java - Giving error when i adding string to child node -

public void createnode(int group){      defaultmutabletreenode root = null;      defaultmutabletreenode child[] = null;      list<string> list = new arraylist<string>();       expressionbuilder builder=new expressionbuilder();      list = builder.getexpression(group,0);      root = new defaultmutabletreenode(groupstring);          defaulttreemodel = new defaulttreemodel(root);       for(int i=0; i<list.size();i++){          child[i] = new defaultmutabletreenode(list.get(i));          defaulttreemodel.insertnodeinto(child[i], root, i);      }  } 

when making child array giving me error of null pointer exception. list populated correctly.

defaultmutabletreenode child[] = null;  

the child array null.

when create array need like:

defaultmutabletreenode child[] = new defaultmutabletreenode[???]; 

so need create array after create list know size make array.