default constructor in Java versus C++

[Matt]

I try to compare the default constructor in Java and C++.

In C++, a default constructor has one of the two meansings
1) a constructor has ZERO parameter

Student()
{ //etc...
}

2) a constructor that all parameters have default values

Student(int age = 10, String name = "Joe")
{ //etc...
}

However, In Java, default constructor means a constructor has ZERO parameter only.

Student()
{ //etc...
}

The following will yield compile errors
Student(int age = 10, String name = "Joe")
{ //etc...
}

Any ideas why Java doesn't support that?

Please advise. Thanks!!

[Tony Morris]

> Any ideas why Java doesn't support that?

This might help.
http://www.google.com/search?hl=en&l...zation+C%2B%2B

--
Tony Morris
http://xdweb.net/~dibblego/

[Ann]

"Matt" <jr********@hot mail.com> wrote in message
news:ba******** *************** **@posting.goog le.com...

I try to compare the default constructor in Java and C++.

In C++, a default constructor has one of the two meansings
1) a constructor has ZERO parameter

Student()
{ //etc...
}

2) a constructor that all parameters have default values

Student(int age = 10, String name = "Joe")
{ //etc...
}

However, In Java, default constructor means a constructor has ZERO parameter only.
Student()
{ //etc...
}

The following will yield compile errors
Student(int age = 10, String name = "Joe")
{ //etc...
}

Any ideas why Java doesn't support that?

Please advise. Thanks!!

Same reason algol60 doesn't support it.
Maybe you can tell us why C++ doesn't support all the
algol60 features.

[Chris Smith]

Matt wrote:

I try to compare the default constructor in Java and C++.

Please note that the two languages use that term in different ways.
It's an implementation-level concept, and naturally implementation-level
concerns will differ when the language is changed. In this case, it's
quite by coincidence that the term happens to have a meaning in both
languages, and it results in some confusion when the C++ meaning is
applied to Java.

You seem to be using the C++ definition of "default constructor" below.
The C++ definition does not apply in Java. In Java, all creations or
objects MUST specify an argument list for the constructor, so there is
no "default" in that sense.

A "default constructor" in Java, though, generally means a constructor
that's generated for you by the compiler. For example, the following
class:

class A { }

has a default constructor, generated automatically by the compiler,
which takes no arguments. The following class:

class B
{
public B() { }
}

is identical in functionality, but it does *not* have a default
constructor. Instead, the no-argument constructor is defined quite
explicitly.
However, In Java, default constructor means a constructor has
ZERO parameter only.
It is true that, in Java, all default constructors have zero parameters.
However, it is not true that all zero-parameter constructors are
"default", and hence not true that default "means" zero-parameter.
The following will yield compile errors
Student(int age = 10, String name = "Joe")
{ //etc...
}

Any ideas why Java doesn't support that?

One of the design goals of Java is to simplify the resolution of syntax.
The task of default parameters is easily accomplished by overloading, so
there is no need for the redundant mechanism.

--
www.designacourse.com
The Easiest Way To Train Anyone... Anywhere.

Chris Smith - Lead Software Developer/Technical Trainer
MindIQ Corporation

[Gary Labowitz]

"Chris Smith" <cd*****@twu.ne t> wrote in message
news:MP******** *************** *@news.altopia. net...
<<snip>>

A "default constructor" in Java, though, generally means a constructor
that's generated for you by the compiler. For example, the following
class:

class A { }

has a default constructor, generated automatically by the compiler,
which takes no arguments. The following class:

class B
{
public B() { }
}

is identical in functionality, but it does *not* have a default
constructor. Instead, the no-argument constructor is defined quite
explicitly.

I don't think so, Chris. In Java, any constructor that takes no parameters
is called a default constructor.
This could be better checked on comp.lang.java. programmer, however.
--
Gary

[Tony Morris]

I don't think so, Chris. In Java, any constructor that takes no parameters
is called a default constructor.
This is not true.
This could be better checked on comp.lang.java. programmer, however.
--

When did a Usenet forum become the definitive source?
Try Java Language Specification Second Edition 8.6.7 Default Constructor.

Chris did mention however that class A{} is functionally equivalent to class
B{public B(){}}.
This is not true.
I'll leave it up to you to figure out why (hint: 8.6.7 also).

--
Tony Morris
http://xdweb.net/~dibblego/

[KiLVaiDeN]

"Matt" <jr********@hot mail.com> wrote in message
news:ba******** *************** **@posting.goog le.com...

I try to compare the default constructor in Java and C++.

In C++, a default constructor has one of the two meansings
1) a constructor has ZERO parameter

Student()
{ //etc...
}

2) a constructor that all parameters have default values

Student(int age = 10, String name = "Joe")
{ //etc...
}

However, In Java, default constructor means a constructor has ZERO parameter only.
Student()
{ //etc...
}

The following will yield compile errors
Student(int age = 10, String name = "Joe")
{ //etc...
}

Any ideas why Java doesn't support that?

Please advise. Thanks!!

it doesn't support that same syntax, but it can be done easily by simply
doing a default constructor ( without any parameter ) and inside declaring
the default values of the variables.

Student() {
age = 10;
name = "Joe";
//etc..
}

I think Java doesn't support the earlier syntax because it makes the syntax
of the constructor different than the one of other functions, and removing
that features, gives a function declaration quasi equal to the one of the
constructors.

K

[Alf P. Steinbach]

* Tony Morris:

I don't think so, Chris. In Java, any constructor that takes no parameters
is called a default constructor.

This is not true.

This could be better checked on comp.lang.java. programmer, however.
--

When did a Usenet forum become the definitive source?
Try Java Language Specification Second Edition 8.6.7 Default Constructor.

You mean §8.8.7.

<url:
http://java.sun.com/docs/books/jls/second_edition/html/classes.doc.htm l#16823>

--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

[Tony Morris]

> > Try Java Language Specification Second Edition 8.6.7 Default
Constructor.

You mean §8.8.7.

<url:

http://java.sun.com/docs/books/jls/second_edition/html/classes.doc.htm l#16823>

That I do.

--
Tony Morris
http://xdweb.net/~dibblego/

[hiwa]

jr********@hotm ail.com (Matt) wrote in message news:<ba******* *************** ***@posting.goo gle.com>...

Java default constructor is one that you don't write. Never.
It doesn't take parameters.

JLS 8.8.7:

If a class contains no constructor declarations, then a default
constructor that takes no parameters is automatically provided.
C++ default constructor is one that you CAN write.
It CAN take parameters.

ISO/ANSI C++ 12.1.5:
A default constructor for a class X is a constructor of class X that
can be called without an argument. If there is no user-declared
constructor for class X, a default constructor is implicitly
declared. An implicitly-declared default constructor is an inline
public member of its class.