没有“抽象”的Java中的运行时多态性？(Run-time Polymorphism in Java without “abstract”?)

 我正在阅读官方的Oracle教程，其中介绍了多态性的概念，以及3个类的类层次结构的示例; 自行车是超类，MountainBike和RoadBike是2个子类。 
 
 它显示了2个子类如何重写在Bicycle中声明的方法“printDescription”，通过声明它的不同版本。 
 
 最后，在最后，教程提到Java虚拟机（JVM）为每个变量中引用的对象调用适当的方法。 
 
 但是，关于多态性的教程没有提到“抽象”类和方法的概念。 除非将Bicycle中的printDescription（）声明为“abstract”，否则如何实现运行时多态性？ 我的意思是，在这个例子中，基于什么提示，编译器决定在编译时不将方法调用绑定到引用类型，并认为它应该留给JVM在运行时处理？ 
 
 以下是使用的示例： 
 
public class Bicycle {
    public int cadence;
    public int gear;
    public int speed;

    public Bicycle(int startCadence, int startSpeed, int startGear) {
      gear = startGear;
      cadence = startCadence;
      speed = startSpeed;
    }

    public void setCadence(int newValue) {
      cadence = newValue;
    }

    public void setGear(int newValue) {
      gear = newValue;
    }

    public void applyBrake(int decrement) {
      speed -= decrement;
    }

    public void speedUp(int increment) {
      speed += increment;
    }

    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 + ". ");
    }

}

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.");
  }

}

public class RoadBike extends Bicycle{

  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.");
  }
}


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();
      }
}

I was going over the official Oracle tutorial where it introduces the idea of polymorphism with the example of a class hierarchy of 3 classes; Bicycle being the superclass, and MountainBike and RoadBike being 2 subclasses.
 
It shows how the 2 subclasses override a method "printDescription" declared in Bicycle, by declaring different versions of it.
 
And finally, toward the end, the tutorial mentions the Java Virtual Machine (JVM) calls the appropriate method for the object that is referred to in each variable.
 
But, nowhere does the tutorial on polymorphism mention the concept of "abstract" classes and methods. How is run-time polymorphism achieved unless printDescription() in Bicycle is declared "abstract"? I mean, given this example, based on what hints does the compiler decide not to bind method invocation to the reference type at compile time, and think that it should leave it for the JVM to deal with at run-time?
 
Below is the example used:
 
public class Bicycle {
    public int cadence;
    public int gear;
    public int speed;

    public Bicycle(int startCadence, int startSpeed, int startGear) {
      gear = startGear;
      cadence = startCadence;
      speed = startSpeed;
    }

    public void setCadence(int newValue) {
      cadence = newValue;
    }

    public void setGear(int newValue) {
      gear = newValue;
    }

    public void applyBrake(int decrement) {
      speed -= decrement;
    }

    public void speedUp(int increment) {
      speed += increment;
    }

    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 + ". ");
    }

}

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.");
  }

}

public class RoadBike extends Bicycle{

  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.");
  }
}


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();
      }
}

原文：https://stackoverflow.com/questions/12709033