Java函数式编程中高阶函数在设计模式中的应用？

// 策略接口
interface Strategy {
    int doSomething(int a, int b);
}

// 具体策略
class AddStrategy implements Strategy {
    @Override
    public int doSomething(int a, int b) {
        return a + b;
    }
}

class MultiplyStrategy implements Strategy {
    @Override
    public int doSomething(int a, int b) {
        return a * b;
    }
}

// 策略模式上下文
class Context {
    private Strategy strategy;

    public Context(Strategy strategy) {
        this.strategy = strategy;
    }

    public int doSomething(int a, int b) {
        return strategy.doSomething(a, b);
    }
}

// 主类
public class Main {
    public static void main(String[] args) {
        Context context = new Context(new AddStrategy());
        System.out.println(context.doSomething(1, 2)); // 输出 3

        context = new Context(new MultiplyStrategy());
        System.out.println(context.doSomething(1, 2)); // 输出 2
    }
}

// 抽象类
abstract class AbstractTemplate {
    public void templateMethod() {
        step1();
        step2();
        hook();
    }

    protected abstract void step1();
    protected abstract void step2();

    // 可选钩子方法
    protected void hook() {}
}

// 具体子类
class ConcreteTemplate extends AbstractTemplate {
    @Override
    protected void step1() {
        System.out.println("Step 1");
    }

    @Override
    protected void step2() {
        System.out.println("Step 2");
    }
}

// 主类
public class Main {
    public static void main(String[] args) {
        ConcreteTemplate template = new ConcreteTemplate();
        template.templateMethod();
        // 输出：
        // Step 1
        // Step 2
    }
}

// 观察者接口
interface Observer {
    void update(Object event);
}

// 被观察者接口
interface Observable {
    void addObserver(Observer observer);
    void removeObserver(Observer observer);
    void notifyObservers(Object event);
}

// 具体观察者
class ConcreteObserver implements Observer {
    @Override
    public void update(Object event) {
        System.out.println("Received event: " + event);
    }
}

// 具体被观察者
class ConcreteObservable implements Observable {

    private List<Observer> observers = new ArrayList<>();

    @Override
    public void addObserver(Observer observer) {
        observers.add(observer);
    }

    @Override
    public void removeObserver(Observer observer) {
        observers.remove(observer);
    }

    @Override
    public void notifyObservers(Object event) {
        for (Observer observer : observers) {
            observer.update(event);
        }
    }
}

// 主类
public class Main {
    public static void main(String[] args) {
        ConcreteObservable observable = new ConcreteObservable();
        ConcreteObserver observer1 = new ConcreteObserver();
        ConcreteObserver observer2 = new ConcreteObserver();

        observable.addObserver(observer1);
        observable.addObserver(observer2);

        observable.notifyObservers("Hello!");
        // 输出：
        // Received event: Hello!
        // Received event: Hello!
    }
}