passing null as a parameter

[Vijayalakshmi Kota]

These r the questions from Abhilash's mockexam site could any one explain why the answers for these questions are :
3A is The code compiles cleanly and shows "String Version"
4A:The code does not compile.
3Q) public class AQuestion
{
public void method(Object o)
{
System.out.println("Object Verion");
}
public void method(String s)
{
System.out.println("String Version");
}
public static void main(String args[])
{
AQuestion question = new AQuestion();
question.method(null);
}
}
4Q) public class AQuestion
{
public void method(StringBuffer sb)
{
System.out.println("StringBuffer Verion");
}
public void method(String s)
{
System.out.println("String Version");
}
public static void main(String args[])
{
AQuestion question = new AQuestion();
question.method(null);
}
}
Thanks for any response
kota
[ June 04, 2003: Message edited by: Vijayalakshmi Kota ]

[preeti khane]

hi,
the first question , method overloading is determined by the compiler as the most specific method that a parameter can be passed into...
Object is a superclass of String hence, String is the most specific method parameter

On the 2 question, both string and StringBuffer class are siblings , ie they are sub classes of Object.. so the compiler finds ambiguity in the methods and cannot determine the closest possible fit.....and hence compile error

[Alton Hernandez]

The code in Question 2 failed, not because of the the formal parameter list, but simply because of the use of null in the method call.
In fact, the code in Question 1 is more ambiguous than 2. String and StringBuffer have no direct relationship.
If you where to change the call to:

question.method("String");

then the code will compile properly.
The problem in 2 is that the null type can be cast into anything, therefore, it would satisfy both method.
Consider this code:

class A {;}
class B {;}
public class AQuestion
{
  public void method(A a)
    { System.out.println("A"); }
  public void method(B b)
    { System.out.println("B"); }
  public static void main(String args[])
  {
    AQuestion question = new AQuestion();
    question.method(null);
  }
}

This code will fail even though class A and class B have no relationship. However, if you were to change the call to

question.method(new A());

then this would compile properly.