//: generics/GenericReading.java
import java.util.*;
public class GenericReading {
static T readExact(List list) {
return list.get(0);
}
static List apples = Arrays.asList(new Apple());
static List fruit = Arrays.asList(new Fruit());
// A static method adapts to each call:
static void f1() {
Apple a = readExact(apples);
Fruit f = readExact(fruit);
f = readExact(apples);
}
// If, however, you have a class, then its 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:
// readExact(List) cannot be
// applied 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();
}
} ///:~