System.out.print("生日");
改为
System.out.println("生日");

因为你参数根本没设进去啊......

  public date(int m,int d,int y){
          day = d;
          month = m;
          year = y;    
  }
  public void printdate(){
      System.out.println(month+"/"+day+"/"+year);
  }