// generics/GenericReading.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.*;
public class GenericReading {
static List apples =
Arrays.asList(new Apple());
static List fruit = Arrays.asList(new Fruit());
static T readExact(List list) {
return list.get(0);
}
// A static method adapts to each call:
static void f1() {
Apple a = readExact(apples);
Fruit f = readExact(fruit);
f = readExact(apples);
}
// A class type is established
// when the class is instantiated:
static class Reader {
T readExact(List list) { return list.get(0); }
}
static void f2() {
Reader fruitReader = new Reader<>();
Fruit f = fruitReader.readExact(fruit);
//- Fruit a = fruitReader.readExact(apples);
// error: incompatible types: List
// cannot be converted to List
}
static class CovariantReader {
T readCovariant(List extends T> list) {
return list.get(0);
}
}
static void f3() {
CovariantReader fruitReader =
new CovariantReader<>();
Fruit f = fruitReader.readCovariant(fruit);
Fruit a = fruitReader.readCovariant(apples);
}
public static void main(String[] args) {
f1(); f2(); f3();
}
}