Markov Model Close to being complete but needs a little help

[Michael Cullen]

I've been working hard at this code to make a Markov Language Generator but am running into some compilation errors as of late specifically on line 107. I am using the Sedgewick and Wayne library if that helps.

//file: Markov.java
//author: Michael Cullen
//date: December 2, 2011
import java.awt.*; 
import java.awt.event.*; 
import java.io.*; 
import java.util.*; 
import javax.swing.*;

public class Markov
{
  private static JFrame frame; 
  private static JTextArea inputText; 
  private static JTextArea outputText; 
  private static JTextField degreeText;
  private static void open() {
    
    FileReader file = DialogIO.openInputFile("Choose File");
    
    char c;
    int hold;
    String text = "";
    try { while(true){
        hold = file.read();
        if(hold == -1)
          break;
        c = (char)hold;
        text+= c;
    }
    }
    catch (IOException e) {
      System.out.println(e);
      System.exit(1);
      return;
    }
    
    inputText.append(text.toString());
    }
  
  private JFileChooser fileChooser = new JFileChooser(".");
  
  private static void save() {
  
  PrintStream printFile = DialogIO.openOutputFile("Save As");
  
  String toPrint = outputText.getText();
  for (int i = 0; i < toPrint.length(); i++)
  { 
    printFile.print(toPrint.charAt(i));
  }
        
  printFile.close();
  
  }
  
  
  private static void generate() {
    
    
    int degree = Integer.parseInt(degreeText.getText());
    
    String s = inputText.getText();
    
    String recent = "";
    
    int num = 0;
    
    HashMap<String, Vector<Integer>> m = new HashMap<String, Vector <Integer>>();
    
    for(int i = 0; i < s.length(); i++)
    {
      char c= s.charAt(i);
      String d = s.substring(i);
      
      if(m.containsKey(recent) == true)
      {
        m.get(recent).addElement((int)s.charAt(i));
      }
      
      else
      {
        m.put(recent, new Vector<Integer>());
        m.get(recent).addElement((int)s.charAt(i));
      }
      
    }
    String n = "";
    Vector word = new Vector();
    while (num != -1){
      
      int i = (int)(Math.random()*(m.get(n).size()));
      
      int k = m.get(n).get(i);
      
      
      
      if (k != -1){
        word.add(num, k);
        num++;
      }
      else
        num = -1;
    }
    char [] words = new char [word.size()];
    for (int j = 0; j < word.size(); j++)
    {
      int p = word.get(j);
      char k = (char)p;
      words[j] = k;
    }
    String output = new String(words);
    outputText.append(output.toString());
  }
  
  public static String elongate(String recent, String d, int degree)
  {
    
    if(recent.length() < degree)
    {
      return recent + d;
    }
    else
      return recent.substring(1) + d; 
  }
  
  private static void createAndShowGUI() { 

  // Create widgets. 
  
  frame = new JFrame("Markov Language Generator"); 
  JPanel mainPanel = new JPanel(); 
  JLabel degreeLabel = new JLabel("Degree"); 
  degreeText = new JTextField(3); 
  JButton openButton = new JButton("Open"); 
  JButton saveButton = new JButton("Save"); 
  JButton generateButton = new JButton(">>>"); 
  inputText = new JTextArea(); 
  inputText.setEditable(true); 
  inputText.setLineWrap(true); 
  JScrollPane inputScrollPane = new JScrollPane(inputText); 
  inputScrollPane.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS); 
  outputText = new JTextArea();
  outputText.setEditable(false); 
  outputText.setLineWrap(true); 
  JScrollPane outputScrollPane = new JScrollPane(outputText); 
  outputScrollPane.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);

  // Register callbacks. 
  //
  openButton.addActionListener(
      new ActionListener() { 
        public void actionPerformed(ActionEvent e) {
          open();
        }
      }
    ); 
  saveButton.addActionListener(
    new ActionListener() { 
      public void actionPerformed(ActionEvent e) {
        save();
      }
    }
  ); 
  generateButton.addActionListener(
    new ActionListener() { 
      public void actionPerformed(ActionEvent e) {
        generate();
      }
    }
  ); 

