java - NullPointerException when using inheritence -

i have scenario can best illustrated following toy example. suppose have 4 classes:

public class animal {     public animal(string a_name){         this.name = a_name;     }     public string name; }  public class mammal extends animal{     public mammal(string a_name, string a_nickname) {         super(a_name);         this.nick_name = a_nickname;     }     string nick_name; }   public class animal_groomer{     public animal_groomer(animal an_animal){         this.the_animal = an_animal;     }     public void print_name() {         system.out.println(the_animal.name);     }     public animal the_animal; }  public class mammal_groomer extends animal_groomer{     public mammal_groomer(mammal a_mammal){         super(a_mammal);     }     public void print_nickname(){         system.out.println(the_animal.nick_name);     }     public mammal the_animal; } 

now if main routine

mammal a_mammal = new mammal("tiger", "bob"); mammal_groomer mg = new mammal_groomer(a_mammal); mg.print_name(); mg.print_nickname(); 

mg.print_name() outputs "tiger", mg.print_nickname() gives me java.lang.nullpointerexception exception. there way me fix constructor or mg.print_nickname() method print nick_name? thank you.

in inheritance, methods overridden while fields , static members gets shadowed/hidden.

so if initialize field in parent, not have impact on child if define same field in child shadowed.

you not initializing child field hiding parent field , has value null. need initialize member of child class separately in child constructor.

public mammal_groomer(mammal a_mammal){     super(a_mammal);     this.the_animal = a_mammal; } 

read more shadowing/hiding , overriding : overriding vs hiding java - confused