请教大师们，我定义了接口，但感觉没用到呀。

package DaRentCar2;

public abstract class Car {
	private String name; 
	private int rentfee;
	private int weight;
	private int person;
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public int getRentfee() {
		return rentfee;
	}
	public void setRentfee(int rentfee) {
		this.rentfee = rentfee;
	}
	public int getWeight() {
		return weight;
	}
	public void setWeight(int weight) {
		this.weight = weight;
	}
	public int getPerson() {
		return person;
	}
	public void setPerson(int person) {
		this.person = person;
	}
	
}

package DaRentCar2;

public interface IPersonCarry {
	void PersonC();
}

package DaRentCar2;

public interface IWeightCarry {
	public abstract void WeightC();
}

package DaRentCar2;

public interface IWeightPersonCarry extends IWeightCarry, IPersonCarry {
	void WeightC();
	
}

package DaRentCar2;

public class Carriage extends Car implements IPersonCarry {

	public Carriage(String name,int rentfee,int weight,int person){
		this.setName(name);
		this.setRentfee(rentfee);
		this.setWeight(weight);
		this.setPerson(person);
		
	}
	
	
	@Override
	public void PersonC() {
		// TODO Auto-generated method stub
		System.out.println("可以载人");
	}

}

package DaRentCar2;

public class Pickup extends Car implements IWeightPersonCarry{
	public Pickup(String name,int rentfee,int weight,int person){
		this.setName(name);
		this.setRentfee(rentfee);
		this.setWeight(weight);
		this.setPerson(person);
		
	}
	
	
	@Override
	public void PersonC() {
		// TODO Auto-generated method stub
		System.out.println("可以载人");

	}

	@Override
	public void WeightC() {
		// TODO Auto-generated method stub
		System.out.println("可以载货");		

	} 
}

package DaRentCar2;

public class Truck extends Car implements IWeightCarry{
	public Truck(String name,int rentfee,int weight,int person){
		this.setName(name);
		this.setRentfee(rentfee);
		this.setWeight(weight);
		this.setPerson(person);
		
	}
	
	
	
	@Override
	public void WeightC() {
		System.out.println("可以载货");		
	}

}

package DaRentCar2;

import java.util.Scanner;

public class Test {

	public static void main(String[] args) {
		Car carriage1 = new Carriage("奥迪A4", 500, 0, 4);
		Car carriage2 = new Carriage("马自达6", 400, 0, 4);
		Car carriage3 = new Carriage("金龙", 800, 0, 20);
		Car pickup = new Pickup("皮卡雪6", 450, 2, 4);
		Car truck1 = new Truck("松花江", 400, 4, 0);
		Car truck2 = new Truck("依维柯", 1000, 20, 0);
		
		System.out.println("欢迎来到大大租车系统");
		System.out.println("是否租车？ 1：是   0：否");
		Scanner in = new Scanner(System.in);
		int choose = in.nextInt();
		if(choose == 0){
			System.out.println("欢迎下次再来，退出租车系统");
			System.exit(0);
		}else if(choose != 0 && choose != 1){
			System.out.println("输入有误，退出系统");
			System.exit(0);
		}else{
			System.out.println("您可租的汽车类型和价目表：");
			System.out.println("序号\t汽车名称\t租金\t容量");
			System.out.println("1.\t"+carriage1.getName()+"\t"+carriage1.getRentfee()+"/天\t"+"载"+carriage1.getPerson()+"人");
			System.out.println("2.\t"+carriage2.getName()+"\t"+carriage2.getRentfee()+"/天\t"+"载"+carriage2.getPerson()+"人");
			System.out.println("3.\t"+carriage3.getName()+"\t"+carriage3.getRentfee()+"/天\t"+"载"+carriage3.getPerson()+"人");
			System.out.println("4.\t"+pickup.getName()+"\t"+pickup.getRentfee()+"/天\t"+"载"+pickup.getPerson()+"人"+" 载"+pickup.getWeight()+"吨");
			System.out.println("5.\t"+truck1.getName()+"\t"+truck1.getRentfee()+"/天\t"+"载"+truck1.getWeight()+"吨");
			System.out.println("6.\t"+truck2.getName()+"\t"+truck2.getRentfee()+"/天\t"+"载"+truck2.getWeight()+"吨");

		System.out.println("请输入您要租的车的数量");
		int num = in.nextInt();
		int n1=0,n2=0,n3=0,n4=0,n5=0,n6=0;
		for(int i =0;i<num;i++){
			System.out.println("请输入您要租车的序号");
			int N = in.nextInt();
			if(N==1){
				n1++;
			}
			if(N==2){
				n2++;
			}
			if(N==3){
				n3++;
			}
			if(N==4){
				n4++;
			}
			if(N==5){
				n5++;
			}
			if(N==6){
				n6++;
			}
		}
			
			int totalPerson = carriage1.getPerson()*n1+carriage1.getPerson()*n2+carriage1.getPerson()*n3
					+pickup.getPerson()*n4;
			int totalWeight = pickup.getWeight()*n4+truck1.getWeight()*n5+truck2.getWeight()*n6;
			
			int totalRentfee = carriage1.getRentfee()*n1+carriage2.getRentfee()*n2+
					carriage3.getRentfee()*n3+pickup.getRentfee()*n4+truck1.getRentfee()*n5+
					truck2.getRentfee()*n6;
			
			System.out.println("请输入租车天数");
			int day = in.nextInt();
			System.out.println("您的账单：");
			System.out.println("***可载人的车有:");
			
			if(n1!=0){ 
				System.out.print(n1+"台"+carriage1.getName()+'\t'); 
				} 
			if(n2!=0){ 
				System.out.print(n2+"台"+carriage2.getName()+'\t'); 
				} 
			if(n3!=0){ 
				System.out.print(n3+"台"+carriage3.getName()+'\t'); 
				} 
			if(n4!=0){ 
				System.out.print(n4+"台"+pickup.getName()+'\t'); 
				} 
			System.out.println("共载人:"+totalPerson+"人"); 
			System.out.println("***可载货的车有:"); 
			if(n4!=0){ 
				System.out.print(n4+"台"+pickup.getName()+'\t'); 
				} 
			if(n5!=0){ 
				System.out.print(n5+"台"+truck1.getName()+'\t'); 
				} 
			if(n6!=0){ 
				System.out.print(n6+"台"+truck2.getName()+'\t'); 
				} 
				System.out.println("共载货:"+totalWeight+"吨"); 
				System.out.println("总租金:"+totalRentfee*day+"元"); }
		}
	}