• 자바는 람다식을 위한 함수형 인터페이스가 정의되어 있어야 함
  • 필요할 때마다 정의해야 하므로 번거로움
  • 자주 사용하는 인터페이스가 java.util.function 패키지에 제공
  • 곧 배울 스트림에서 유용하게 사용

☕ Main.java

				Runnable dogButtonFunc = () -> System.out.println("멍멍");
        Runnable catButtonFunc = () -> System.out.println("야옹");
        Runnable cowButtonFunc = () -> System.out.println("음메");

        dogButtonFunc.run();
        catButtonFunc.run();
        cowButtonFunc.run();

☕ Button.java

public class Button {
    private String text;
    public Button(String text) { this.text = text; }
    public String getText() { return text; }

    private Runnable onClickListener;
    public void setOnClickListener(Runnable onClickListener) {
        this.onClickListener = onClickListener;
    }
    public void onClick () {
        onClickListener.run();
    }
}

☕ Main.java

				System.out.println("\\n- - - - -\\n");

				Button dogButton = new Button("강아지");
        dogButton.setOnClickListener(dogButtonFunc);

        Button catButton = new Button("고양이");
        catButton.setOnClickListener(() -> {
            System.out.println("- - - - -");
            System.out.println(catButton.getText() + " 울음소리: 야옹야옹");
            System.out.println("- - - - -");
        });

        dogButton.onClick();
        catButton.onClick();

				Supplier<String> appName = () -> "얄코오피스";
        Supplier<Double> rand0to10 = () -> Math.random() * 10;
        Supplier<Boolean> randTF = () -> Math.random() > 0.5;

				String supp1 = appName.get();
        Double supp2 = rand0to10.get();
        Boolean supp3 = randTF.get();

				System.out.println("\\n- - - - -\\n");

				Consumer<Integer> sayOddEven = i -> System.out.printf(
                "%d은 %c수입니다.%n", i, "짝홀".charAt(i % 2)
        );
        
        Consumer<Button> clickButton = b -> b.onClick();
        
        BiConsumer<Button, Integer> clickButtonNTimes = (b, n) -> {
            for (int i = 0; i < n; i++) { b.onClick(); }
        };

        sayOddEven.accept(3);
        sayOddEven.accept(4);
        clickButton.accept(catButton);
        clickButtonNTimes.accept(dogButton, 5);

				System.out.println("\\n- - - - -\\n");

				Function<Integer, Boolean> isOdd = i -> i % 2 == 1;
        Function<String, Button> getButton = s -> new Button(s);

        BiFunction<Unit, Horse, Integer> getfullHP = (u, h) -> {
            h.setRider(u);
            return u.hp;
        };

        BiFunction<String, Runnable, Button> getButtonWFunc = (s, r) -> {
            Button b = new Button(s);
            b.setOnClickListener(r);
            return b;
        };

        Boolean isOdd3 = isOdd.apply(3);
        Boolean isOdd4 = isOdd.apply(4);

        Button goatButton = getButton.apply("염소");

        Integer unitFullHP = getfullHP.apply(
                new MagicKnight(Side.RED), new Horse(80)
        );

        getButtonWFunc
                .apply("오리", () -> System.out.println("꽥꽥"))
                .onClick();

				Predicate<Integer> isOddTester = i -> i % 2 == 1;
        Predicate<String> isAllLowerCase = s -> s.equals(s.toLowerCase());

        BiPredicate<Character, Integer> areSameCharNum = (c, i) -> (int) c == i;
        BiPredicate<Unit, Unit> areSameSide = (u1, u2)  -> u1.getSide() == u2.getSide();

        boolean isOddT3 = isOddTester.test(3);
        boolean isOddT4 = isOddTester.test(4);
        boolean isAL1 = isAllLowerCase.test("Hello");
        boolean isAL2 = isAllLowerCase.test("world");

        boolean areSameCN1 = areSameCharNum.test('A', 64);
        boolean areSameCN2 = areSameCharNum.test('A', 65);

        boolean areSameSide1 = areSameSide.test(
                new Knight(Side.RED), new Knight(Side.BLUE)
        );
        boolean areSameSide2 = areSameSide.test(
                new Swordman(Side.BLUE), new MagicKnight(Side.BLUE)
        );

				UnaryOperator<Integer> square = i -> i * i;
        UnaryOperator<Swordman> respawn = s -> {
            if (s instanceof MagicKnight) return new MagicKnight(s.getSide());
            if (s instanceof Knight) return  new Knight(s.getSide());
            return new Swordman(s.getSide());
        };

        Integer squared = square.apply(3);
        Swordman respawned1 = respawn.apply(new Knight(Side.BLUE));
        Swordman respawned2 = respawn.apply(new MagicKnight(Side.RED));