null pointer exception with nested ? : syntax

[jamesd128]

Could someone please explain to me why executing this code results in a NullPointerException?

String s = "fds";
Integer t = s.equals("d") ? 1 : s.equals("fds") ? null : 2;

[Norm]

One way to get the exception with that statement is if s has a null value.

What happens if you replace the null with a number?

[Fubarable]

Could someone please explain to me why executing this code results in a NullPointerException?

String s = "fds";
Integer t = s.equals("d") ? 1 : s.equals("fds") ? null : 2;

The nature of the terniary operator is funny. I think it tries to change all possible results to the same type, and so I believe that the line causes an internal call to:

      Integer.valueOf(null);

which will cause a NPE.

[Norm]

Here's the javap output:

D:\JavaDevelopment\Testing\ForumQuestions8>javap -c TestCode11
Compiled from "TestCode11.java"
public class TestCode11 extends java.lang.Object{
public TestCode11();
  Code:
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."<init>":()V
   4:   return

public static void main(java.lang.String[])   throws java.lang.Exception;
  Code:
   0:   ldc     #2; //String fds
   2:   astore_1
   3:   aload_1
   4:   ldc     #3; //String d
   6:   invokevirtual   #4; //Method java/lang/String.equals:(Ljava/lang/Object;
)Z
   9:   ifeq    16
   12:  iconst_1
   13:  goto    36
   16:  aload_1
   17:  ldc     #2; //String fds
   19:  invokevirtual   #4; //Method java/lang/String.equals:(Ljava/lang/Object;
)Z
   22:  ifeq    29
   25:  aconst_null
   26:  goto    33
   29:  iconst_2
   30:  invokestatic    #5; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Int
eger;
   33:  invokevirtual   #6; //Method java/lang/Integer.intValue:()I
   36:  invokestatic    #5; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Int
eger;
   39:  astore_2
   40:  iconst_0
   41:  invokestatic    #7; //Method java/lang/System.exit:(I)V
   44:  return

[Fubarable]

Here's the javap output:

LOL, I just did the same thing, and in fact it's the first time that I created a byte code file with javap. It appears to me that I'm correct, that Integer.intValue(...) is called which maps to Integer.valueOf(...) which is called on the null resulting in a NPE.

[jamesd128]

The explanation that the terniary operator will try to take the valueOf on all the result types doesn't make sense though.

First, Integer.valueOf(null) throws a NumberFormatException not a NPE.

Second, that wouldn't explain why this line works fine:
Integer t = s.equals("fds") ? null : 2;
but this one doesn't:
Integer t = s.equals("d") ? 1 : s.equals("fds") ? null : 2;

The NPE is only thrown in the nested version.

[DarrylBurke]

As Norm and Fubarable pointed out, it's the intValue called on the null -- the result of auto-unboxing -- that gives rise to the NPE.

As framed, the type returned from your nested ternary is interpreted by the compiler to be the primitive type int. This int is autoboxed to Integer on assignment.

This doesn't throw an NPE since the type of the ternary value expression is Integer and no auto-unboxing is attempted.

Integer t = s.equals("d") ? Integer.valueOf(1) : s.equals("fds") ? null : Integer.valueOf(2);

Neither does this:

Integer t = s.equals("d") ? Integer.valueOf(1) : s.equals("fds") ? null : 2;

nor this:

Integer t = s.equals("fds") ? null : s.equals("d") ? 1 :  2;

db