Inheritance II: Extends, Casting, Higher Order Functions

10.1 Implementation Inheritance: Extends

extend 语法：

public class RotatingSLList<Blorp> extends SLList<Blorp>{
       public void rotateRight() {
              Blorp oldBack = removeLast();
              addFirst(oldBack);
	}
}

Constructor Behavior

extend 的子类需要在构造函数中调用父类的构造函数，通过super(). 如果省略了 java 会帮忙自动调用。

public VengefulSLList() {
   deletedItems = new SLList<Item>();
}

public VengefulSLList() {
   super();
   deletedItems = new SLList<Item>();
}

如果子类构造函数需要调用父类带参数的构造函数，那么这时候必须主动调用super(...), 否则 java 只会自动调用不带参数的super().

public VengefulSLList(Item x) {
   super(x);
   deletedItems = new SLList<Item>();
}

public VengefulSLList(Item x) {
   deletedItems = new SLList<Item>();
}

10.2 Encapsulation

Module: A set of methods that work together as a whole to perform some task or set of related tasks. A module is said to be encapsulated if its implementation is completely hidden, and it can be accessed only through a documented interface.

10.3 Casting

Java 中的每个变量都有一个静态类型。这是在声明变量时指定的类型，并在编译时进行检查。每个变量也有一个动态类型；此类型在实例化变量时指定，并在运行时检查。

Compile-Time Type Checking and Expressions

SLList<Integer> sl = new VengefulSLList<Integer>();
VengefulSLList<Integer> vsl = new SLList<Integer>();

第一行 compile time type 右边是 VengefulSLList 左边是 SLList, VengefulSLList 是 SLList, 所以 compile check pass.

第二行 compile time type 右边是 SLList 左边是 VengefulSLList, SLList 不 VengefulSLListst, 所以 compile check fail.


public static Dog maxDog(Dog d1, Dog d2) { … }

Poodle frank  = new Poodle("Frank", 5);
Poodle frankJr = new Poodle("Frank Jr.", 15);

Dog largerDog = maxDog(frank, frankJr);
Poodle largerPoodle = maxDog(frank, frankJr);

maxDog() 返回的 compile time type 是 Dog.

在最后一行会报错，因为 Dog 不是 Poodle.

casting

java 支持强制类型转换，来改变 compile time type。

10.4 Higher order Function in java

java7 包括之前的版本，变量不能是函数指针。

需要用 interface 来实现 HoF:

public interface IntUnaryFunction {
	int apply(int x);
}

public class TenX implements IntUnaryFunction {
	public int apply(int x) {
   		return 10 * x;
	}
}

public class HoFDemo {
	public static int do_twice(IntUnaryFunction f, int x) {
   		return f.apply(f.apply(x));
	}
	
	public static void main(String[] args) {
   		System.out.println(do_twice(new TenX(), 2));
	}
}