Add Type Parameter vs Wildcard in Java Generics (#12988)
* Add Type Parameter vs Wildcard in Java Generics * Fix Logger variable and code format * Update Cat class * Add missing import * Add warning comment
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apeterlic
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package com.baeldung.generics;
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abstract class Animal {
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protected final String type;
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protected final String name;
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protected Animal(String type, String name) {
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this.type = type;
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this.name = name;
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}
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abstract String makeSound();
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public String getType() {
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return type;
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}
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public String getName() {
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return name;
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}
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}
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package com.baeldung.generics;
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import java.util.ArrayList;
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import java.util.List;
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import org.slf4j.Logger;
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import org.slf4j.LoggerFactory;
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class AnimalDemo {
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private static final Logger logger = LoggerFactory.getLogger(AnimalDemo.class);
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public static void main(String[] args) {
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List<Cat> cats = new ArrayList<>();
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cats.add(new Cat("Persian", "Bono"));
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cats.add(new Cat("Egyptian", "Lulu"));
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cats.add(new Cat("Siamese", "Ra"));
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order(cats);
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logger.info("Ordered cats: {}", cats);
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}
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public static <T extends Animal & Comparable<T>> void order(List<T> list) {
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list.sort(Comparable::compareTo);
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}
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}
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package com.baeldung.generics;
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import java.util.Objects;
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class Cat extends Animal implements Comparable<Cat> {
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public Cat(String type, String name) {
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super(type, name);
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}
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@Override
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public String makeSound() {
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return "Meow";
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}
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/**
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* Warning: Inconsistent with the equals method.
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*/
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@Override
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public int compareTo(Cat cat) {
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return this.getName().length() - cat.getName().length();
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}
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@Override
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public String toString() {
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return "Cat{" + "type='" + type + '\'' + ", name='" + name + '\'' + '}';
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}
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@Override
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public int hashCode() {
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return Objects.hash(type, name);
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}
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@Override
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public boolean equals(Object o) {
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if(o == this) return true;
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if(!(o instanceof Cat)) return false;
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Cat cat = (Cat) o;
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return type.equals(cat.type) && name.equals(cat.name);
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}
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}
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package com.baeldung.generics;
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import java.util.List;
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import java.util.stream.Collectors;
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import java.util.stream.Stream;
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public class CollectionUtils {
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private CollectionUtils() {
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}
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public static <T> void print(T item) {
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System.out.println(item);
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}
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public static void swap(List<?> list, int src, int des) {
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swapHelper(list, src, des);
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}
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private static <E> void swapHelper(List<E> list, int src, int des) {
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list.set(src, list.set(des, list.get(src)));
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}
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public static <E> List<E> mergeTypeParameter(List<? extends E> listOne, List<? extends E> listTwo) {
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return Stream.concat(listOne.stream(), listTwo.stream())
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.collect(Collectors.toList());
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}
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public static <E> List<? extends E> mergeWildcard(List<? extends E> listOne, List<? extends E> listTwo) {
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return Stream.concat(listOne.stream(), listTwo.stream())
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.collect(Collectors.toList());
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}
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public static long sum(List<Number> numbers) {
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return numbers.stream()
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.mapToLong(Number::longValue)
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.sum();
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}
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public static <T extends Number> long sumTypeParameter(List<T> numbers) {
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return numbers.stream()
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.mapToLong(Number::longValue)
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.sum();
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}
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public static long sumWildcard(List<? extends Number> numbers) {
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return numbers.stream()
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.mapToLong(Number::longValue)
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.sum();
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}
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public static void addNumber(List<? super Integer> list, Integer number) {
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list.add(number);
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}
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}
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package com.baeldung.generics;
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import java.util.ArrayList;
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import java.util.List;
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import org.slf4j.Logger;
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import org.slf4j.LoggerFactory;
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class CollectionUtilsDemo {
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private static final Logger logger = LoggerFactory.getLogger(CollectionUtilsDemo.class);
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public static void main(String[] args) {
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CollectionUtils.print("Baeldung");
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List<Number> numbers1 = new ArrayList<>();
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numbers1.add(5);
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numbers1.add(10L);
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List<Number> numbers2 = new ArrayList<>();
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numbers2.add(15f);
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numbers2.add(20.0);
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List<Number> numbersMerged = CollectionUtils.mergeTypeParameter(numbers1, numbers2);
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logger.info("Merged numbers: {}", numbersMerged);
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List<Number> numbers = new ArrayList<>();
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numbers.add(5);
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numbers.add(10L);
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numbers.add(15f);
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numbers.add(20.0);
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logger.info("Sum: {}", CollectionUtils.sum(numbers));
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logger.info("Sum (wildcard): {}", CollectionUtils.sumWildcard(numbers));
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logger.info("Sum (type parameter): {}", CollectionUtils.sumTypeParameter(numbers));
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List<Integer> integers = new ArrayList<>();
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integers.add(5);
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logger.info("Sum integers (wildcard): {}", CollectionUtils.sumWildcard(integers));
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CollectionUtils.addNumber(numbers, 4);
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CollectionUtils.addNumber(integers, 5);
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logger.info("Before swap: {}", numbers);
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CollectionUtils.swap(numbers, 0, 1);
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logger.info("After swap: {}", numbers);
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}
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}
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@ -0,0 +1,14 @@
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package com.baeldung.generics;
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public class Dog extends Animal {
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public Dog(String type, String name) {
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super(type, name);
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}
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@Override
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public String makeSound() {
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return "Wuf";
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}
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}
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