Why can we call abstract method from an abstract class before sub-class initialization

Question

We can initialize an abstract class through initialization of a concrete sub class. Therefore, should be able to call the abstract methods of an abstract class only after the initialization of a subclass However, the following initialization block in abstract class Mammal can call an abstract method before the initialization of the sub child class.

If you call the constructor of Platypus with new Platypus(), it outputs: "yummy!", "Mammal constructor", "Platypus constructor" in the same order. I thought an instance of a class gets created when the constructor is called. But the initialization block in Mammal is able to call the method before Platypus initialization.

abstract class Mammal {

    {
        System.out.println(chew()); 
    }

    abstract CharSequence chew();

    public Mammal() {
       System.out.println("Mammal constructor"); 
    }
}

public class Platypus extends Mammal {

    public Platypus() {
        System.out.println("Platypus constructor");
    }
    
    String chew() {
        return "yummy!";
    }


}

When the class is loaded by the JVM, its code becomes available. Code is not per-instance, so it's already there before the class instance has been reserved in memory. If it helps you understand it, you can think of the code as "static": there's only one chunk code for all the instances of a class. It does not duplicate the code for each class instance. — m0skit0, Apr 24 '23 at 08:37
Because Java doesn't work like C++, which does behave the way you expect. — user207421, Apr 24 '23 at 09:57

score 4 · Accepted Answer · answered Apr 24 '23 at 08:43

There is nothing special about abstract methods here. You can also just call non-abstract methods in the same way:

class Platypus {
    {
        System.out.println(chew());
    }
    
    public Platypus() {
        System.out.println("Platypus constructor");
    }

    String chew() {
        return "yummy!";
    }
}

I thought an instance of a class gets created when the constructor is called.

No, the constructor is not responsible for creating the object. An object is created first, and then its constructor is called to initialise the object.

This can be seen in the bytecode too. The bytecode for new Platypus() looks something like this:

NEW yourpackage/Platypus
DUP
INVOKESPECIAL yourpackage/Platypus.<init> ()V

The new instruction creates the object, and then <init>, which is the JVM name for constructors, is invoked.

So here, we can say that chew() is called before the object is fully initialised. The object already exists, so calling chew is technically possible.

However, calling instance methods when the object is not fully initialised is usually not a good idea. The method might depend on the fact that the object is fully initialised (e.g. accessing fields that are expected to not be null). See also: Can I call methods in constructor in Java?

Why can we call abstract method from an abstract class before sub-class initialization

1 Answers1