Question

I have a question about code style:

Let’s assume i have a String containing some information (like “Information1” or “Information2”). Based on this i want to create objects with a factory. Obviously i could write something like this:

if(string.equals("Information1")){
  Factory.createInformation1Object();
}
if(string.equals("Information2")){
  Factory.createInformation2Object();
}
if(string.equals("Information3")){
  Factory.createInformation3Object();
}

Now i was wondering if there is a better (and prettier) way to do this. I really like the multiple dispatch idea of the visitor pattern, but i cant see a way to apply this easily to this particular problem.

Answer 1

You can use the abstract factory pattern.

Create an abstract Factory class [or interface], and classes that extends it: MyObject1Factory, MyObject2Factory, …

On preprocessing, populate a Map<String,Factory> from a String to the coresponding Factory instance, this is done only once in your application.

When you need to create a new instance – invoke map.get(string).create() to create the relevant object, of the relevant type.

Edit: Small example with code:

Your classes are:

public static class MyBase { 
    @Override
    public String toString() {
        return "Base";
    }
}
public static class Class1 extends MyBase { 
    @Override
    public String toString() {
        return "Class1";
    }
}
public static class Class2 extends MyBase { 
    @Override
    public String toString() {
        return "Class2";
    }
}

And your factories will be:

public static abstract class MyFactory {
    public abstract MyBase build();
}
public static class MyFactory1 extends MyFactory {
    @Override
    public Class1 build() {
        return new Class1();
    }
}
public static class MyFactory2 extends MyFactory {
    @Override
    public Class2 build() {
        return new Class2();
    }
}

populate a map only once in the program lifetime:

    Map<String,MyFactory> map = new HashMap<String, Test.MyFactory>();
    map.put("class1", new MyFactory1());
    map.put("class2", new MyFactory2());

and when you need a new object, invoke with:

    MyBase obj = map.get(s).build();
    System.out.println(obj);

(*) Note: The static keyword for classes are here because I created them as inner classes – of course you need to ommit it if it is not your case….