What happens when a subclass does not define a constructor in Java?

I have a few things I want to know First, if you don't have a constructor:

class NoCons { int x; }

When I do the new nocons (), the default constructor is called What did it do? It sets x to 0, otherwise it will happen elsewhere?

What if I have this situation?

class NoCons2 extends NoCons { int y; }

What happens when I call the new nocons2()? Nocons2's default constructor is called, and then nocons2's constructor? Do they set their respective x and Y fields to 0?

What about this version

class Cons2 extends NoCons { int y; public Cons2() {} }

Now I have a constructor, but it doesn't call the constructor of the superclass How does x initialize? What if I have this situation?

class Cons { int x; public Cons() {} }
class NoCons2 extends Cons { int y;  }

Will the constructor be called?

I can try all these examples, but I don't know when to run the default constructor What is the general way to think about this so that I know what will happen in the future?

Solution

When the Java class does not explicitly define a constructor, a public no args default constructor is added:

class Cons { int x; }

amount to:

class Cons { int x; public Cons() {} }

The constructor of a subclass does not explicitly define the constructor of the parent class it calls. It will automatically call the default constructor of the parent class before the parent class So suppose:

class A { public A() { System.out.println("A"); } }

So this:

class B extends A { public B() { System.out.println("B"); } }

Fully equivalent to:

class B extends A { public B() { super(); System.out.println("B"); } }

And the output in both cases will be:

A
B

So when you do:

new NoCons2();

The order is:

>Nocons's default constructor call, although this is technically the first part of (2); Next, the default constructor of > nocons2 is called