// Arrange widgets.

mainPanel.setLayout(new GridBagLayout()); 
mainPanel.setBorder(BorderFactory.createEmptyBorder(10,10,10,10)); 
GridBagConstraints c = new GridBagConstraints();

c.gridx = 0; c.gridy = 0; c.weightx = 0; c.weighty = 0; 
c.gridwidth = 1; c.gridheight = 2; 
c.fill = GridBagConstraints.NONE; 
c.insets = new Insets(0,0,0,0); 
mainPanel.add(openButton,c);

c.gridx = 1; c.gridy = 0; c.weightx = 0; c.weighty = 0; 
c.gridwidth = 1; c.gridheight = 1; 
c.fill = GridBagConstraints.NONE; 
c.insets = new Insets(0,0,0,0); 
mainPanel.add(degreeLabel,c);

c.gridx = 1; c.gridy = 1; c.weightx = 0; c.weighty = 0; 
c.gridwidth = 1; c.gridheight = 1; 
c.fill = GridBagConstraints.HORIZONTAL; 
c.insets = new Insets(5,5,5,5); 
mainPanel.add(degreeText,c);

c.gridx = 2; c.gridy = 0; c.weightx = 0; c.weighty = 0; 
c.gridwidth = 1; c.gridheight = 2; 
c.fill = GridBagConstraints.NONE; 
c.insets = new Insets(0,0,0,0); 
mainPanel.add(saveButton,c);

c.gridx = 0; c.gridy = 2; c.weightx = 1; c.weighty = 1; 
c.gridwidth = 1; 
c.fill = GridBagConstraints.BOTH; 
c.insets = new Insets(0,0,0,0); 
mainPanel.add(inputScrollPane,c);

c.gridx = 1; c.gridy = 2; c.weightx = 0; c.weighty = 0; 
c.gridwidth = 1; 
c.fill = GridBagConstraints.NONE; 
c.insets = new Insets(0,0,0,0); 
mainPanel.add(generateButton,c);

c.gridx = 2; c.gridy = 2; c.weightx = 1; c.weighty = 1; 
c.gridwidth = 1; 
c.fill = GridBagConstraints.BOTH; 
c.insets = new Insets(0,0,0,0); 
mainPanel.add(outputScrollPane,c);

// Finalize geometry. 
frame.add(mainPanel); 
frame.pack(); 
frame.setSize(600,400); 
frame.setVisible(true); 
frame.setResizable(true); 
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}

public static void main(String[] args) { 

  javax.swing.SwingUtilities.invokeLater(new Runnable() {
    public void run() {
      createAndShowGUI();
    }
  });
}
}

[Matthew Brown]

word is a Vector, which means that word.get(j) returns an Object, and you can't assign that to an int variable.

The best solution to this would be to use a generic collection: Vector<Integer> (or even better, ArrayList<Integer>, since Vectors are pretty obsolete now). That way it will make sure that you only put integers into the Vector, and you can assign it to an int when you extract a value. You're using generics elsewhere in the code, so I suspect you just forgot to use them here?

[Michael Cullen]

I'm not fully getting how to implement ArrayList<Integer> as I haven't learned about this yet. I get what you're saying about word.get(j) returning and object that can't be assigned but I'm not sure where to use the ArrayList<Integer> code

thanks for the help you've given me so far I do really appreciate it.

[Matthew Brown]

Well, ArrayList is a modern replacement for Vector. To use that, you'd just replace every use of Vector with ArrayList.

