Hi! Ayo kita nerusake sinau babagan generik. Sampeyan wis entuk akeh kawruh babagan pelajaran sadurunge (babagan nggunakake varargs nalika nggarap generik lan babagan erasure jinis ), nanging ana topik penting sing durung dianggep - wildcards . Iki minangka fitur generik sing penting banget. Dadi luwih supaya kita wis darmabakti pawulangan kapisah kanggo iku! Sing jarene, ora ana sing rumit babagan wildcard. Sampeyan bakal weruh langsung :) ![Wildcards ing generik - 1]()
Ayo ndeleng conto:
Ayo ndeleng conto:
public class Main {
public static void main(String[] args) {
String str = new String("Test!");
// No problem
Object obj = str;
List<String> strings = new ArrayList<String>();
// Compilation error!
List<Object> objects = strings;
}
}
Stringobyek menyang Objectobyek. Ora ana masalah ing kene - kabeh bisa digunakake kaya sing dikarepake. Nanging ing kahanan kapindho, compiler ngasilake kesalahan. Nanging kita uga nindakake perkara sing padha, ta? Wektu iki kita mung nggunakake koleksi sawetara obyek. Nanging kenapa kesalahan kasebut kedadeyan? Apa bedane? Apa kita ngirim siji Stringobyek menyang Objectutawa 20 obyek? Ana bedane penting antarane obyek lan koleksi obyek . Yen Bkelas iku anak saka Akelas, banjur Collection<B>dudu anak saka Collection<A>. Iki kok kita padha ora bisa kanggo cast kita List<String>menyang aList<Object>. Stringiku anak saka Object, nanging List<String>dudu anak saka List<Object>. Iki bisa uga ora katon super intuisi. Kenging punapa pangripta basa menika damel kados makaten? Ayo bayangake manawa kompiler ora menehi kesalahan:
List<String> strings = new ArrayList<String>();
List<Object> objects = strings;
objects.add(new Object());
String s = strings.get(0);
List<Object>referensi sing nuduhake strings, kita bisa nambah obyek lawas Objecting stringskoleksi kasebut! Mangkono, kita wis kelangan jaminan yen koleksi kita mung ngemot Stringobyek sing ditemtokake dening argumen tipe ing invocation tipe umum . Ing tembung liyane, kita wis ilang kauntungan utama generik - jinis safety. Lan amarga compiler ora mungkasi kita nindakake iki, kita bakal entuk kesalahan mung ing wektu mbukak, sing tansah luwih elek tinimbang kesalahan kompilasi. Kanggo nyegah kahanan kaya iki, kompiler menehi kesalahan:
// Compilation error
List<Object> objects = strings;
List<String>dudu keturunan List<Object>. Iki minangka aturan ironclad kanggo generik, lan kudu eling nalika nggarap. Ayo nerusake. Upaminipun kita duwe hierarki kelas cilik:
public class Animal {
public void feed() {
System.out.println("Animal.feed()");
}
}
public class Pet extends Animal {
public void call() {
System.out.println("Pet.call()");
}
}
public class Cat extends Pet {
public void meow() {
System.out.println("Cat.meow()");
}
}
iterateAnimals()cara sing gampang. Cara kasebut kudu njupuk koleksi kewan apa wae ( Animal, Pet, Cat, Dog), ngulang kabeh unsur, lan nampilake pesen ing konsol sajrone saben pengulangan. Ayo nyoba nulis cara kaya mangkene:
public static void iterateAnimals(Collection<Animal> animals) {
for(Animal animal: animals) {
System.out.println("Another iteration in the loop!");
}
}
List<Cat>, List<Dog>lan List<Pet>dudu keturunan List<Animal>! Iki tegese yen kita nyoba nelpon iterateAnimals()metode kasebut kanthi dhaptar kucing, kita entuk kesalahan kompilasi:
import java.util.*;
public class Main3 {
public static void iterateAnimals(Collection<Animal> animals) {
for(Animal animal: animals) {
System.out.println("Another iteration in the loop!");
}
}
public static void main(String[] args) {
List<Cat> cats = new ArrayList<>();
cats.add(new Cat());
cats.add(new Cat());
cats.add(new Cat());
cats.add(new Cat());
// Compilation error!
iterateAnimals(cats);
}
}
public static void iterateAnimals(Collection<? extends Animal> animals) {
for(Animal animal: animals) {
System.out.println("Another iteration in the loop!");
}
}
Animalobyek utawa koleksi obyek saka sembarang kelas sing mudhun saka Animal(? ngluwihi Animal). Ing tembung liyane, cara bisa nampa koleksi Animal, Pet, Dog, utawa Catobyek — iku ndadekake ora prabédan. Ayo gawe uwong yakin yen bisa:
public static void main(String[] args) {
List<Animal> animals = new ArrayList<>();
animals.add(new Animal());
animals.add(new Animal());
List<Pet> pets = new ArrayList<>();
pets.add(new Pet());
pets.add(new Pet());
List<Cat> cats = new ArrayList<>();
cats.add(new Cat());
cats.add(new Cat());
List<Dog> dogs = new ArrayList<>();
dogs.add(new Dog());
dogs.add(new Dog());
iterateAnimals(animals);
iterateAnimals(pets);
iterateAnimals(cats);
iterateAnimals(dogs);
}
Another iteration in the loop!
Another iteration in the loop!
Another iteration in the loop!
Another iteration in the loop!
Another iteration in the loop!
Another iteration in the loop!
Another iteration in the loop!
Another iteration in the loop!
Animal, kanthi Petkelas ing ngisor, lan kelas Catlan Doging tingkat sabanjure. Saiki sampeyan kudu nulis ulang iterateAnimals()cara supaya bisa digunakake karo sembarang jinis kewan, kajaba asu . Yaiku, kudu nampa Collection<Animal>,Collection<Pet>utawa Collection<Car>, nanging ngirim ora bisa karo Collection<Dog>. Kepiye carane bisa nggayuh iki? Iku misale jek sing maneh ngadhepi prospek nulis cara kapisah kanggo saben jinis: / Carane liya kita nerangake kanggo compiler apa kita arep kelakon? Iku bener cukup prasaja! Sawise maneh, wildcards teka kanggo mbantu kita ing kene. Nanging wektu iki kita bakal nggunakake jinis wildcard liyane - wildcard wates ngisor , kang ditulis nggunakake super .
public static void iterateAnimals(Collection<? super Cat> animals) {
for(int i = 0; i < animals.size(); i++) {
System.out.println("Another iteration in the loop!");
}
}
<? super Cat>ngandhani kompiler yen iterateAnimals()metode kasebut bisa ditampa minangka input koleksi Catobyek utawa leluhur kelas Catminangka input. Ing kasus iki, Catkelas, wong tuwane Pet, lan wong tuwane, Animalkabeh cocog karo katrangan iki. Kelas kasebut Dogora cocog karo watesan kita, mula nyoba nggunakake metode kanthi List<Dog>argumen bakal nyebabake kesalahan kompilasi:
public static void main(String[] args) {
List<Animal> animals = new ArrayList<>();
animals.add(new Animal());
animals.add(new Animal());
List<Pet> pets = new ArrayList<>();
pets.add(new Pet());
pets.add(new Pet());
List<Cat> cats = new ArrayList<>();
cats.add(new Cat());
cats.add(new Cat());
List<Dog> dogs = new ArrayList<>();
dogs.add(new Dog());
dogs.add(new Dog());
iterateAnimals(animals);
iterateAnimals(pets);
iterateAnimals(cats);
// Compilation error!
iterateAnimals(dogs);
}
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