OnJava8-Examples/generics/GenericReading.java

// 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<Apple> apples =
    Arrays.asList(new Apple());
  static List<Fruit> fruit = Arrays.asList(new Fruit());
  static <T> T readExact(List<T> 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> {
    T readExact(List<T> list) { return list.get(0); }
  }
  static void f2() {
    Reader<Fruit> fruitReader = new Reader<>();
    Fruit f = fruitReader.readExact(fruit);
    //- Fruit a = fruitReader.readExact(apples);
    // error: incompatible types: List<Apple>
    // cannot be converted to List<Fruit>
  }
  static class CovariantReader<T> {
    T readCovariant(List<? extends T> list) {
      return list.get(0);
    }
  }
  static void f3() {
    CovariantReader<Fruit> fruitReader =
      new CovariantReader<>();
    Fruit f = fruitReader.readCovariant(fruit);
    Fruit a = fruitReader.readCovariant(apples);
  }
  public static void main(String[] args) {
    f1(); f2(); f3();
  }
}