Polymorphism, Constructors and Methods

[Clemonte Johnstone]

Dear All,

The source of this question comes from the Online Udemy Course "Pass the Oracle Certified Associate(OCA): Java SE 8 Programmer I EXAM"
Instructor: Udayan Khattry


Please could someone kindly explain how on earth this program prints -1 when executed?

class Base1 {
	int id = 1000; 									// Line n1

	Base1() {
		Base1(); 									// Line n2
	}

	void Base1() { 									// Line n3
		System.out.println(++id); 					        // Line n4
	}
}

class Derived1 extends Base1 {
	int id = 2000; 									// Line n5

	Derived1() {} 									// Line n6

	void Base1() { 									// Line n7
		System.out.println(--id); 					        // Line n8
	}
}

public class _14_Question_Polymorphism_And_Constructors {
	public static void main(String[] args) {
		Base1 base = new Derived1(); 				        // Line n9
	}
}

Thanks in advance for any help that you can give.

[Stephan van Hulst]

The main method calls the Derived1 constructor.

The Derived1 constructor calls the Base1 constructor.

Base1 initializes its own id field to 1000.

The Base1 constructor calls the Base1() method that's overridden in Derived1.

The Base1() method in Derived1 prints --id. id hasn't been initialized yet, so it prints -1.

The Base1() method returns to the Base1 constructor.

The Base1 constructor returns to the Derived1 constructor.

Derived1 initializes its own id field (which was -1) to 2000.

The Derived1 constructor returns to the main method.

The main method finishes running.

In short, the solution the problem is to realize that the Base1 constructor runs before the Derived1.id field is initialized. Superclass constructors always run before fields in subclasses are initialized.

The lesson is that you must never call overridable methods from constructors.