String and null pointers in Java

Question

The following code makes me mad:

private String blahBlah(){
    return null;
}

@Test
public void myTest(){
    System.out.println(blahBlah()); //Good, output "null"
    Object obj = blahBlah();
    System.out.println(obj.toString()) //Good as above
    //System.out.println(blahBlah().toString()); //Bad, NullPointerException
    //System.out.println(((Object)blahBlah()).toString()); //Bad as above
}

Can anyone explain the above behavior?

UPDATE:

The above code is NOT the truth. What I actually experienced is that I received NullPointerException and I track back to the call of toString(), and I tried different workarounds including in-statement casting but it does'nt work. But after I use seperated cast I accidentally removed the toString() call so it WORKED.

Erm... You sure the third line of myTest doesn't throw an exception? — luiscubal
– luiscubal, Commented Feb 10, 2012 at 2:13
Sorry, I have changed from the observed code. I will fix it. — Earth Engine
– Earth Engine, Commented Feb 10, 2012 at 2:21

Mike Samuel · Accepted Answer · 2012-02-10 02:32:16Z

1

Easy.

You can print a null; you just can't de-reference a null value.

edited Feb 10, 2012 at 2:32

Mike Samuel

121k30 gold badges230 silver badges255 bronze badges

answered Feb 10, 2012 at 2:13

duffymo

310k46 gold badges376 silver badges570 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Earth Engine Over a year ago

This explain is enough for the direct call of toString(). However, how would you explain the difference of the in-statement cast and the separated cast?

Mike Samuel Over a year ago

@EarthEngine, duffymo is correct. The simplest explanation to your code example above is that blahBlah() is not consistently returning null or you are not running the code in that sample. Have you ruled both of those out in a debugger?

warbio · Accepted Answer · 2012-02-10 02:14:03Z

0

Because you are returning a string pointing at null instead of pointing at a string with the value null. Try changing return null to return "".

answered Feb 10, 2012 at 2:14

warbio

4665 silver badges16 bronze badges

Comments

Mike Samuel · Accepted Answer · 2012-02-10 02:27:58Z

0

You can use String.valueOf(Object) to get the toString output from an object, or null if the value passed in is null.

http://docs.oracle.com/javase/7/docs/api/java/lang/String.html#valueOf(java.lang.Object)

public static String valueOf(Object obj)
Returns the string representation of the Object argument.

Parameters:
obj - an Object.

Returns:
if the argument is null, then a string equal to "null"; otherwise, the value of obj.toString() is returned.

answered Feb 10, 2012 at 2:27

Mike Samuel

121k30 gold badges230 silver badges255 bronze badges

Collectives™ on Stack Overflow

String and null pointers in Java

3 Answers 3

2 Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

2 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related