1. static 與 instance 的陷阱

class Counter {
    static int count = 0;
    public Counter() { count++; }
}

public class Test4 {
    public static void main(String[] args) {
        new Counter();
        new Counter();
        Counter c = new Counter();
        System.out.println(c.count);
    }
}

2. 多型 (Polymorphism)

class Animal {
    void sound() { System.out.println("Animal sound"); }
}
class Dog extends Animal {
    void sound() { System.out.println("Bark"); }
}

public class Test5 {
    public static void main(String[] args) {
        Animal a = new Dog();
        a.sound();
    }
}

3. Exception 順序考題

public class Test6 {
    public static void main(String[] args) {
        try {
            int x = 5 / 0;
        } catch (ArithmeticException e) {
            System.out.println("Arithmetic");
        } catch (Exception e) {
            System.out.println("General");
        }
    }
}

4. StringBuilder vs String

public class Test7 {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder("Hi");
        String s = "Hi";
        System.out.println(sb.equals(s));
        System.out.println(sb.toString().equals(s));
    }
}