方法重写与递归简介

Java 中的输入/输出与流

Alex Liu

Software Development Engineer

定义基类

Animal 类表示通用动物

class Animal {
  void makeSound() {
      System.out.println("Animal sound");
  }
  public static void main(String[] args) {
      Animal a = new Animal();
      a.makeSound(); // Output: Animal sound
  }
}

Animal sound

理解继承 — extends

// Use `extends` to create a subclass of `Animal` named `Dog`
class Dog extends Animal {
    void bark() {System.out.println("Bark");}
}
public static void main(String[] args) {
    Dog d = new Dog(); // Create new instance of Dog object which extends Animal
    d.makeSound();
    d.bark();
}

逐行打印两种声音：

Animal sound
Bark

方法重写（Java）

class Cat extends Animal {
    @Override // Use `@Override` to override the behavior of `.makeSound()`
    void makeSound() {
        System.out.println("Meow");
    }
}
public static void main(String[] args) {
    Cat c = new Cat(); // Create a new instance of Cat object which extends Animal
    c.makeSound(); // Call the overrided method `makeSound()`
}

Meow

理解递归

方法通过自我调用来解决问题
必须有基例以防无限递归

public class RecursionExample {
    static void countdown(int n) {
        // Base case
        if (n == 0) return; 
        System.out.println(n);
        // Recursive call
        countdown(n - 1); 
    }
}

递归示例用法

使用示例

public static void main(String[] args) {
  countdown(5);
}

递归在达到基例0时停止
```
5
4
3
2
1
```

总结

extends 启用继承
@Override 修改继承的方法行为
Recursion 将问题拆解为更小步骤
- 基例防止无限递归

Let's practice!

Java 中的输入/输出与流