Java™ 教程（多态性）

多态性多态性的字典定义是指生物学中的原理，其中生物体或物种可以具有许多不同的形式或阶段，这个原则也可以应用于面向对象的编程和像Java语言之类的语言，类的子类可以定义它们自己的唯一行为，但仍然共享父类的一些相同功能。可以通过对Bicycle类的微小修改来演示多态性，例如，可以将printDescription方法添加到显示当前存储在实例中的所有数据的类中。public void printDescription(){ System.out.println("\nBike is " + “in gear " + this.gear + " with a cadence of " + this.cadence + " and travelling at a speed of " + this.speed + “. “);}要演示Java语言中的多态特性，请使用MountainBike和RoadBike类扩展Bicycle类，对于MountainBike，添加一个suspension字段，这是一个字符串值，表示自行车是否有前减震器，或者，自行车有一个前后减震器。这是更新的类：public class MountainBike extends Bicycle { private String suspension; public MountainBike( int startCadence, int startSpeed, int startGear, String suspensionType){ super(startCadence, startSpeed, startGear); this.setSuspension(suspensionType); } public String getSuspension(){ return this.suspension; } public void setSuspension(String suspensionType) { this.suspension = suspensionType; } public void printDescription() { super.printDescription(); System.out.println(“The " + “MountainBike has a” + getSuspension() + " suspension.”); }}请注意重写的printDescription方法，除了之前提供的信息之外，还有关于suspension的其他数据包含在输出中。接下来，创建RoadBike类，因为公路车或赛车的轮胎很薄，所以添加一个属性来跟踪轮胎的宽度，这是RoadBike类：public class RoadBike extends Bicycle{ // In millimeters (mm) private int tireWidth; public RoadBike(int startCadence, int startSpeed, int startGear, int newTireWidth){ super(startCadence, startSpeed, startGear); this.setTireWidth(newTireWidth); } public int getTireWidth(){ return this.tireWidth; } public void setTireWidth(int newTireWidth){ this.tireWidth = newTireWidth; } public void printDescription(){ super.printDescription(); System.out.println(“The RoadBike” + " has " + getTireWidth() + " MM tires.”); }}请注意，再次重写了printDescription方法，这次，显示有关轮胎宽度的信息。总而言之，有三个类：Bicycle、MountainBike和RoadBike，这两个子类重写printDescription方法并打印唯一信息。这是一个创建三个Bicycle变量的测试程序，每个变量都分配给三个自行车类别中的一个，然后打印每个变量。public class TestBikes { public static void main(String[] args){ Bicycle bike01, bike02, bike03; bike01 = new Bicycle(20, 10, 1); bike02 = new MountainBike(20, 10, 5, “Dual”); bike03 = new RoadBike(40, 20, 8, 23); bike01.printDescription(); bike02.printDescription(); bike03.printDescription(); }}以下是测试程序的输出：Bike is in gear 1 with a cadence of 20 and travelling at a speed of 10. Bike is in gear 5 with a cadence of 20 and travelling at a speed of 10. The MountainBike has a Dual suspension.Bike is in gear 8 with a cadence of 40 and travelling at a speed of 20. The RoadBike has 23 MM tires.Java虚拟机（JVM）为每个变量中引用的对象调用适当的方法，它不会调用由变量类型定义的方法，此行为称为虚方法调用，并演示Java语言中重要多态特征的一个方面。上一篇：重写和隐藏方法下一篇：使用super关键字