47 lines
1.4 KiB
Java
47 lines
1.4 KiB
Java
// generics/GenericReading.java
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// ©2016 MindView LLC: see Copyright.txt
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// We make no guarantees that this code is fit for any purpose.
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// Visit http://mindviewinc.com/Books/OnJava/ for more book information.
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import java.util.*;
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public class GenericReading {
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static <T> T readExact(List<T> list) {
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return list.get(0);
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}
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static List<Apple> apples = Arrays.asList(new Apple());
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static List<Fruit> fruit = Arrays.asList(new Fruit());
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// A static method adapts to each call:
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static void f1() {
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Apple a = readExact(apples);
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Fruit f = readExact(fruit);
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f = readExact(apples);
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}
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// If, however, you have a class, then its type is
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// established when the class is instantiated:
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static class Reader<T> {
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T readExact(List<T> list) { return list.get(0); }
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}
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static void f2() {
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Reader<Fruit> fruitReader = new Reader<>();
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Fruit f = fruitReader.readExact(fruit);
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// Fruit a = fruitReader.readExact(apples); // Error:
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// readExact(List<Fruit>) cannot be
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// applied to (List<Apple>).
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}
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static class CovariantReader<T> {
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T readCovariant(List<? extends T> list) {
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return list.get(0);
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}
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}
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static void f3() {
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CovariantReader<Fruit> fruitReader =
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new CovariantReader<>();
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Fruit f = fruitReader.readCovariant(fruit);
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Fruit a = fruitReader.readCovariant(apples);
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}
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public static void main(String[] args) {
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f1(); f2(); f3();
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}
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}
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/* Output: (None) */
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