高级应用：内部类的设计模式与内部类与外部类之间的交互

public class Outer {
    private int outerVar;
    public Outer(int outerVar) {
        this.outerVar = outerVar;
    }
    public void outerMethod() {
        System.out.println("Outer method");
    }
    public class Inner {
        private int innerVar;
        public Inner(int innerVar) {
            this.innerVar = innerVar;
        }
        public void innerMethod() {
            System.out.println("Inner method");
            System.out.println("Outer variable: " + outerVar); // 访问外部类的私有成员
            outerMethod(); // 调用外部类的方法
        }
    }
}
public class Main {
    public static void main(String[] args) {
        Outer outer = new Outer(10);
        Outer.Inner inner = outer.new Inner(5);
        inner.innerMethod();
    }
}

// 迭代器模式
public class MyCollection {
    private List<String> list;
    public MyCollection() {
        list = new ArrayList<>();
    }
    public void add(String element) {
        list.add(element);
    }
    public Iterator<String> getIterator() {
        return new MyIterator();
    }
    private class MyIterator implements Iterator<String> {
        private int index;
        @Override
        public boolean hasNext() {
            return index < list.size();
        }
        @Override
        public String next() {
            return list.get(index++);
        }
    }
}
// 工厂模式
public class AnimalFactory {
    public Animal createAnimal(String type) {
        if ("dog".equalsIgnoreCase(type)) {
            return new Dog();
        } else if ("cat".equalsIgnoreCase(type)) {
            return new Cat();
        } else {
            throw new IllegalArgumentException("Invalid animal type: " + type);
        }
    }
    private class Dog implements Animal {
        @Override
        public void makeSound() {
            System.out.println("Dog barks");
        }
    }
    private class Cat implements Animal {
        @Override
        public void makeSound() {
            System.out.println("Cat meows");
        }
    }
}
// 观察者模式
public interface Observer {
    void update(String message);
}
public class Subject {
    private List<Observer> observers;
    public Subject() {
        observers = new ArrayList<>();
    }
    public void addObserver(Observer observer) {
        observers.add(observer);
    }
    public void removeObserver(Observer observer) {
        observers.remove(observer);
    }
    public void notifyObservers(String message) {
        for (Observer observer : observers) {
            observer.update(message);
        }
    }
}
// 单例模式
public class Singleton {
    private Singleton() {
    }
    public static Singleton getInstance() {
        return Holder.INSTANCE;
    }
    private static class Holder {
        private static final Singleton INSTANCE = new Singleton();
    }
}