Java autoboxing and ternary operator madness

问题:

Just spent a frustrating couple of hours debugging this code:

    LinkedHashMap<String, Integer> rsrqs = new LinkedHashMap<String, Integer>();
    Integer boxedPci = 52;
    Integer boxedRsrq = boxedPci != null ? rsrqs.get(boxedPci.toString()) : -1;

The above produces a NullPointerException. The below code doesn't:

    LinkedHashMap<String, Integer> rsrqs = new LinkedHashMap<String, Integer>();
    Integer boxedPci = 52;
    Integer boxedRsrq = boxedPci != null ? rsrqs.get(boxedPci.toString()) : Integer.valueOf(-1);

The only difference is wrapping the -1 with Integer.valueOf(). I'm sure I'm going to smack my forehead once somebody explains why this code behaves the way it does.. but can someone explain to me why this code behaves the way it does :)?

-- Edit

On second thought, I suspect that the NPE is coming from the rsrqs.get() returning null, which I think java is attempting to unbox into an int, before boxing back to an Integer. The Integer.valueOf() forces Java to do the unbox-box step. Moral of the story; don't just ignore those boxing warnings in Eclipse ;)

回答1:

Ternary expressions, like any expression, have a type that is determined by the compiler. If the two sides of the ternary expression have what looks like different types, then the compiler will try and find a common base type using the least ambiguous of the two options. In your case, the -1 is least ambiguous, and so the type of the ternary expression is int. Sadly, the compiler doesn't use type inference based on the receiving variable.

The expression rsrqs.get(boxedPci.toString()) is then evaluated and forced into type int to match the ternary expression, but because it's null it throws the NPE.

By boxing the -1, the value of the ternary expression is Integer, and so you're null-safe.

回答2:

The explanation can be concluded from the information in java language specification: 15.25. Conditional Operator ? :.

From the table there, you get the information, that, if the second operand (rsrqs.get(boxedPci.toString())) is of type Integer and the third operand is of type int, the result will be of type int.

That however means, that

Integer boxedRsrq = boxedPci != null ? rsrqs.get(boxedPci.toString()) : -1;

is semantically the same as

Integer boxedRsrq = boxedPci != null ? ((int)rsrqs.get(boxedPci.toString())) : -1;

But that means you get a NullPointerException, if you get null from the map, which obviously happens.

If you cast the third operand to Integer, the second operand will never be cast to int and no NPE happens.

回答3:

1

is an int, not an Integer. So, Java is going to un-box your Integer to int, which causes the NullPointerException. When you auto-unbox a null Integer, it results in a NullPointerException. ( reference here )

But when you use

 Integer.valueOf(-1) 

it doesn't need to auto-unbox it, which leads to no exceptions.

回答4:

Well, Integer.valueOf(String) returns an Integer and -1 is a primitive int. The first example is forced to unbox because one term is a primitive. You could also have used

Integer boxedRsrq = boxedPci != null ? 
    rsrqs.get(boxedPci.toString()) : (Integer) -1;

which would have boxed the -1.