java - 创建对象的基本java

Question

我是java新手，

现在我正在学习java link

当我开始了解对象和类时，

第一部分:

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);
    }
}

这里创建了一个类名“Bicycle”，没有main方法，它包含字段和方法。

第二部分:

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();
    }
} 

这里包含main方法并创建了两个对象，但是

为什么在这里创建另一个类“BicycleDemo”

我只是困惑，这是一个简单的问题，

如果有人解释一下，作为像我这样的初学者，这将会有所帮助。

谢谢

Answer 1

BicycleDemo包含public static void main(String [] args)方法，它就像所有Java程序的入口点。

本质上，BicycleDemo充当驱动程序类，供您测试Bicycle类的功能。

看看this了解为什么会有BicycleDemo

合并为一个BicycleDemo.java

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();
    }
}

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);
    }
}

选项 2: Bicycle 中的 main 方法

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);
    }

    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();
    }
}