A really simple program in 30 lines of code to decide the grade level that a book/paragraph/sentence should be offered to.

Reader.java

import javax.swing.JOptionPane;

public class Reader {

    public Reader() {
        String text = JOptionPane.showInputDialog("Please enter a few sentences to parse.");
        int level = (int) read(text);
        
        JOptionPane.showMessageDialog(null, "The text is " + (int) Math.ceil(level) + (level == 1 ? "st" : (level == 2 ? "nd" : (level == 3 ? "rd" : "th"))) + " grader level.");
    }

    public static void main(String[] args) {
        new Reader();
    }
    
    public float read(String text) {
        String[] t = text.split(" ");
        float avg = 0;
        
        for(String word : t) {
            float length = word.length();
            if(avg == 0)
                avg = length;
            avg = (avg + length) / 2;
        }
        
        return avg;
    }
    
}