Why can't we "new" a Statement object?

[tenxian]

Why do we have to use the Connection object to produce a Statement
object? Similar question like, Why do we have to use the DriverManager
to produce a Connection object? Can't we create a Connection object
via Connection myConn=new Connection()?

 

[Daniel Pitts]

Why do we have to use the Connection object to produce a Statement
object? Similar question like, Why do we have to use the DriverManager
to produce a Connection object? Can't we create a Connection object
via Connection myConn=new Connection()?

The pattern is called "Factory"

Connection may not be a concrete type. It might not even be a class!
You don't know what type of Connection you're going to get unless you
know what Driver you're using. etc...

Same goes with Statement. Different drivers may create different
Connections, which create different Statements, but all *you* know about
are the static abstract base classes/interfaces.

 

[Arne Vajhøj]

Why do we have to use the Connection object to produce a Statement
object? Similar question like, Why do we have to use the DriverManager
to produce a Connection object? Can't we create a Connection object
via Connection myConn=new Connection()?

You can:

Connection myConn = new FoobarDatabaseSpecificConnection();

but you will hopefully never be able to get that type
of code through code review, because the code is
Foobar database specific.

It is a core concept in Java that database code should
be database independent.

DriverManager.getConnection enables you to use the
same code for different databases (the connection URL
can be put in a configuration file).

Arne

 

[Roedy Green]

Why do we have to use the Connection object to produce a Statement
object? Similar question like, Why do we have to use the DriverManager
to produce a Connection object? Can't we create a Connection object
via Connection myConn=new Connection()?

This derives from the fact that every SQL database may implement
things quite differently. All that gets nailed down is the INTERFACE,
not the classes or even an abstract class basic implementation.

The code you run for Statement.someMethod is completely different for
Oracle vs MySQL .

New presumes a common class and common implementation. When you don't
have that you need a pluggable mechanism and a factory to get you the
objects you need. That's one thing the driver does.

You will see the same sort of thing in JCE where you can have
different implementors.

JavaMail allows different implementations so it too likely works this
way though it has been a while since I fiddled with those details.

 

[tenxian]

The pattern is called "Factory"

Connection may not be a concrete type. It might not even be a class!
You don't know what type of Connection you're going to get unless you
know what Driver you're using. etc...

Same goes with Statement. Different drivers may create different
Connections, which create different Statements, but all *you* know about
are the static abstract base classes/interfaces.

What is a Factory in Java?

 

[Arne Vajhøj]

What is a Factory in Java?

Same as in other languages.

See http://en.wikipedia.org/wiki/Factory_method_pattern !

Arne

 

[Roedy Green]

What is a Factory in Java?

see http://mindprod.com/jgloss/factorymethod.html

It is a design pattern for constructing objects.