What Is a Class?什么是类？

In the real world, you'll often find many individual objects all of the same kind.在现实世界中，您经常会发现许多相同类型的单独对象。There may be thousands of other bicycles in existence, all of the same make and model.可能还有成千上万辆其他的自行车，都是同样的品牌和型号。Each bicycle was built from the same set of blueprints and therefore contains the same components.每辆自行车都是由同一套蓝图制成的，因此包含相同的部件。In object-oriented terms, we say that your bicycle is an instance of the class of objects known as bicycles.在面向对象的术语中，我们说您的自行车是被称为自行车的对象类的实例。A class is the blueprint from which individual objects are created.类是创建单个对象的蓝图。

The following Bicycle class is one possible implementation of a bicycle:以下Bicycle类别是自行车的一种可能实现：

class Bicycle {

    int cadence = 0;
    int speed = 0;
    int gear = 1;

    void changeCadence(int newValue) {
         cadence = newValue;
    }

    void changeGear(int newValue) {
         gear = newValue;
    }

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

    void applyBrakes(int decrement) {
         speed = speed - decrement;
    }

    void printStates() {
         System.out.println("cadence:" +
             cadence + " speed:" + 
             speed + " gear:" + gear);
    }
}

The syntax of the Java programming language will look new to you, but the design of this class is based on the previous discussion of bicycle objects.Java编程语言的语法对您来说是新的，但是这个类的设计是基于前面对bicycle对象的讨论。The fields cadence, speed, and gear represent the object's state, and the methods (changeCadence, changeGear, speedUp etc.) define its interaction with the outside world.字段cadence、speed和gear表示对象的状态，而方法（changeCadence、changeGear、speedUp等）定义了它与外部世界的互动。

You may have noticed that the Bicycle class does not contain a main method.您可能已经注意到Bicycle类不包含main方法。That's because it's not a complete application; it's just the blueprint for bicycles that might be used in an application.那是因为它不是一个完整的应用程序；这只是自行车的蓝图，可能会在应用程序中使用。The responsibility of creating and using new Bicycle objects belongs to some other class in your application.创建和使用新Bicycle对象的责任属于应用程序中的其他类。

Here's a BicycleDemo class that creates two separate Bicycle objects and invokes their methods:下面是一个BicycleDemo类，它创建两个单独的Bicycle对象并调用它们的方法：

class BicycleDemo {
    public static void main(String[] args) {

        // Create two different 
        // Bicycle objects
        Bicycle bike1 = new Bicycle();
        Bicycle bike2 = new Bicycle();

        // Invoke methods on 
        // those objects
        bike1.changeCadence(50);
        bike1.speedUp(10);
        bike1.changeGear(2);
        bike1.printStates();

        bike2.changeCadence(50);
        bike2.speedUp(10);
        bike2.changeGear(2);
        bike2.changeCadence(40);
        bike2.speedUp(10);
        bike2.changeGear(3);
        bike2.printStates();
    }
}

The output of this test prints the ending pedal cadence, speed, and gear for the two bicycles:此测试的输出打印两辆自行车的结束踏板步频、速度和档位：

cadence:50 speed:10 gear:2
cadence:40 speed:20 gear:3