NullPointerException

[Shweta R Dhaneshwar]

Please help me clear this doubt.Check out the code and its o/p.

 1. public class TestDogs {
2. public static void main(String [] args) {
3. Dog [][] theDogs = new Dog[3][];
4. System.out.println(theDogs[2][0].toString());
5. }
6. }
7.
8. class Dog {}
what is the result? (Choose one.)
A. null
B. theDogs
C. Compilation fails
D. An exception is thrown at runtime 

here the correct ans is option D.

one more...

 1. public class X {
2. public static void main(String [] args) {
3. String names [] = new String[5];
4. for (int x=0; x < args.length; x++)
5. names[x] = args[x];
6. System.out.println(names[2]);
7. }
8. }
and the command line invocation is
java X a b
what is the result? (Choose one.)
A. names
B. null
C. Compilation fails
D. An exception is thrown at runtime 

for this the correct option is B.

consider this...

Myclass d=null;
 System.out.println(d); 

here o/p is null.

I am a little confused...when do we get o/p null and when do we get NullPointerException?

[Srinivasa Raghavan]

Check this piece of code

String x = null; 
System.out.println ( x );
System.out.println( x.trim() );

Try to run it and surely you 'll get to know ( This a better way to learn ). If you face any problem again , please update this thread.
[ July 28, 2005: Message edited by: Srinivasa Raghavan ]

[deshdeep divakar]

Hi Buddy
Srinivasa Raghavan
When I assigned String x = "null";
the

 import java.io.*;
 
class tryit
{
public static void main(String [] args)
{
String x = "null"; 
System.out.println ( x );
System.out.println( x.trim() );
}
} 

Output is :null null does this mean that in case when
we say String x="null"; reference variable x value
does'nt have any spaces while saying String x=null;
does have space.Plz elaborate on this...

[deshdeep divakar]

Hi Buddy
Srinivasa Raghavan
When I assigned String x = "null";
the

 import java.io.*;
 
class tryit
{
public static void main(String [] args)
{
String x = "null"; 
System.out.println ( x );
System.out.println( x.trim() );
}
} 

Output is :null null does this mean that in case when
we say String x="null"; reference variable x value
does'nt have any spaces while saying String x=null;
does have space.Plz elaborate on this...

[Srinivasa Raghavan]

I'm sorry if the code mis leaded you.

I ment to say you 'll get a null pointer exception if you are trying to access any method in an object which is null.

 
Foo fooObject = null;
fooObject.doComplexStuff();// Will throw NullPointerExec

[ July 28, 2005: Message edited by: Srinivasa Raghavan ]

[Devender Thareja]

If you try to print a null object reference, it will print "null".
If you try to invoke a method on a Object reference which is null, it will throw null poiner exception. Same will happen if you try to access a field from null object reference.
[ July 28, 2005: Message edited by: Devender Thareja ]

[Fes D Gaur]

For the first question:
When arrays are declared whether at class level or local (method) level, their elements are given default values. In this case the array object is being declared as type Dog. The default values in the array object is null. When you call toString() or trim() on a reference that is not pointing to an object a NullPointer Exception is generated at run time.

For the second question: since the array values are by default null and since you are passing only two arguments the array element at index 2 will be null.

Last question simply is printing null because that reference is null.

Hope this helps.

Fes

[Veer Batra]

 
public class TestDogs {//line 1
public static void main(String [] args) {// line 2
Dog [][] theDogs = new Dog[3][]; //line 3
System.out.println(theDogs[2][0]);// Line 4
System.out.println(theDogs[2][0].toString());// line 5
} // line 6
} // line 7
class Dog {} // line 8

If line 4 is there then it gives nullPointerException on line 4. If line 4 commented out then it gives same exception on line 5. Line 4 which is not invoking any method, why it should give nullPointerException on that. Should it not print null and give exception on line 5 only?

[Srinivasa Raghavan]

Veer,

Line 4 which is not invoking any method

Are you sure ?

System.out.println(theDogs[2][0]);

Think what will happen when the above line gets executed .. & whant gets printed & from where it came ?
[ July 29, 2005: Message edited by: Srinivasa Raghavan ]

[Devender Thareja]

Originally posted by Veer Batra:

 
public class TestDogs {//line 1
public static void main(String [] args) {// line 2
Dog [][] theDogs = new Dog[3][]; //line 3
System.out.println(theDogs[2][0]);// Line 4
System.out.println(theDogs[2][0].toString());// line 5
} // line 6
} // line 7
class Dog {} // line 8

If line 4 is there then it gives nullPointerException on line 4. If line 4 commented out then it gives same exception on line 5. Line 4 which is not invoking any method, why it should give nullPointerException on that. Should it not print null and give exception on line 5 only?

<code>
Dog [][] theDogs = new Dog[3][]; //line 3
</code>

Here you are telling compiler that Dog is an array of 3 arrays. You haven't initialized those 3 arrays yet. Once you create those arrays, then only it will initialize it with null reference of Dog object.

Try replacing line 3 with following and see what happens:
<code>
Dog [][] theDogs = new Dog[3][3]; //line 3
</code>

Now your line 4 should print null.