Constructors

[shuaib ahmad]

class sample
{
sample(String s)
{
System.out.println("String");
}
sample(Object o)
{
System.out.println("Object");
}
}
class constructor
{
public static void main(String arg[])
{
sample s1=new sample(null);
}
}
y the answer is String , and y it is not declared as error

[nupur dhawan]

Originally posted by shuaib ahmad:
class sample
{
sample(String s)
{
System.out.println("String");
}
sample(Object o)
{
System.out.println("Object");
}
}
class constructor
{
public static void main(String arg[])
{
sample s1=new sample(null);
}
}
y the answer is String , and y it is not declared as error

Null can be a String value as well as an Object value.
When null is passed as an argument, the constructor with the most appropriate version of the argument is executed ... null is most specifically a String .
If the constructor with the string parameter was missing , the other constructor would have been executed, resulting in an object.

[David Poglitsch]

From the JLS (section 3.9)

while null might appear to be a keyword, it is technically the null literal

So null is a String (literal), not an object.

[Valentin Crettaz]

Sorry but null is not a String. However, System.out.println does print the string "null" whenever a null reference is passed.
From JLS 3.10.7 The Null Literal

The null type has one value, the null reference, represented by the literal null, which is formed from ASCII characters. A null literal is always of the null type.

As far as I know, String is not the null type :roll:
literal does not always mean "String", there are character literals ('a', 'b', ...), integer literals (1,2,3,...), float literals (1.2f,...), etc.
From JLS 3.10 Literals

A literal is the source code representation of a value of a primitive type, the String type, or the null type.

[ July 31, 2002: Message edited by: Valentin Crettaz ]

[David Poglitsch]

Yeah, knew that last sentence was a mistake after I posted it (Where's that "Unpost" button when you need it!)
Also from JLS 15.18.1.1 String Conversion

If the reference is null, it is converted to the string "null"

So "sample s1 = new sample(null)" invokes the sample method for strings.
Adding:
sample s2 = null; // or Object s2 = null;
sample s3 = new sample(s2);
will invoke the sample method for objects.
Dave
P.S. Bartenders rock!

[Valentin Crettaz]

Yeah, knew that last sentence was a mistake after I posted it (Where's that "Unpost" button when you need it!)
When you click on the "Edit" icon above the message you posted, there is a "Delete" checkbox. Just check it and confirm...
P.S. Bartenders rock!
Yihaaah