NullPointerException when assigning numbers

There is a strange thing happening when I execute the following code:

开发者_JAVA百科

private void doStuff(Long inLong) {
    long theNumber = inLong;

    /* ... */
}

Sometimes I see a NullPointerException in the logs at the assignment line and I can't understand why it happens. Any idea?

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You passed in null. Check what you are passing in.

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For long theNumber = inLong;, the long value of inLong is fetched by implicitly calling inLong.longValue(). This is called auto-unboxing (sometimes more general auto-boxing). When inLong is null, you therefore get a NullPointerException just like calling any other method on null.

Therefore, you should think of some alternative:

If you do not need an undefined value, you should make sure the caller never passes null and assert this via:

private void doStuff(Long inLong) {
  assert inLong != null; 
  long theNumber = inLong;
  /* ... */
} 

(or use a Nonnull-Checker). If the method were public and you cannot be sure that null is not passed, rather do:

public void doStuff(Long inLong) {
  if (inLong == null) { throw new IllegalArgumentException(); } 
  long theNumber = inLong;
  /* ... */
} 

If you neither need undefined values nor use the value in a collection (or in any other generic parameter), rather use the method private void doStuff(long inLong) instead. If you do need a Long object, you can, of course, still use the parameter of type long and do (auto-)boxing inside of doStuff to get an according Long object.

If you do need undefined values, you should check for it and do what is necessary:

private void doStuff(Long inLong) {
  if (inLong == null) { 
    handleUndef(); 
  } else {
    long theNumber = inLong;
    /* ... */
  }
} 

Just setting the value to 0 instead of some more elaborate handleUndef() is dubious in my opinion, because then you could have used the parameter of type long (see above) in the first place.

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inLong is probably null. Then you try to assign a null reference and unbox it into a primitive type.

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because inLong can be null and will not be automatically mapped to 0.
I guess what you want to do is this:

theNumber = 0; // or Long.MIN_VALUE or Long.MAX_VALUE or whatever you prefer
if (inLong != null) {
    theNumber = inLong;
}
// ... 

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That's because the inLong parameter is null. Whenever you assign a Long object to a long variable, Java automatically tries to unbox it to the proper type, but it fails if value of the Long variable is null.

Just put a null-check before that assignment and you'll get rid of that error, but you may have to raise an exception or deal with the null argument appropriately.

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if inLong is null, then NPE is the expected behaviour.

long theNumber = inLong;

is semantically equivalent to

long theNumber = inLong.longValue();

which should make the cause of the NPE obvious.