Want to know the internal code inferred by the compiler while unboxing

Question

Code:-

Integer value =null;
int a = value;

Output:-

Exception in thread "main" java.lang.NullPointerException

I understand that unboxing failed because there is not int value for null reference.But can anyone tell me the method invoked which lead to nullPointerException

Because the runtime was trying to invoke `value.intValue()` internally to get the result, but `value` is `null`. — Luiggi Mendoza, Nov 27 '13 at 20:32
@ferrerverck a quick search on a SEO leds me to this Q/A: http://stackoverflow.com/q/3352791/1065197 — Luiggi Mendoza, Nov 27 '13 at 20:37
@Cruncher didn't say it is a dup, but also covers what OP asked. — Luiggi Mendoza, Nov 27 '13 at 20:39
@LuiggiMendoza I know, just weighing in as it was a reply to somebody asking to check if it was a dup — Cruncher, Nov 27 '13 at 20:41
@SotiriosDelimanolis, the log actually doesn't tell. However, by the time when i answer i didn't see `Luiggi Mendoza`'s comment. Otherwise i wouldn't try to dig up the JLS specification and answer it. But down voting this question is not completely understood by me — Sage, Nov 27 '13 at 20:48
At first glance it doesn't seem like a good question, but it's subtler than it appears and is by no means obvious (and doesn't deserve the downvotes). I edited the title to be more descriptive of the problem and to hopefully be more searchable. — Reinstate Monica -- notmaynard, Nov 27 '13 at 20:54

Sage · Accepted Answer · 2013-11-27T20:46:03.653

4

While assigning the reference type Integer to corresponding primitive type int Unboxing conversion happens. It is defined in jls-5.1.8 Unboxing Conversion:

If r is a reference of type Integer, then unboxing conversion converts r into r.intValue()

So, when you are trying to unboxing it, the value.intValue() gets called resulting in NPE, as value is null

edited Nov 27 '13 at 20:46

answered Nov 27 '13 at 20:35

Sage

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score 2 · Answer 2 · answered Nov 27 '13 at 20:35

You first have to understand that the code you have is actually compiled to something a little different.

[s_delima@ml-l-sotiriosd bin]$ /usr/java/latest/bin/javap -c Test.class 
Compiled from "Test.java"
public class Test {
  ...   

  public static void main(java.lang.String[]) throws java.io.IOException;
    Code:
       0: aconst_null   
       1: astore_1      
       2: aload_1       
       3: invokevirtual #19                 // Method java/lang/Integer.intValue:()I
       6: istore_2      
       7: return        
}

You will notice that there is an invocation of java/lang/Integer.intValue(). Since your Integer variable is referencing null, you will get a NullPointerException when the invocation tries to dereference it.

Want to know the internal code inferred by the compiler while unboxing

2 Answers2