Hmmmmm..... I suppose

CODE
ArrayList<String> sentences = new ArrayList<String>();
String temp;
int numLines;
JFileChooser chooser = new JFileChooser();
File infile = null;
FileReader reader = null;
if (chooser.showOpenDialog(null) == JFileChooser.APPROVE_OPTION)
{
infile = chooser.getSelectedFile();
reader = new FileReader(infile);
}
Scanner input = new Scanner(infile);
while (input.hasNextLine())
{temp = input.nextLine();
sentences.add(temp);
numLines++;}
Random r = new Random();
System.out.println(sentences.get(r.nextInt(sentences.size())));
Breakdown :
1. JFileChooser allows the user to select a file to read the sentences from
2. It assign the File the selected file and the reader is assign a file to read!
3. Scanner is then created using the FIle selected, and using the hasNextLine() method, we make a while loop to keep cycling through every line adding it into the arraylist, but increase numLines as we go through it.
4. We then create a random object. (You will have to import Random) ArrayList has a method get() that allows us to retrieve something at that element. In this case we are going to get the string, and print it. Hence why it is in a System.out.println call
Hope this helps!
Goodluck!