2 complex problems

[Gaurav Goyal]

plz provide justification on these 2 problems
1) String s = "abcde";
StringBuffer s1 = new StringBuffer("abcded");
if ( s.equals(s1) )
s1 = null;
if ( s1.equals(s) )
s = null;
why does not the compiler give an error?
can a null string buffer be compared or used ?

2) can a method return null?
if yes, what happens in each case when a method returns null and the return type is i) void ii) String iii) int
public String method()
{
return null;
}

[Mikael Jonasson]

1.
The compiler should not give an error, as long as equels accepts the type send (I believe that equals take Object, so there is no problem). Given that the equals won't return true, neither reference will ever become null. If you change the code so they do, all you get is a runtime-error indicating null pointer.
2.
Yes, a method can return null, if if the returntype is a reference. If the returntype isd void, you can only return from the function, not return something (null count's as something being returned). If the returntype is int, you can only return numbers.
Look at the following

public Customer Find(int customerId)
{
   Customer found=null;
   for (int i=0;i<customers.length;i++)>
   {
      if (customers[i].customerId==customerId)
         found=customers[i];
   }
   return found;
}

If no customer matching the customerId is found, null is returned, else the matching customer is returned. So you see, not only is it allowed, somtimes it's recomended to return null.
Ofcourse the method-example you gave can be a usefull way of implenting some abstract method you don't like to code right now, but need to test other parts of the program.
/Mike