Thread: can Java create a user define type of classes?

Hi,

FYI I am very new to Java programming from a long time Delphi programmer...

Does java allow a mechanism to create a user defined type for a "class type"?

I am looking for a java equivalent of delphi "class of" declaration.

Delphi example...

type   // define types for the following block of codes...
    TMyClasses = class of TMyClass;
    TMyClass = class(TObject)
    private
        FMyOtherClass: TMyClasses
    public
        class function TableName: string; virtual; abstract;  // defines virtual method that must be overridden
        property MyOtherClass: TMyClasses  read  FMyOtherClass  write  FMyOtherClass;
        .... my class properties and methods...
        end;
 
    TMyClass2 = class(TMyClass)  // inheritance
    public
        class function TableName: string; override;  // ie. implemented as "Items" in source
        end ...
    TMyClass3 = class(TMyClass)
    public
        class function TableName: string; override;  // ie. implemented as "Employee" in source
        end 
    TMyClass21 = class(TMyClass2); ...
    public
        class function TableName: string; override;  // ie. implemented as "Junk" in source
        end 
    TMyClass31 = class(TMyClass3); ...
    public
        class function TableName: string; override;  // ie. implemented as "Foo" in source
        end 
 
    TBroClass = class(TObject)...
 
var
    MyClass: TMyClass;
    OtherClass: TMyClass;
 
...
MyClass := TMyClass.Create;  // instantiate an TMyClass object
 
// and I can do the following
MyClass.MyOtherClass := TMyClass2;   // assignment of just the class type
ShowMessage(MyClass.MyOtherClass.TableName);  // this will display a dialog box with message "Items"
OtherClass := MyClass.MyOtherClass.Create; // this line will instantiate TMyClass2
 
MyClass.MyOtherClass := TMyClass31;
ShowMessage(MyClass.MyOtherClass.TableName);  // this will display a dialog box with message "foo"
OtherClass := MyClass.MyOtherClass.Create; // this line will instantiate TMyClass31
 
MyClass.MyOtherClass := TBroClass;  // this will have a compiler error for type incompatability

So, does java has something similar?

Thank you,

I think you're just talking about simple instantiation. I think that's implied in Java. Recommended reading: Lesson: Classes and Objects (The Java™ Tutorials > Learning the Java Language)

You also might want to check out the Class API: Class (Java Platform SE 6)

But I'm still not really sure what you're actually looking for. Maybe post what you think the Java code might look like, if it existed?

Sounds like you're looking for is covariance in polymorphism. There are two main ways to achieve this, either through generics or via reflection.

Without a more concrete example it's difficult to determine which you'll want, but it sounds like you're looking for generics.

public class A<T extends Number> // some generic parameter T of class A. T is some sub-class of Number
{
    T data; // class A holds some data of type T
}