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【练习7-1(TruckDemo.java)】扩展Vehicle类

为演示继承的强大功能，我们将扩展在第4章中最先开发的Vehicle类。它封装了汽车的信息，包括乘载人数、油箱容量和燃油利用率。我们可以把Vehicle类作为起点来创建更具体的类。例如，卡车是汽车的一种。卡车的一个重要属性就是它的运载能力。因此要创建Truck类，可扩展Vehicle类，添加存储运载能力的实例变量。下面的Truck类实现了这一点。在实现过程中，Vehicle类中的实例变量将被设为private，并提供访问器方法来获取或设置它们的值。

package javaone.a.beginners.guide.chapterseven;// Try This 7-1
//
// Build a subclass of Vehicle for trucks/
class Vehicle{private int passengers; // number of passengersprivate int fuelcap;    // fuel capacity in gallonsprivate int mpg;        // fuel consumption in miles per gallon// This is a constructor for VehicleVehicle(int p, int f, int m){passengers = p;fuelcap = f;mpg = m;}// Return the rangeint range(){return mpg * fuelcap;}// Compute fuel needed for a given distancedouble fuelneeded(int miles){return (double) miles / mpg;}// Accessor methods for instance variables.public int getPassengers() {return passengers;}public void setPassengers(int passengers) {this.passengers = passengers;}public int getFuelcap() {return fuelcap;}public void setFuelcap(int fuelcap) {this.fuelcap = fuelcap;}public int getMpg() {return mpg;}public void setMpg(int mpg) {this.mpg = mpg;}
}// Extend Vehicle to create a Truck specialization
class Truck extends Vehicle{private int cargocap;   // cargo capacity in pounds// This is a constructor for TruckTruck(int p, int f, int m, int c){/*Initialize Vehicle members using Vehicle's constructor.*/super(p,f,m);cargocap = c;}// Accessor methods for cargocap.int getCargo(){return cargocap;}void putCargo(int c){cargocap = c;}
}public class ChapterSevenProgramOne {public static void main(String[] args) {// construct some trucksTruck semi = new Truck(2, 200, 7, 44000);Truck pickup = new Truck(3, 28, 15, 2000);double gallons;int dist = 252;gallons = semi.fuelneeded(dist);System.out.println("Semi can carry " + semi.getCargo() + " pounds.");System.out.println("To go " + dist + " miles semi needs " + gallons + " gallons of fuel.\n");gallons = pickup.fuelneeded(dist);System.out.println("Pickup can carry " + pickup.getCargo() + " pounds.");System.out.println("To go " + dist + " miles pick up needs " + gallons + " gallons of fuel.\n");}}

7.15 自测题

1. 超类有权访问子类的成员吗?子类有权访问超类的成员吗?

答案：没有，超类不知道子类的存在。子类可以访问超类的任何非私有成员。

2. 创建一个名为Circle的TwoDShape的子类，它包括一个计算圆面积的area方法和一个使用super初始化TwoDShape部分的构造函数。

package javaone.a.beginners.guide.chapterseven;class TwoDShapeTen{private double width;private double height;private String name;// A default constructor.TwoDShapeTen(){width = height = 0.0;name = "none";}// Parameterized constructor.TwoDShapeTen(double w, double h, String n){width = w;height = h;name = n;}// Construct object with equal width and height.TwoDShapeTen(double x, String n){width = height = x;name = n;}// Construct an object from an object.TwoDShapeTen(TwoDShapeTen ob){width = ob.width;height = ob.height;name = ob.name;}// Accessor methods for width and height.public double getWidth() {return width;}public void setWidth(double width) {this.width = width;}public double getHeight() {return height;}public void setHeight(double height) {this.height = height;}String getName() {return name;}void showDim(){System.out.println("Width and height are " + width + " and " + height);}double area(){System.out.println("area() must be overridden");return 0.0;}
}// A subclass of TwoDShape for circles.
class Circle extends TwoDShapeTen{// A default constructor.Circle(){super();}// Construct CircleCircle(double x){super(x, "circle"); // call superclass constructor}// Construct an object from an object.Circle(Circle ob){super(ob);  // pass object to TwoDShape constructor}double area(){return (getWidth() / 2) * (getWidth() / 2) * 3.14;}
}public class ChapterSevenExerciseTwo {
}

3. 如何防止子类访问超类的成员?
答案：可以把超类的成员声明为私有的。

4. 描述本章中两个super版本的目的及用法。

答案：super关键字具有两种形式。第一种用来调用超类的构造函数，其基本形式如下：

super(param-list);

第二种用来访问超类的成员，其基本形式如下：

super.member

5. 已知层次结构如下：

class Alpha { ...

class Beta extends Alpha { ...

class Gamma extends Beta { ...

当初始化一个Gamma对象时，这些类的构造函数的调用顺序是什么?

答案：构造函数总是按照派生的顺序被调用，因此，当创建Gamma对象时，调用顺序是Alpha、Beta、Gamma。