To get rid of your particular problem, change line 88 to:

Vector<Integer> word = new Vector<Integer>();

but you could equivalently use:

ArrayList<Integer> word = new ArrayList<Integer>();

In fact, there's an even better way:

List<Integer> word = new ArrayList<Integer>();

This is what's known as "programming to an interface". Both Vector and ArrayList implement the List interface. The advantage of this style is that you can interchange between different types of List whenever you want. Similarly, you could rewrite line 68 more flexibly as:

Map<String, List<Integer>> m = new HashMap<String, List<Integer>>();

[Winston Gutkowski]

Matthew Brown wrote:Well, ArrayList is a modern replacement for Vector. To use that, you'd just replace every use of Vector with ArrayList.

Actually, that will only work if the code doesn't make use of any Vector-specific methods, such as elementAt(); but I certainly agree that it's worth making the change.

Winston

[Michael Cullen]

I am getting null pointer exception at line 97

any ideas?

//file: Markov.java
//author: Matt Nacier & Coleman Younger
//date: December 2, 2011
import java.awt.*; 
import java.awt.event.*; 
import java.io.*; 
import java.util.*; 
import javax.swing.*;

public class Markov
{
private static JFrame frame; 
private static JTextArea inputText; 
private static JTextArea outputText; 
private static JTextField degreeText;
private static void open() {

FileReader file = DialogIO.openInputFile("Choose File");

char c;
int value;
String text = "";
try { while(true){
value = file.read();
if(value == -1)
break;
c = (char)value;
text = text + c;
}
}
catch (IOException e) {
System.out.println(e);
System.exit(1);
return;
}

inputText.append(text.toString());
}

private JFileChooser fileChooser = new JFileChooser(".");

private static void save() {

PrintStream printFile = DialogIO.openOutputFile("Save As");

String toPrint = outputText.getText();
for (int i = 0; i < toPrint.length(); i++)
{ 
printFile.print(toPrint.charAt(i));
}

printFile.close();

}


private static void generate() {


int degree = Integer.parseInt(degreeText.getText());

String s = inputText.getText();

String last = "";

int num = 0;

HashMap<String, Vector<Integer>> m = new HashMap<String, Vector <Integer>>();

for(int i = 0; i < s.length(); i++)
{
char c = s.charAt(i);

if (m.containsKey(last))
m.get(last).addElement((int)s.charAt(i));
else{
m.put(last, new Vector<Integer>());
m.get(last).addElement((int)s.charAt(i));}
last = stringElongate(last, c, degree);
}

m.get(last).addElement(-1);


Vector<Integer> v = new Vector<Integer>();
v.addElement(-1);
m.put(last, v);
//System.out.println("generate: m = " + m.toString()); 
StringBuffer sb = new StringBuffer();
last = "";

int done = 0;
while (done != -1) {

v = m.get(last);

int k = (int)(Math.random() * v.size()); 

char randomChar = (char)v.elementAt(k).intValue(); 

done = (int) randomChar;

if((int)randomChar != -1) {

sb.append(randomChar); 
last = stringElongate(last, randomChar, degree); 
}

}

outputText.setText(sb.toString()); 

}



public static String stringElongate(String last, char c, int degree)
{

if(last.length() < degree)
return last + c;

else
return last.substring(1) + c;

}

private static void createAndShowGUI() { 

// Create widgets. 

frame = new JFrame("Markov Language Generator"); 
JPanel mainPanel = new JPanel(); 
JLabel degreeLabel = new JLabel("Degree"); 
degreeText = new JTextField(3); 
JButton openButton = new JButton("Open"); 
JButton saveButton = new JButton("Save"); 
JButton generateButton = new JButton(">>>"); 
inputText = new JTextArea(); 
inputText.setEditable(true); 
inputText.setLineWrap(true); 
JScrollPane inputScrollPane = new JScrollPane(inputText); 
inputScrollPane.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS); 
outputText = new JTextArea();
outputText.setEditable(false); 
outputText.setLineWrap(true); 
JScrollPane outputScrollPane = new JScrollPane(outputText); 
outputScrollPane.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);

