java - 有人能解释一下为什么这个java类不能编译吗？

Question

我是一个java新手，我试图编译它，但它无法编译。有人可以帮忙吗？它说 void 是表达式的非法开头。我从 https://docs.oracle.com/javase/tutorial/java/concepts/class.html 得到这个。当我删除 main 方法及其右括号时，它会进行编译。为什么是这样？我怎样才能用类中包含的 main 方法来编写这个？

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

    }

}

}

Answer 1

您将 main 方法放在了错误的位置。 main 方法不应包含该类的方法和成员。

您应该提取主要方法:

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[])
    {
        // here you can create an instance of the class and test it
        Bicycle bike = new Bicycle ();
        bike.changeCadence(4);
        bike.changeGear(5);
        bike.speedUp(3);
        bike.printStates();
    }

}