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chapter 14
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‎src/main/java/s4j/java/chapter14/Example.java‎

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package s4j.java.chapter14;
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import s4j.java.chapter12.Customer;
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import s4j.java.chapter12.DiscountedCustomer;
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import s4j.java.chapter12.PricedItem;
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import java.util.ArrayList;
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import java.util.List;
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public class Example {
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public static void main(String... args) {
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polymorphismInGenericLists();
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exampleOfBoundedTypes();
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assignmentRules();
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}
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private static void polymorphismInGenericLists() {
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List<Customer> customers = new ArrayList<>();
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// customers.add(new HockeyPuck()); // compiler failure
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Customer customerA = new Customer("Bob", "15 Fleetwood Mack Road");
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Customer customerB = new DiscountedCustomer("Derick Jonar", "23 Woodland Way");
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customerA.add(new PricedItem(10D));
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customerB.add(new PricedItem(10D));
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customers.add(customerA);
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customers.add(customerB);
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for (Customer customer : customers) {
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System.out.println(customer.getName() + " : " + customer.total());
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}
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}
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public static void exampleOfBoundedTypes() {
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// what's the difference between these two?
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Stack<Integer> a = new NumberStack<>();
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a.push(12);
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Integer popA = a.pop();
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Stack<Integer> b = new ListStack<>();
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b.push(12);
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Integer popB = b.pop();
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// Stack<String> b = new NumberStack<>(); // compiler failure
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}
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private static void assignmentRules() {
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// simple sub-type assignment
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A a = new A();
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B b = new B();
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a = b;
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// b = a; // compiler failure
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// not the same rules for generics
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List<B> listA = new ArrayList<>();
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List<A> listB = new ArrayList<>();
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// listA = listB; // compiler failure
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// listB = listA; // compiler failure
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}
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public static <A extends Comparable> List<A> sort(List<A> list) {
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list.sort((first, second) -> first.compareTo(second));
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return list;
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}
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public static class NumberStack<T extends Number> extends ListStack<T> { }
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private static class HockeyPuck {}
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private static class A {}
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private static class B extends A {}
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}

‎src/main/java/s4j/java/chapter14/Existential.java‎

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package s4j.java.chapter14;
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import java.util.ArrayList;
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import java.util.List;
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public class Existential {
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private final List<? extends Object> foo = new ArrayList<>();
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}

‎src/main/java/s4j/java/chapter14/ListStack.java‎

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package s4j.java.chapter14;
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import java.util.ArrayList;
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import java.util.Arrays;
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import java.util.List;
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public class ListStack<T> implements Stack<T> {
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static <A> A[] toArray(Stack<A> stack) {
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throw new UnsupportedOperationException();
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}
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static <A extends Comparable<A>> A[] toSortedArray(Stack<A> stack) {
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throw new UnsupportedOperationException();
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}
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public static <T> void copy(ListStack<? extends T> source, ListStack<? super T> destination) {
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for (T t : source.elements) {
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destination.elements.add(t);
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}
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}
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private final List<T> elements = new ArrayList<>();
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@Override
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public void push(T t) {
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elements.add(0, t);
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System.out.println(Arrays.toString(elements.toArray()));
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}
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@Override
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public T pop() {
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if (elements.isEmpty())
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throw new IndexOutOfBoundsException();
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return elements.remove(0);
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}
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public static void main(String... args) {
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Stack<String> stack = new ListStack<>();
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stack.push("C");
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stack.push("B");
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stack.push("A");
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System.out.println(stack.pop());
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System.out.println(stack.pop());
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System.out.println(stack.pop());
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// System.out.println(stack.pop());
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List<String> a = new ArrayList<>();
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List<? extends CharSequence> b = new ArrayList<>();
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List<? extends Object> c = new ArrayList<>();
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b = a;
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c = b;
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c = a;
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}
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}

‎src/main/java/s4j/java/chapter14/LowerBounds.java‎

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package s4j.java.chapter14;
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import java.util.ArrayList;
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import java.util.Collections;
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import java.util.List;
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public class LowerBounds {
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public static <A extends Comparable<? super A>> void sort(List<A> list) {
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Collections.sort(list);
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}
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public static void main(String... args) {
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List<Animal> zoo = new ArrayList<>();
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zoo.add(new Wolf());
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zoo.add(new Lion());
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zoo.add(new Wolf());
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System.out.println(zoo);
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sort(zoo);
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System.out.println(zoo);
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List<Cat> cattery = new ArrayList<>();
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sort(cattery);
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}
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interface Animal extends Comparable<Animal> {
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default int compareTo(Animal o) {
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return this.getFoodChainRanking().compareTo(o.getFoodChainRanking());
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}
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Integer getFoodChainRanking();
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}
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static class Lion implements Animal {
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@Override
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public Integer getFoodChainRanking() {
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return 10;
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}
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}
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static class Wolf implements Animal {
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@Override
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public Integer getFoodChainRanking() {
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return 8;
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}
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}
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static class DomesticPet {}
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static class Cat extends DomesticPet implements Comparable<DomesticPet>{
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@Override
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public int compareTo(DomesticPet o) {
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return 0;
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}
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}
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}

