java - What is the proper way to access an objects instance-objects instance-variable -

let's have 2 constructor classes; person , arm.

public class person {   private arm arm; //instance variable    public person(arm arm) //constructor   {     this.arm = arm;   } } 

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public class arm {   private int fingers; //instance variable    public arm(int fingers) //constructor   {     this.fingers = fingers;   }    public int getfingers()   {     return this.fingers();   }  } 

every person object has arm object instance-variable. every arm object has int instance-variable called fingers.

what proper way access fingers variable?

1. create getfingers() method, in person class:

//in person.java public int getfingers() {   return this.arm.getfingers(); //this refrencing getfingers() method in arm class } 

so can access this:

arm myarm = new arm(5); person me = new person(myarm);  system.out.println(me.getfingers()); 

2. create method in person returns arm.

   public arm getarm() {   return this.arm; } 

so can use getfingers method arm class this:

arm myarm = new arm(5); person me = new person(myarm);  system.out.println(me.getarm().getfingers()) 

normally suggest law of demeter comes play here.

• each unit should have limited knowledge other units: units "closely" related current unit.
• each unit should talk friends; don't talk strangers.
• only talk immediate friends.

if write:

a = new a(); d d = a.getb().getc().getd(); 

then allow access d instance of a but breaks encapsulation , reveals a has b, b has c etc. etc. might argue redundant in case, since expect person have arm.

an alternative approach think want top-level object (person) do, , tell you. e.g. if want fingers can tell them grasp (perhaps) then:

person.pickup(object) 

and person can determine how (it choose arm, or it's feet or mouth in extreme situations!). rather fingers yourself, you're telling object you. oop about.