threads in a loop with each one a different e but the same NumberFormat

This appears to be the source of intermittent issues that your code is experiencing. According to the documentation of NumberFormat , the class is not thread-safe, so concurrent access must be synchronized externally:

Number formats are generally not synchronized. It is recommended to create separate format instances for each thread. If multiple threads access a format concurrently, it must be synchronized externally.

Answer 2

From the JavaDoc for DecimalFormat

Synchronization

Decimal formats are generally not synchronized. It is recommended to create separate format instances for each thread. If multiple threads access a format concurrently, it must be synchronized externally.

By the way, which version of Java is this? I don't see a DecimalFormat.fastFormat() method in the docs for either Java 6 or 7.