switch object creation refactor

Question

Suppose I have classes A,B that each of them extends some class X. And I want a method to create A or B based on some parameter value(value is a result of some other logic).

Can I do it without a switch statement?

i.e.:

class X {...}
class A extends X {...}
class B extends X {...}

So naive would be to make a class:

class Z {
    X createObject(int type) {
        switch(type)
            case 1: return new A();
            ...
            case 2: return new B();
}

Answer 1

Yes, you can do it without a switch statement. I suggest using either an array or Map and Supplier.

Map<Integer, Supplier<X>> map = new HashMap<>();
map.put(1, A::new); // () -> new A()
map.put(2, B::new); // () -> new B()

X createObject(int type) {
    Supplier<X> supplier = map.get(type);
    if (supplier == null) return null;
    return supplier.get();
}

Answer 2

You can of course do it without a switch statement.

If you have only a few cases you could use the ternary operator.

  public static X createObject(int type) {
    return type == 1 ?
      new A() :
      type == 2 ?
        new B() :
        null;
  }

You could use also a more generic approach:

  private static final Map<Integer, Supplier<X>> FACTORIES;
  static {
    FACTORIES = new HashMap<>();
    FACTORIES.put(1, A::new);
    FACTORIES.put(2, B::new);
  }

  public static X createObject(int type) {
    return Optional.ofNullable(FACTORIES.get(type))
      .map(Supplier::get)
      .orElse(null);
  }

Since your using integers to identify the type, you could use very easy an array:

  private static final Supplier<X>[] FACTORIES = new Supplier[] { A::new, B::new };

  public static X createObject(int type) {
    return type > 0 && type <= FACTORIES.length ?
      FACTORIES[type - 1].get() :
      null;
  }