Polymorphism
“多态性”的字典定义是指 bio 学中的一种原理,其中 bio 或物种可以具有许多不同的形式或阶段。该原理也可以应用于面向对象的编程和 Java 语言之类的语言。一个类的子类可以定义自己的独特行为,但可以共享父类的某些相同功能。
可以通过对Bicycle
类进行较小的修改来证明多态性。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 语言中的多态功能,请将Bicycle
类扩展为MountainBike
和RoadBike
类。对于MountainBike
,为suspension
添加一个字段,该字段是String
的值,该值指示自行车是否具有前减震器Front
。或者,自行车具有前后减震器Dual
。
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.");
}
}
请注意被覆盖的printDescription
方法。除了之前提供的信息外,有关悬架的其他数据也包括在输出中。
接下来,创建RoadBike
类。由于公路或赛车自行车的轮胎稀疏,因此请添加属性以跟踪轮胎宽度。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.");
}
}
请再次注意,printDescription
方法已被覆盖。这次,显示有关轮胎宽度的信息。
总而言之,共有三类:Bicycle
,MountainBike
和RoadBike
。这两个子类重写printDescription
方法并输出唯一信息。
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之一。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“虚拟方法调用”,并且说明了 Java 语言中重要的多态性功能的一个方面。