مبدأ PECS

ما الذي تعنيه PECS

public class Main {
    public static void main(String[] args) {
        System.out.println("Producer Extends, Consumer Super");
    }
}

طريقة copy التقليدية

import java.util.ArrayList;
import java.util.List;

public class Main {
    static <T> void copy(List<? extends T> src, List<? super T> dest) {
        for (T t : src) dest.add(t);
    }
    public static void main(String[] args) {
        List<Integer> src = List.of(1, 2, 3);
        List<Object> dest = new ArrayList<>();
        copy(src, dest);
        System.out.println(dest);
    }
}

لماذا يضيف محرفا البدل مرونة

import java.util.ArrayList;
import java.util.List;

public class Main {
    static <T> void copy(List<? extends T> src, List<? super T> dest) {
        for (T t : src) dest.add(t);
    }
    public static void main(String[] args) {
        List<Integer> ints = List.of(10, 20);
        List<Number> nums = new ArrayList<>();
        copy(ints, nums);
        System.out.println(nums);
    }
}

مثال على منتِج

import java.util.List;

public class Main {
    static double sum(List<? extends Number> producer) {
        double total = 0;
        for (Number n : producer) total += n.doubleValue();
        return total;
    }
    public static void main(String[] args) {
        System.out.println(sum(List.of(1, 2, 3)));
        System.out.println(sum(List.of(1.1, 2.2)));
    }
}

مثال على مستهلِك

import java.util.ArrayList;
import java.util.List;

public class Main {
    static void pushDigits(List<? super Integer> consumer) {
        for (int i = 0; i < 5; i++) consumer.add(i);
    }
    public static void main(String[] args) {
        List<Number> nums = new ArrayList<>();
        pushDigits(nums);
        System.out.println(nums);
    }
}

دوران معًا: استخدم نوعًا عاديًا

import java.util.ArrayList;
import java.util.List;

public class Main {
    static <T> void rotate(List<T> list) {
        if (list.isEmpty()) return;
        T last = list.remove(list.size() - 1);
        list.add(0, last);
    }
    public static void main(String[] args) {
        List<Integer> list = new ArrayList<>(List.of(1, 2, 3));
        rotate(list);
        System.out.println(list);
    }
}

تطبيق PECS في Comparator

import java.util.Comparator;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        Comparator<Object> byHash = Comparator.comparingInt(Object::hashCode);
        List<String> words = List.of("a", "bb", "ccc");
        String max = java.util.Collections.max(words, byHash);
        System.out.println(max);
    }
}

تطبيق PECS في Stream.collect

import java.util.List;
import java.util.function.Consumer;

public class Main {
    public static void main(String[] args) {
        Consumer<Object> printer = o -> System.out.println("got: " + o);
        List<String> items = List.of("x", "y");
        items.forEach(printer); // forEach takes Consumer<? super T>
    }
}

المنتِجات والمستهلِكات في Function

import java.util.function.Function;

public class Main {
    static <T, R> R apply(Function<? super T, ? extends R> fn, T value) {
        return fn.apply(value);
    }
    public static void main(String[] args) {
        Function<Object, Integer> len = o -> o.toString().length();
        System.out.println(apply(len, "hello"));
    }
}

أنواع الإرجاع: تجنّب محارف البدل

import java.util.ArrayList;
import java.util.List;

public class Main {
    // Good: concrete return type
    static List<Number> doubled(List<? extends Number> src) {
        List<Number> out = new ArrayList<>();
        for (Number n : src) out.add(n.doubleValue() * 2);
        return out;
    }
    public static void main(String[] args) {
        System.out.println(doubled(List.of(1, 2, 3)));
    }
}

دمج كامل باستخدام PECS

import java.util.ArrayList;
import java.util.List;

public class Main {
    static <T> void merge(List<? extends T> a, List<? extends T> b, List<? super T> out) {
        for (T t : a) out.add(t);
        for (T t : b) out.add(t);
    }
    public static void main(String[] args) {
        List<Integer> a = List.of(1, 2);
        List<Integer> b = List.of(3, 4);
        List<Number> out = new ArrayList<>();
        merge(a, b, out);
        System.out.println(out);
    }
}

تحقق سريع

مراجعة

public class Main {
    public static void main(String[] args) {
        System.out.println("PECS recap: Producer Extends, Consumer Super");
    }
}