Question

I have a public constructor which takes a parameter (int age) to create an object. I want to check if the passed parameter is legal or not, for example age cannot be negative. If its illegal, then don’t create an object/instance. If legal, no problem.

I can only think of one way to do this –

Make constructor private. Create a static method with parameter (int age) to do all the checking and return a null if you pass it an illegal value. If you pass it a legal value, then create an object and return its reference.
Is there any other way of doing it ? Maybe from inside the constructor itself ?

EDIT :
I thought of one problem with the above method. The factory method/object creator method can only be a static method for obvious reasons. What happens if the factory method has to access a member variable (to do some checking) to create an object ? Then, we will be forced to make that member variable static. This may not be okay in all cases.

Does it make sense ?

Answer 1

Is there any other way of doing it ? Maybe from inside the constructor itself ?

Yes. I suggest to throw an Exception from constructor

public class Person
{


    int age;
    public Person(int age) throws Exception
    {
       if(age <= 0)
       {

          throw new Exception("Age is not allowed");
       }
       // Do some stuffs
       this.age = age;
    }

}

Edit:

You can also use IllegalArgumentException as suggested by Till Helge Helwig

public class Person
{


    int age;
    public Person(int age) throws IllegalArgumentException
    {
       if(age <= 0)
       {

          throw new IllegalArgumentException("Age is not allowed");
       }
       // Do some stuffs
       this.age = age;
    }

}