package com.hu; 

public class Vehicle {
   public int num;   
   public String mode;   
   public String type; 
         
   public void ability(){ 
          System.out.println("交通工具具有载人出行的能力");                
          System.out.println("num:"+num+"\nmode:"+mode+"\ntype:"+type);            
          }        
  }   
                
public class Car extends Vehicle { 
   public int num=6;     
   public String mode="陆行";      
   public String type="汽车";  
           
   public void ability(){
               System.out.println("汽车具有载人陆行的能力");                  
               System.out.println("num:"+num+"\nmode"+mode+"\ntype"+type);                
               }                    
               @Override                        
               public String toString() {                        
               return "Car [num=" + num + ", mode=" + mode + ", type=" + type + "]";                        
               } 
               
               public class Initail {   public static void main(String[] args) { 
                              Vehicle vehicle = new Vehicle();                
                              Vehicle car = new Car();  
                                                                            
                               vehicle.ability();                  
                               System.out.println();   
                                                                    
                               car.ability();                  
                               System.out.println();                                    
                               System.out.println(car.num+"\n"+car.mode+"\n"+car.type);//就是这行
                                               
                               System.out.println();                                                    
                               System.out.println(car);                        
                               }                
                               }

为什么

System.out.println(car.num+"\n"+car.mode+"\n"+car.type);

这句输出的是

0

null

null

而不是

6

陆行

汽车