// generics/Holder.java

// (c)2021 MindView LLC: see Copyright.txt

// We make no guarantees that this code is fit for any purpose.

// Visit http://OnJava8.com for more book information.

import java.util.Objects;

public class Holder<T> {

private T value;

public Holder() {}

public Holder(T val) { value = val; }

public void set(T val) { value = val; }

public T get() { return value; }

@Override public boolean equals(Object o) {

return o instanceof Holder &&

Objects.equals(value, ((Holder)o).value);

}

@Override public int hashCode() {

return Objects.hashCode(value);

}

public static void main(String[] args) {

Holder<Apple> apple = new Holder<>(new Apple());

Apple d = apple.get();

apple.set(d);

// Holder<Fruit> Fruit = apple; // Cannot upcast

Holder<? extends Fruit> fruit = apple; // OK

Fruit p = fruit.get();

d = (Apple)fruit.get(); // Returns 'Object'

try {

Orange c = (Orange)fruit.get(); // No warning

} catch(Exception e) { System.out.println(e); }

// fruit.set(new Apple()); // Cannot call set()

// fruit.set(new Fruit()); // Cannot call set()

System.out.println(fruit.equals(d)); // OK

}

}

/* Output:

java.lang.ClassCastException: Apple cannot be cast to

Orange

false

*/