Java – why can’t I extend instantiatable classes with new value components while retaining the CompareTo contract?

Each valid Java for Joshua Bloch:

Could you please explain the above problems through examples and challenges? Can you explain what Joshua means to "value components" and whether other types of components are available

Can you explain what Joshua meant when second class was the first class?

Solution

of course. Consider these two classes – I've omitted all getters, installers, etc., but I want you to get drift:

class NamedThing {
    String name;
}

class Person extends NamedThing {
    Date dateOfBirth;
}

Ignore whether this is a good inheritance example - this is a simple example

It's natural for namedthing to implement name - based comparisons in alphabetical order

It is also natural for people to compare names first (so consistent in this regard), and then check that one birth date is earlier than another

Now imagine:

NamedThing person1 = new Person("Jon",date1);
NamedThing person2 = new Person("Jon",date2);
NamedThing thing = new NamedThing("Jon");

int comparison1 = person1.compareTo(thing);
int comparison2 = person2.compareTo(thing);
int comparison3 = person1.compareTo(person2);
int comparison4 = thing.compareTo(person1);

What do you want all these results to be? If person CompareTo is smart enough to only apply its date processing to instances of person, so you may want comparison1 and comparison2 to be 0, but compare3 to be non-zero

Presumably compare4 must be 0 because it uses namedthing CompareTo, which compares only names

Fundamentally, trying to compare different types of instances is problematic It eventually becomes clearer, with an external definition of comparison, which defines the comparison it will use Therefore, you can have a comparator < person > that accepts only person references and uses names and dates, and comparator < namedthing > compares only by name The behavior will have symmetry and clarity

You are out of context It is: "treat instances of the second class as instances of the first class" – for example

// First class = Animal
// Second class = Zebra
Animal person = new Zebra();