‎src/main/java/s4j/java/chapter14/Stack.java‎

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package s4j.java.chapter14;
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public interface Stack<T> {
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void push(T t);
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T pop();
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}

‎src/main/java/s4j/java/chapter14/Variance.java‎

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package s4j.java.chapter14;
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@SuppressWarnings("all")
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public class Variance {
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static class A {}
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static class B extends A {}
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static class Container<T> {
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}
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public static void main(String... args) {
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basicSubTyping();
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containerSubTyping();
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}
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private static void basicSubTyping() {
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A a = new A();
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B b = new B();
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a = b;
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// b = a; // compiler failure
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}
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private static void containerSubTyping() {
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Container<A> a = new Container<>();
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Container<B> b = new Container<>();
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// a = b; // compiler failure
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}
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}

‎src/main/java/s4j/java/chapter14/Wildcards.java‎

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package s4j.java.chapter14;
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import java.util.ArrayList;
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import java.util.List;
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import static s4j.java.chapter14.Zoo.Animal;
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import static s4j.java.chapter14.Zoo.Lion;
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@SuppressWarnings("all")
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public class Wildcards {
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public static void main(String... args) {
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cantAddToList();
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// printObjects(new ArrayList<String>()); // compiler error
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printObjects(new ArrayList<Object>());
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// print anything (List<?>)
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printUnknown(new ArrayList<String>());
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printUnknown(new ArrayList<Lion>());
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// printAnimals(new ArrayList<String>());
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printAnimals(new ArrayList<Lion>());
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// addNumbers(new ArrayList<String>());
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addNumbers(new ArrayList<Integer>());
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addNumbers(new ArrayList<Number>());
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addNumbers(new ArrayList<Object>());
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}
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// can't add anything to this collection (except null), see http://docs.oracle.com/javase/tutorial/extra/generics/wildcards.html
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private static void cantAddToList() {
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List<?> list = new ArrayList<String>();
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// list.add(new Object()); // compiler error
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// list.add("Hello"); // compiler error
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list.add(null); // ok
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}
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static void printObjects(List<Object> list) {
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for (Object element : list) {
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System.out.println(element);
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}
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}
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static void printUnknown(List<?> list) {
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for (Object element : list) {
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System.out.println(element);
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}
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}
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static void printAnimals(List<? extends Animal> animals) {
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for (Animal animal : animals) {
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System.out.println(animal);
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}
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}
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static void addNumbers(List<? super Integer> numbers) {
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for (int i = 0; i < 100; i++) {
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numbers.add(i);
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}
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}
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}

‎src/main/java/s4j/java/chapter14/Zoo.java‎

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package s4j.java.chapter14;
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import java.util.ArrayList;
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import java.util.List;
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@SuppressWarnings("all")
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public class Zoo {
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public static <A extends Comparable<? super A>> void sort(List<A> list) {
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}
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// public static <A extends Comparable<?>, U> void scalaLikeSort(List<A> list) {
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public static <A extends Comparable<?>> void scalaLikeSort(List<A> list) {
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// eg. Collections.sort(list);
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}
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public static void main(String... args) {
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List<Lion> enclosure = new ArrayList<>();
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enclosure.add(new Lion());
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enclosure.add(new Lion());
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sort(enclosure);
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List<Animal> zoo = new ArrayList<>();
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zoo.add(new Zebra());
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zoo.add(new Lion());
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zoo.add(new Lion());
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sort(zoo);
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}
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static class Animal implements Comparable<Animal> {
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@Override
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public int compareTo(Animal o) {
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return 0;
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}
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}
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static class Lion extends Animal { }
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static class Zebra extends Animal { }
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}

‎src/main/scala/s4j/scala/chapter14/Animal.scala‎

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package s4j.scala.chapter14
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class Animal extends Comparable[Animal] {
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def compareTo(o: Animal): Int = 0
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}
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class Lion extends Animal
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class Zebra extends Animal

‎src/main/scala/s4j/scala/chapter14/Existential.scala‎

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package s4j.scala.chapter14
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class Existential {
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// this is described as an existential type
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val foo: List[_ <: AnyRef] = List()
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}

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