java - how to make List of of mutable objects Immuatable -

hi asked interview question have list of objects in immutable class, class immuatable, can modified , how can prevent it. gave below solution.

public final class a{    final list<b> listofb; // b mutable     public list<b> getlistofb(){       return collections.unmodifiablelist(this.listofb);    } } 

now says after getting getlistofb() can change 'b' instances , wanted me avoid also. said.

public list<b> getlistofb(){    list<b> ret;    for(b b: this.listofb){        ret.add(b.clone()); // make deep copy of 'b' return list    }    return ret; }  

the interviewer did not respond saying right or wrong.

this solution definately works. there better way of doing it, approach clumsy , requires additional memory.

ps: assume b cannot made immutable.

assuming b interface or implementation of interface a, wrap each element of listofb using java.net.proxy of b or a respectively, intercepts modifying calls throwing unsupportedoperationexception instead.

more detailed, replace each item of listofb java.net.proxy wrapped instance, implementing a. whenever fetches item of listofb, obtain wrapped instance instead. whenever calls setter on such wrapped item, intercept call , throw unsupportedoperationexception. hope have idea mean. it's collections.unmodifiablexxx() does. if have knowledge of methods may modify state of item of listofb, b not have pojo. make sure wrapper replaces modifying methods throwing exception.