Java ternary operator and NullPointerException

Question

hi，I encountered a problem in works，the question like this:

Integer test1=null;
Integer test2=null;

i call them like this:

test2=(test1==null?test1:(test1+1));
test2=(test1==null?test1:-1);

then the java throw a NullPointerException,so iwrite another code like this:

test2=(test1==null?test1:test1);

this code is ok.

but,why?

@Heuster Exactly, and the answer is a) entirely non-obvious and b) very hard to google for. — Marko Topolnik, Aug 08 '13 at 11:28
Look out for the rules in [JLS Conditional Operator](http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.25) as to how the expression is evaluated. — Rohit Jain, Aug 08 '13 at 11:29

Brian Agnew · Accepted Answer · 2013-08-08T11:36:34.423

Here

test2=(test1==null?test1:(test1+1));

the expression is going to return an int, not an Integer.

So the JVM will unbox test1, and later box the result. If you unbox a null, it will throw an NPE.

e.g. try this:

    Integer a = null;
    int b = a;

You'll get an NPE at the second line. But surely this is a simple integer assignment!? Not so. It's unboxing the null.

I would strongly advise against mixing Integers and int when null is involved. It is a complete nightmare to resolve these issues (I'm looking at such stuff right now involving method calls returning ints and Integers, and passing null around. A simple method return blows up unexpectedly).

If you have an 'optional' integer result, then I would recommend not using null, and rather some sort of Optional wrapper.

score 2 · Answer 2 · answered Aug 08 '13 at 11:36

2

test2=(test1==null?test1:(test1+1));

is equivalent to

   if(test1==null)
{
    test2 = test1;
}

else{

   test2 = test1 + 1;    
}

Please note you can never do any operation on null (Here you are doing test1 + 1 which is equivalent to null + 1 . Hence the Null Pointer Exception)

Happy Coding :)

answered Aug 08 '13 at 11:36

Abhishek Singh

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Well, this is not what's happening here. Since `test1` is `null`, your `if-else` block would not throw `NPE`, whereas, it does in the conditional operator case. – Rohit Jain Aug 08 '13 at 11:40
I wrote this just to explain him the case – Abhishek Singh Aug 08 '13 at 12:30
I am sorry i shouldn't have used 'equivalent to' – Abhishek Singh Aug 08 '13 at 12:32

score 0 · Answer 3 · answered Aug 08 '13 at 11:33

0

Do something like:

test2=(test1==null?test1: new Integer(test1+1));
test2=(test1==null?test1: new Integer(-1));

answered Aug 08 '13 at 11:33

blackSmith

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Java ternary operator and NullPointerException

3 Answers3