下面是一道关于继承的题目，我总是没发理解，不知道怎么做，有能帮忙解释一下的吗？


要求，每个类单独保存为一个文件，文件名要和类名相同；其中，Transportation类为Bicycle类的超类，Bicycle类为MultispeedBicycle类的超类。在相应的缺省构造方法中将属性owner初始化为你的姓名，number初始化为你的学号，size初始化为你的机器号，level初始化为10 。编译通过后可编译运行文件MultispeedBicycleTest.java测试。



/*程序入口*/
public class MultispeedBicycleTest {
    public static void main(String[] args) {
        System.out.println("-------缺省构造方法测试-------");
        MultispeedBicycle m1 = new MultispeedBicycle();
        m1.show();
   
       /* System.out.println("-------get、set方法测试-------");
        m1.setOwner("黄老邪");
        m1.setNumber("8888");
        m1.setSize(88);
        m1.setLevel(8);
        System.out.println("拥有者：" + m1.getOwner());
        System.out.println("编号：" + m1.getNumber());
        System.out.println("大小：" + m1.getSize());
        System.out.println("级别：" + m1.getLevel());
        
        System.out.println("-------带参数构造方法测试-----------");
        MultispeedBicycle m2 = new MultispeedBicycle("老顽童","7777",77,7);
        m2.show();
        */
    }
}
第一个类
public class Transportation {

     private String owner;
     private String number;
     
     public Transportation()
     {this.owner="11";
      this.number="25";}

       public Transportation(String owner, String number)
       {this.owner=owner;
       this.number=number;}

       public String getOwner()
       {return this.owner;}
 
       public void setOwner(String owner)
       { this.owner=owner;}

       public String getNumber()
       {return this.number;}

      public void setNumber(String number)
      {this.number=number;}


     public void show()
     {System.out.println("姓名："+owner);}

}
继承了第一个类的第二个类
public class Bicycle extends Transportation {


    private int size;



public Bicycle()
{}


public Bicycle(String owner,String number,int size)
{      this.owner=owner;
       this.number=number;
       this.size=size;
   
   
    }

public int getSize(int size)
{return this.size;}
 
public void show()
{}
  
}
继承了第二个类的第三个类
public class MultispeedBicycle extends Bicycle {
   private int level;

    public MultispeedBicycle() {
    }
   
   

public MultispeedBicycle(int level)
{      
       this.level=level;}

public int getLevel()
{return level;}

public void setLevel(int level)
{this.level=level;}
public void show()
{}

}

java不能多重继承的