How I can I return a null value from an object constructor?


The simple answer is : no, you can't.

The explanation why is simple. You don't return any value whatsoever from an object constructor. The object has already been created - in the constructor you're just initializing the object's state. So there isn't any way to return a null value.

However, if you want to throw a spanner in the works, and stop someone using your object (which is usually the intent of returning a null value, or to indicate an error), why not throw a NullPointerException ?

public MyClass()
    // Something might go wrong, so throw a null pointer exception
    throw new NullPointerException() ;