Question

If I have an abstract class in java named Foo and it has an implementor named Bar then I want to know the following.

lets say Foo looks something like

public abstract class Foo {

  Service serviceFoo

  ...

}

And Bar is

public class Bar extends Foo {
...
}

Also, lets assume I have an instance with Foo, named foo, currently that has serviceFoo instantiated

If I then declare:

Foo foo = new Bar();

will this create a a new instance of Bar that has serviceFoo instantiated or not? E.g. will that field be inherited and instantiated or just inherited?

Answer 1

When you call new Bar();, the constructor for Foo is called implicitly. If Foo‘s constructor instantiates serviceFoo then so will Bar. If Foo relies on someone else to instantiate serviceFoo then Bar will do the same.

Existing instances of either Foo or Bar have no bearing on what goes on when a new instance is made. Only what code gets executed in the constructor (or what gets passed in as a parameter) has an impact on the new object.