在不知道类名称的情况下调用构造函数（java）

Question

This problem easier to understand with code than with words:

Map<Integer, Parent> objectMap = new HashMap<Integer, Parent>();

Parent myParent;
Child1 myChild1;
Child2 myChild2;
//A lot more myChilds

objectMap.put(1, myChild1);
objectMap.put(2, myChild2);
//Place all the myChilds

myChild1 = new Child1();  //Constructor is expensive, object may not get used
myChild2 = new Child2();  //Constructor is expensive, object may not get used
//Call constructor for all of myChilds

Parent finalObject;

int number = 1; //This can be any number

finalObject = objectMap.get(number);

As you see, I don't know in advance which class will finalObject be. The code works without problem, but here is my question:

How can I avoid calling both constructors?

As only myChild1 or myChild2 will be used and the constructor methods are quite expensive, I want to only call the one that will actually get used.

Something like

finalObject.callConstructor();

in the last line

Any ideas?

Thanks in advance.

EDIT: What I want to know is how to call the constructor without knowing the name of the class . Check the updated code.

Answer 1

How about this?

Parent finalObject;

if (condition) {
    finalObject = new Child1();
} else {
    finalObject = new Child2();
}

Or, even better, this?

Parent finalObject = condition? new Child1() : new Child2();

Answer 2

Don't construct both objects. Only construct the object you need.

Parent finalObject;
if (condition) {
    finalObject = new Child1();
} else {
    finalObject = new Child2();
}

Answer 3

You should use the factory pattern :

public interface Factory<T> {
  T create();
}

...

Factory<T> factory;
if (condition) {
  factory = FACTORY1;
} else {
  factory = FACTORY2;
}
object = factory.create()

Answer 4

First check your condition and then inside the if or else statement call the constructor. if(condition) finalobject = new myChild1(); else finalobject = new myChild2();