스파르타 spring 2기 TIL day13

Java wildcard에 대해

// ============================================
// STUDY: note about wildcard
// ============================================

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

class WildCard {
    public static void main(String[] args) {
        // Variable is a box that you can put in a data.
        // 
        // And a type is a contraint on what type can be put in.
        //
        // And List<? extends Number> means
        //
        // This variable can contain a list that holds any class that extends Number.
        //
        List<? extends Number> list;

        // So, all of these below compiles
        list = new ArrayList<Integer>();
        list = new ArrayList<Short>();
        list = new ArrayList<Long>();
        list = new ArrayList<Double>();
        list = new ArrayList<Float>();

        // But below code doesn't compile.
        // list.add(Integer.valueOf(10));

        // And why is that? Let's say we decided to put a list that holds floats
        List<Float> floats = new ArrayList<Float>();
        list = floats;

        // But if we add int now, that would mess up that float list.
        // list.add(Integer.valueOf(10));
        //
        // So java compiler prevents us from doing that.

        // How about this example?
        class Dog {
            String name;

            Dog(String name) {
                this.name = name;
            }

            void bark() {
                System.out.println("bark");
            }
        }

        List<? extends Dog> dogList = new ArrayList<Dog>();

        // Below code doesn't compile. Why?
        // dogList.add(new Dog("max"));

        // Let's declare a new CoolDog class
        class CoolDog extends Dog {
            CoolDog(String name) {
                super(name);
            }

            void doCoolTrick() {
                System.out.println("Did a cool trick");
            }
        }

        // and let's declare two lists
        List<CoolDog> coolDogs = new ArrayList<CoolDog>();
        List<Dog> lameDogs = new ArrayList<Dog>();

        // like we said before, dogList can hold both lameDogs and coolDogs
        dogList = lameDogs;
        dogList = coolDogs;

        // but now if we try to put plain dog into a dogList, that would cause problems
        // dogList.add(new Dog("Taffy"));

        // see, Taffy is not a CoolDog, so it can't doCoolTrick. Even though underlying coolDogs elements
        // are required to doCoolTrick.
        //
        // So java compiler prevents us from doing that.
    }
}