OnJava8-Examples/generics/GenericReading.java at master · CSapprenticeWT/OnJava8-Examples · GitHub

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// generics/GenericReading.java

// 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();

}

OnJava8-Examples/generics/GenericReading.java at master · CSapprenticeWT/OnJava8-Examples · GitHub

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