Question

I have the following example:

package cage;

import java.util.HashSet;
import java.util.Set;

import animals.Animal;

public class Cage<T extends Animal> {  //A cage for some types of animals
    private Set<T> set = new HashSet<T>();

    public void add(T animal)  {
        set.add(animal);
    }

    public void showAnimals()  {
        for (T animal : set) {
            System.out.println(animal.getName());
        }
    }

    public void transferTo(Cage<? super T> cage) {
        cage.set.addAll(this.set);
    }
}

Main class:

package exe;

import cage.Cage;
import animals.Animal;
import animals.Ape;
import animals.Lion;
import animals.Rat;

public class Main {

    public static void main(String[] args) {
        System.out.println("Test with super........");
        Cage<Animal> animals = new Cage<Animal>();
        Cage<Lion> lions = new Cage<Lion>();
        animals.add(new Rat(true, 4, "Rat", true)); // OK to put a Rat into a Cage<Animal> 
        lions.add(new Lion(true, 4, "King", 9));
        lions.transferTo(animals); // invoke the super generic method -> animals is SUPER of lion.
        animals.showAnimals();
    }
}

In the class cage there is an invocation

cage.set.addAll(this.set);

Why does it work to invoke “cage.set…” with the dot notation althought I have neither a getSet-method nor is “set” static? What’s the technical background?

Answer 1

You’re just using field access. Having a getSet() method would make no difference, as the Java compiler won’t use an accessor method automatically for you.

I suspect what you’re missing is that private access isn’t determined by the object whose member you’re trying to access – the code in Cage has access to the private members of any other Cage, including the set field.

Section 6.6 of the Java Language Specification describes access control, including:

Otherwise, if the member or constructor is declared private, then access is permitted if and only if it occurs within the body of the top level class (§7.6) that encloses the declaration of the member or constructor.