Why am I getting a NullPointerException here?

Question

Long n = null;
for (Long price : new Long[]{null, 2L, 10L}) {          
n = (n != null) ? 0L : price;   
}

I'm stumped, why am I getting a NPE when I execute this code ? Seems to be just a simple assignment of n = price where price is null. Don't worry about what it means, it makes no sense.

Answer 1

In the line n = (n != null) ? 0L : price; n = (n != null) ? 0L : price; , you have a long and a Long as the alternatives to your ?: statement. Java is going to construe this as of type long and will try to unbox the Long , instead of box the long . When price is null, as it is in the first iteration, this generates an NPE.

Answer 2

This is because price is being unboxed to match the 0L return possibility in the ternary operator.

Try instead:

Long n = null;
for (Long price : new Long[]{null, 2L, 10L}) {          
    n = (n != null) ? new Long(0L) : price;   
}

And all will run smoothly.

Answer 3

If you look at the byte code this happens

 49:    invokevirtual   #26; //Method java/lang/Long.longValue:()J
 52:    invokestatic    #20; //Method java/lang/Long.valueOf:(J)Ljava/lang/Long;

when doing the comparison the Long.longValue is being executed on each item in that array causing your NullPointerException . The problem is that the ternary expression in combination with AutoBoxing is obscuring whats really occurring.

Answer 4

The line

n = (n != null) ? 0L : price;   

gets compiled into

n = Long.valueOf(n == null ? price.longValue() : 0L);

The statement price.longValue() throws NullPointerException when price is null . Try replacing the array with new Long[] {1L, 2L, 3L} and see it work.

Answer 5

尝试

 n = (price != null) ? price : 0L;

Answer 6

It is trying to coerce price to a primitive long , to match the type of 0L . This generates the NPE if price is null .

This expression works, because it avoids the implicit type coercion:

n = (n != null) ? new Long( 0L ) : price ;

But as others have said in the comments, what you're doing here doesn't seem to make much sense anyway.