// Car，Bus,Truck的基类

public class Vehicle{
    protected int wheelCount;   
}


// 所有类共同的接口
public interface Countable{
    public final int a=9;
    
    public int getWheelCount();
}

public class Bus extends Vehicle implements Countable{
    public Bus(){
        wheelCount=6;
    }

    @Override
    public int getWheelCount() {
        return wheelCount;
    }
}

public class Car extends Vehicle implements Countable{
    public Car(){
        wheelCount=4;
    }

    @Override
    public int getWheelCount() {
        return wheelCount;
    }
}

public class Truck extends Vehicle  implements Countable{
    public Truck(){
        wheelCount=10;
    }

    @Override
    public int getWheelCount() {
        return wheelCount;
    }
}

public class Motor  implements Countable{

    @Override
    public int getWheelCount() {
        return 2;
    }
}

public class Tank implements Countable{
    @Override
    public int getWheelCount() {
        return 0;
    }
}

public class Test{
    public static void main(String[] args){
        
        // 统一了诸类接口，所有实例都可放入一个数据结构，方便存储和计算
        Countable[] arr=new Countable[7];
        
        if(arr[1]==null){
            System.out.println("arr[1]==null");
        }
        
        arr[0]=new Bus();
        arr[1]=new Car();
        arr[2]=new Motor();
        arr[3]=new Tank();
        arr[4]=new Truck();
        arr[5]=new Car();
        arr[6]=new Car();
        
        int sum=0;
        for(int i=0;i<arr.length;i++){
            // 同一接口强制子类实现固定方法，方便了代码书写和测试
            sum+=arr[i].getWheelCount();
        }
        System.out.println("Sum="+sum);
        
        for(Countable c:arr){
            // 同一接口强制子类实现固定方法，方便了代码书写和测试
            sum+=c.getWheelCount();
        }
        System.out.println("Sum="+sum);
    }
}