Practice›Java Fundamentals›Module 9. Generics›9.3 Wildcards

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9.3 Wildcards — writing with ? super, and PECS

Lesson

The mirror image

The lower bounded wildcard List<? super Integer> means "a list of some unknown type that is Integer or a supertype of it" — so it accepts List<Integer>, List<Number>, or List<Object>. Now adding an Integer is safe: whatever the list really holds, an Integer fits. Such a parameter is called a consumer.

PECS: Producer Extends, Consumer Super

static void addNumbers(List<? super Integer> list) {
    list.add(1);   // safe: an Integer fits any supertype list
    list.add(2);
}

addNumbers(listOfNumber);   // [1, 2]
addNumbers(listOfObject);   // [1, 2]

The rule of thumb is PECS: if the parameter produces values you read, use ? extends; if it consumes values you write, use ? super. Reading from a ? super Integer list only guarantees Object — so read with extends, write with super.

▶ Try it: run this — the same method fills a List<Number> and a List<Object>.

import java.util.ArrayList;
import java.util.List;
public class Main {
    static void addNumbers(List<? super Integer> list) {
        list.add(1);
        list.add(2);
    }
    public static void main(String[] args) {
        List<Number> nums = new ArrayList<Number>();
        addNumbers(nums);
        System.out.println(nums);
        List<Object> objs = new ArrayList<Object>();
        addNumbers(objs);
        System.out.println(objs);
    }
}

List<? super Integer> accepts...

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PECS stands for...

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Type exactly what this program prints.

import java.util.ArrayList;
import java.util.List;
public class Main {
    static void add(List<? super Integer> l) {
        l.add(7);
    }
    public static void main(String[] a){
        List<Number> n = new ArrayList<Number>();
        add(n);
        System.out.println(n);
    }
}

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Match each need to the right wildcard.

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True or false?

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