What is the most effective way of writing a factory method?

Question

In most of the cases when we write a factory method it is a bunch of if conditions which can keep on growing. What is the most effective way of writing such a method (with the least if conditions)?

public A createA(final String id) {
    if (id.equals("A1")) {
      return new A1();
    }
    else if (id.equals("A2")) {
      return new A2();
    }
    return null;
  }

On stackoverflow, this question is likely to be closed as being subjective. For this kind of programming style question, http://programmers.stackechange.com would have been a better fit. — oberlies, Apr 18 '14 at 19:49

score 12 · Accepted Answer · answered Apr 17 '14 at 06:44

12

You could use a Map<String, Supplier<A>>:

Map<String, Supplier<A>> map = new HashMap<>();
map.put("A1", () -> new A1());
map.put("A2", () -> new A2());

...

public A createA(final String id) {
    Supplier<A> supplier = map.get(id);
    if (supplier != null) {
        return supplier.get();
    }
    throw new IllegalArgumentException("unknown id: " + id);
}

This uses a standard Java 8 Supplier interface, with a Java 8 lambda syntax, but you could of course define your own Supplier interface, and create instances using anonymous inner classes.

answered Apr 17 '14 at 06:44

JB Nizet

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Not too familiar with lambdas, but is this lazy or eager initialization? – Fuhrmanator Apr 18 '14 at 19:46
It's lazy. The get() call calls the body of the lambda. Passing () -> new A1(); where a Supplier should go is roughly equivalent to (deep breath) new Supplier () { public A get() { return new A1(); } }. Supplier is a [functional interface](http://java.dzone.com/articles/introduction-functional-1). – Calum Apr 18 '14 at 19:50
@Calum This is technically eager. An anonymous class is constructed immediately, which creates new instances of some class via the _get()_ method, but at no point is the creation of an object delayed until it is needed. – Marco May 02 '15 at 19:32
I meant to express that it's delaying the construction of A1 instances until they're needed but I take your meaning. It doesn't solve the problem of returning a single A1 instance to all callers if that's what's being asked. – Calum May 16 '15 at 08:06

score 1 · Answer 2 · answered Apr 17 '14 at 06:43

I like a archetype map approach - just fill the archetypes up at some stage - either in a static block or the class constructor. Code looks roughly like this

  Map<String, A> archetypes;

  public A createA(final String id)
  {
     A a = archetypes.get(id);
     return (a!=null)?a.copy():null;
  }

Fuhrmanator · Answer 3 · 2014-04-19T01:04:56.727

-1

This method is lazy initialization. Assuming you have classes that correspond to the name in id:

public A createA(final String id) {
  // warning, should not use an id string that is tainted (eg from user input)
  return (A) Class.forName(id).newInstance();
}

edited Apr 19 '14 at 01:04

answered Apr 18 '14 at 19:40

Fuhrmanator

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Creating classes by name should be avoided if possible. It leads to lots of problems if you have non flat classloaders like in OSGi or JavaEE. – Christian Schneider Feb 22 '22 at 13:50

What is the most effective way of writing a factory method?

3 Answers3