The initial implementation of a "builder" to build a tree from a formatted text file

QuestionNodeTreeBuilder.java

import java.io.*;
import java.util.*;

import com.mycompany.guesstheanimal.exceptions.InvalidTreeParsingException;

public class QuestionNodeTreeBuilder implements TreeBuilder<QuestionNode>{
    public static final String NODE_DATA_SPLITTER = QuestionNodeTreeWriter.NODE_DATA_SPLITTER;
    public static final String QUESTION_PREFIX = QuestionNodeTreeWriter.QUESTION_PREFIX;
    public static final String ANSWER_PREFIX = QuestionNodeTreeWriter.ANSWER_PREFIX;
    
    private Iterator<String> input;
    private QuestionNode root;
    private int lineCounter;
    
    public QuestionNodeTreeBuilder(Iterator<String> input) {
        this.lineCounter = 0;
        this.input = input;
        this.root = null;
    }
    
    public QuestionNodeTreeBuilder(File file) throws FileNotFoundException {
        this(new Scanner(file));
    }
    
    public QuestionNode build() throws InvalidTreeParsingException {
        if (invalidRoot())
            buildRootAndTree();
        return root;
    }
    
    private boolean invalidRoot() {
        return root == null;
    }
    
    private void buildRootAndTree() throws InvalidTreeParsingException {
        root = buildSubTree();
    }
    
    private QuestionNode buildSubTree() throws InvalidTreeParsingException {
        if(input.hasNext()) {
            String[] s = readNextFromInput();
            String prefix = s[0];
            String data = s[1];
            if(isQuestion(prefix))
                return new QuestionNode(data, buildSubTree(), buildSubTree());
            return new QuestionNode(data);
        }
        invalidStateReached();
        return null;
    }
    
    private String[] readNextFromInput() {
        lineCounter++;
        return parseData(input.next());
    }
    
    private void invalidStateReached() throws InvalidTreeParsingException {
        String msg = "Missing answer node at line # " + lineCounter; 
        throw new InvalidTreeParsingException(msg, lineCounter);
    }
    
    private String[] parseData(String s) {
        return s.trim().split(NODE_DATA_SPLITTER, 2);
    }
    
    private boolean isQuestion(String prefix) {
        return prefix.equals(QUESTION_PREFIX);
    }
}