在名为“ 面向对象编程概念 ”的课程中，对面向对象概念的介绍以自行车类为例，其中赛车，山地自行车和双人自行车为子类。这是一个Bicycle类的可能实现的示例代码，以概述类的声明。本课程的后续部分将重复并逐步解释类声明。目前，不要担心细节。

    1. public class Bicycle {
    3.     // the Bicycle class has
    4.     // three fields
    5.     public int cadence;
    6.     public int gear;
    7.     public int speed;
    9.     // the Bicycle class has
    10.     // one constructor
    11.     public Bicycle(int startCadence, int startSpeed, int startGear) {
    12.         gear = startGear;
    13.         cadence = startCadence;
    14.         speed = startSpeed;
    15.     }
    17.     // the Bicycle class has
    18.     // four methods
    19.     public void setCadence(int newValue) {
    20.         cadence = newValue;
    21.     }
    23.     public void setGear(int newValue) {
    24.         gear = newValue;
    25.     }
    27.     public void applyBrake(int decrement) {
    28.         speed -= decrement;
    29.     }
    31.     public void speedUp(int increment) {
    32.         speed += increment;
    33.     }
    35. }

    Bicycle的一个子类，MountainBike类的类声明可能是这样：

    1. public class MountainBike extends Bicycle {
    3.     // the MountainBike subclass has
    4.     // one field
    5.     public int seatHeight;
    7.     // the MountainBike subclass has
    8.     // one constructor
    9.     public MountainBike(int startHeight, int startCadence,
    10.                         int startSpeed, int startGear) {
    11.         super(startCadence, startSpeed, startGear);
    12.         seatHeight = startHeight;
    13.     }   
    15.     // the MountainBike subclass has
    16.     // one method
    17.     public void setHeight(int newValue) {
    18.         seatHeight = newValue;
    19.     }   
    21. }

    MountainBike继承Bicycle的所有字段和方法，并添加字段seatHeight及其设置方法（山地自行车的座椅可以根据地形要求上下移动）。