java - Where is the NullPointerException coming from? -

i'm trying write method converts 2 digit number 2000 + number, returns other numbers , returns null when null passed argument.

this implementation works intended

private integer convertto4digits(integer modelyear) {     boolean istwodigit = modelyear != null && modelyear < 100;     if (istwodigit) {         return 2000 + modelyear;     } else {         return modelyear;     } } 

but 1 fails npe in return statement when called null.

private integer convertto4digits(integer modelyear) {     return (modelyear != null && modelyear < 100) ? (2000 + modelyear) : modelyear; } 

or bug? i'm using eclipse keple jdk 1.7.0_04

i think answer can found in chapter 15.25 of jls

if 1 of second , third operands of primitive type t, , type of other result of applying boxing conversion (§5.1.7) t, type of conditional expression t.

so when either second or third operand primitive type expression's type primitive. if pass null reference branch : modelyear executed. since 1 operand primitive must unboxed. causes npe.

you can see if take @ generated byte code

private convertto4digits(ljava/lang/integer;)ljava/lang/integer;  l0   linenumber 46 l0   aload 1   ifnull l1   aload 1   invokevirtual java/lang/integer.intvalue()i   bipush 100   if_icmpge l1   sipush 2000   aload 1   invokevirtual java/lang/integer.intvalue()i   iadd   goto l2  l1   linenumber 47 l1   aload 1   invokevirtual java/lang/integer.intvalue()i  l2   linenumber 46 l2   invokestatic java/lang/integer.valueof(i)ljava/lang/integer;   areturn  

your own answer solves problem because casting second operand integer

(modelyear != null && modelyear < 100) ? (integer) (2000 + modelyear) : modelyear; 

and therefore neither second nor thrid operand of primitive type. rule of jls posted above not applied , npe gone.