why to use factory?

[Pavel Kubal]

Hi ranchers, what's the point in using calling like this?

instance = MyFactory.getNewInstance();

[Keith Lynn]

There are some classes that are defined as abstract, and the only way you can
obtain an instance of the class is to use a factory method.

[Paul Bourdeaux]

Factory is used when the implementation of the class is unknown until run time. You usually have an interface, then multiple classes that implement the interface. Here is a practical example I remember from school.

public interface SortUtiilty {
    public void sort(Collection c);
}
 
public class TreeSort() implements SortUtility {
    public void sort(Collection c) {
        // sorts the collection using a tree sort
        ...
    }
}
 
public class BinarySort() implements SortUtility {
    public void sort(Collection c) {
        // sorts the collection using a binarysort
        ...
    }
}

Here is the factory code.

public class SortFactory {
    public static final int TREE = 0;
    public static final int BINARY = 1;
 
    public SortUtility getTreeSort() {
        return new TreeSort();
    }
 
    public SortUtility getBinarySort() {
        return new BinarySort();
    }
}

Now in you application, you can determine what kind of sort utility you want to use at run time...

SortUtility su;
if (userResponse == SortFactory.TREE) {
    su = SortFactory.getTreeSort();
} else if (userResponse == SortFactory.BINARY) {
    su = SortFactory.getBinarySort();
}
su.sort(collection);

[Pavel Kubal]

Well if I understood you correctly. Factory is used for instance when an instance is created via relfection?