Super Constructor

[Nisheeth Kaushal]

Hi Friends
I have a difficulty here in this particular quesiton
class Vehicle {
String str ;
public Vehicle() {
}

public Vehicle ( String s ) {
str = s;
}
}
public class Car extends Vehicle {
public static void main (String args[] ) {
final Vehicle v = new Vehicle ( " Hello" );
v = new Vehicle ( " How are you");
v.str = "How is going";
System.out.println( "Greeting is : " + v.str );
}
}
A) Compiler error while subclassing the Vehicle
B) Compiler error , you cannot assign a value to final variable
B) Prints Hello
C) Prints How is going
This class is executing easily, but i think that the subclass will see for a default constructor, and if it does not find it, it will throw an error.
This particular program is producing error, but b'coz of fina variable not for the reason which i want.
Please help me,
Thanx in advance.
Nisheeth

[Rajinder Yadav]

In this case a default constructor for class Car is provided for you automatically, so you will not get a compile time or runtime error! Only if you declared a constructor with an argument would you need to explicitly declared a default constructor (if it's needed).
Study the following code, both class Automobile and class Truck are missing a default constructor, but the code builds and runs fine!

public class Test {
static public void main(String[] arg) {

// Truck s = new Truck(); // this line would not compile!

// This is fine, we have a constructor to build this object!!
Truck t = new Truck("Dodge Ram");
System.out.println(t.getName());
}
}
class Automobile {
String name;
Automobile(String s) {
name = s;
}

String getName() {
return name;
}
}
class Truck extends Automobile {
Truck(String s) {
super(s);
}
}
[ January 22, 2002: Message edited by: Rajinder Yadav ]

[R K Singh]

HI Kaushal
sorry.. but I am not getting your prob

[mark stone]

yes you are right that the statement
final Vehcile v = new Vehcile.......
is not correct. You make a class final where you declare it.
second you asked about the default constructor for Vehcile. well it is there, though it is empty but that is ok.
public Vehcile() {}
but this default constructor is not being invoked. only the constructor which takes string parameters is invoked. (2 times)
i hope now question is clear.

Originally posted by Nisheeth Kaushal:
Hi Friends
I have a difficulty here in this particular quesiton
class Vehicle {
String str ;
public Vehicle() {
}

public Vehicle ( String s ) {
str = s;
}
}
public class Car extends Vehicle {
public static void main (String args[] ) {
final Vehicle v = new Vehicle ( " Hello" );
v = new Vehicle ( " How are you");
v.str = "How is going";
System.out.println( "Greeting is : " + v.str );
}
}
A) Compiler error while subclassing the Vehicle
B) Compiler error , you cannot assign a value to final variable
B) Prints Hello
C) Prints How is going
This class is executing easily, but i think that the subclass will see for a default constructor, and if it does not find it, it will throw an error.
This particular program is producing error, but b'coz of fina variable not for the reason which i want.
Please help me,
Thanx in advance.
Nisheeth

[Paul Salerno]

Mark,

second you asked about the default constructor for Vehcile. well it is there, though it is empty but that is ok.
public Vehcile() {}
but this default constructor is not being invoked. only the constructor which takes string parameters is invoked. (2 times)

Mark I thought that the no args default constructor Vehicle() would only "be there" if there were no other constructor specified. Here there is a constructor for Vehicle which takes a string arg. Also how does this get invoked twice?
TIA

[Rob Ross]

Originally posted by mark stone:
yes you are right that the statement
final Vehcile v = new Vehcile.......
is not correct. You make a class final where you declare it.

Actually Mark, this line is fine. You can declare a variable to be final. This allows the variable to be initialized once, then prevented from having its value changed. The error in this code is in the line
v = new Vehicle ( " How are you");
when the final variable v, which has already been initialized, is re-assigned a new value.

second you asked about the default constructor for Vehcile. well it is there, though it is empty but that is ok.
public Vehcile() {}
but this default constructor is not being invoked. only the constructor which takes string parameters is invoked. (2 times)
i hope now question is clear.

In the original example, the Vehicle class has two explicit constructors - one is the no-args constructor, and one is a consructor that takes a String argument. If you comment out the second assignement to v, this program works fine and a Vehicle object is instantiated.
The Car class has no explicit constructors, so the compiler is adding one for you - a no-arg constructor:
Car() { super();}
This means that the only way to instantiate a Car object is with a no-arg constructor. You can write
Car myCar = new Car();
but not
Car myCar = new Car("Toyota");
because the Car class has no constructor that takes a string argument.

Rob
[ January 22, 2002: Message edited by: Rob Ross ]

[Paul Salerno]

Rob,
Is it suffice to say that if class Vehicle only had Vehicle(String s) type of constructor, that you would not be able to instantiate the Car class utilizing the default: Car myCar = new Car()

[Nisheeth Kaushal]

Hi all,
Thanx to all,
I wud like to ask one more question to Mark that as he said that Default constructor is not invoked even once.
But if u use something like this.
public Vehicle() {
System.out.println("Default Constructor");
}
This will print the above statement, so i think that the instantiation of default constructor is based on the object that u create in the subclass.
So, as we are not creating any vehicle no arg object, a super will be embed and call the default constructor of the superclass.
Tx
Nisheeth