// Register callbacks. 
//
openButton.addActionListener(
new ActionListener() { 
public void actionPerformed(ActionEvent e) {
open();
}
}
); 
saveButton.addActionListener(
new ActionListener() { 
public void actionPerformed(ActionEvent e) {
save();
}
}
); 
generateButton.addActionListener(
new ActionListener() { 
public void actionPerformed(ActionEvent e) {
generate();
}
}
); 

// Arrange widgets.

mainPanel.setLayout(new GridBagLayout()); 
mainPanel.setBorder(BorderFactory.createEmptyBorder(10,10,10,10)); 
GridBagConstraints c = new GridBagConstraints();

c.gridx = 0; c.gridy = 0; c.weightx = 0; c.weighty = 0; 
c.gridwidth = 1; c.gridheight = 2; 
c.fill = GridBagConstraints.NONE; 
c.insets = new Insets(0,0,0,0); 
mainPanel.add(openButton,c);

c.gridx = 1; c.gridy = 0; c.weightx = 0; c.weighty = 0; 
c.gridwidth = 1; c.gridheight = 1; 
c.fill = GridBagConstraints.NONE; 
c.insets = new Insets(0,0,0,0); 
mainPanel.add(degreeLabel,c);

c.gridx = 1; c.gridy = 1; c.weightx = 0; c.weighty = 0; 
c.gridwidth = 1; c.gridheight = 1; 
c.fill = GridBagConstraints.HORIZONTAL; 
c.insets = new Insets(5,5,5,5); 
mainPanel.add(degreeText,c);

c.gridx = 2; c.gridy = 0; c.weightx = 0; c.weighty = 0; 
c.gridwidth = 1; c.gridheight = 2; 
c.fill = GridBagConstraints.NONE; 
c.insets = new Insets(0,0,0,0); 
mainPanel.add(saveButton,c);

c.gridx = 0; c.gridy = 2; c.weightx = 1; c.weighty = 1; 
c.gridwidth = 1; 
c.fill = GridBagConstraints.BOTH; 
c.insets = new Insets(0,0,0,0); 
mainPanel.add(inputScrollPane,c);

c.gridx = 1; c.gridy = 2; c.weightx = 0; c.weighty = 0; 
c.gridwidth = 1; 
c.fill = GridBagConstraints.NONE; 
c.insets = new Insets(0,0,0,0); 
mainPanel.add(generateButton,c);

c.gridx = 2; c.gridy = 2; c.weightx = 1; c.weighty = 1; 
c.gridwidth = 1; 
c.fill = GridBagConstraints.BOTH; 
c.insets = new Insets(0,0,0,0); 
mainPanel.add(outputScrollPane,c);

// Finalize geometry. 
frame.add(mainPanel); 
frame.pack(); 
frame.setSize(600,400); 
frame.setVisible(true); 
frame.setResizable(true); 
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}

public static void main(String[] args) { 

javax.swing.SwingUtilities.invokeLater(new Runnable() {
public void run() {
createAndShowGUI();
}
});
}
}

[Michael Cullen]

Actually 97 was the line where we were experiencing the problem

[Matthew Brown]

Here's how you can go about tracking down the error:

The only way line 97 can throw a NullPointerException is if v is null (then calling v.size() will throw it). You can confirm that by printing out the value just before, if you want (or stepping through with a debugger if you're using an IDE).

So then look at where v is assigned. That's on line 95. m.get(last) must be returning null. So next, find out what last is at that point. It's "" the first time round the loop, but I don't know if it's erroring the first time round - you need to find that out. Once you've tracked down the offending value of last, there are three possible sources of the error:

- You shouldn't be calling m.get(last) with that value of last
- m doesn't contain a value for that value of last, but it should do
- Calling it is OK, but you need to be able to cope with it returning null

You'll have to decide which of those it is, as you're the one who knows what the program is trying to do.

(I'd also suggest indenting your code properly, as it will be much easier to follow!)