Calling a Non-Static Method in a Null Object JAVA?

Question

**public void testCompareTo() {
    System.out.println("compareTo");
    Patient p = null;
    Patient instance = null;
    int expResult = 0;
    int result = instance.compareTo(p);
    assertEquals(expResult, result);
}**

Is there a way to call a method in a null object or do I have to do an Exception here?

Eran · Accepted Answer · 2014-09-29 17:31:39Z

3

No, you can't call a non-static method on a null object, since that would cause a NullPointerException.

You must verify that instance is not null before calling instance.compareTo(p).

edited Sep 29, 2014 at 17:31

answered Sep 29, 2014 at 17:29

Eran

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4 Comments

ControlAltDel Over a year ago

Actually, semantically this is not true. What you mean is that calling any method on a null object will ALWAYS throw a NullPointerException

user3133474 Over a year ago

Is there a way to set a value for the variable "result"?.If I can't call compareTo and without changing the values of and 'p' and 'instance'

Eran Over a year ago

@T.J.Crowder That's interesting to know. I never tried to call a static method through a null object reference. Then again, I try not to call static methods though an object reference, so it's not a surprise I never tried it.

Eran Over a year ago

@user3133474 You can initialize it to a default value and then assign to it the result of compareTo if instance != null. Or you can use the new Optional class introduced in Java 8.

sumanta · Accepted Answer · 2014-09-29 17:38:23Z

0

NullPointerException is inevitable in this case as java can not get any reference from a null object, which makes it obvious to throw a NullPointerException

answered Sep 29, 2014 at 17:38

sumanta

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Comments

TheLostMind · Accepted Answer · 2014-09-29 17:57:06Z

0

null objects by definition are... nothing. You can't invoke an instance level method on a null object without causing a NullPointerException.

edited Sep 29, 2014 at 17:57

TheLostMind

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answered Sep 29, 2014 at 17:30

Edward Smith

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7 Comments

T.J. Crowder Over a year ago

"You can't invoke anything on a null object without causing a NullPointerException" You can, you know: A static method. (Bizarre but true.)

Edward Smith Over a year ago

You're technically invoking the method on the Class, not the object.

T.J. Crowder Over a year ago

Yes, quite true. You're using the variable/data member (e.g., that's how the compiler figures out what class is relevant), but not an object.

Edward Smith Over a year ago

Having just tested it, I didn't realize that at run time, a null instance.someStaticMethod() won't cause a NPE. Learned something today.... something... pretty useless :).

T.J. Crowder Over a year ago

It is indeed (useless, I mean). :-) The bytecode even goes to the trouble of reading the value from the variable first, before...not using it.

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