Question

I’m getting a NullPointerException error in Eclipse. Code as it stands right now:

Java:

import java.io.File;
import java.io.IOException;
import java.util.Scanner;
import java.util.ArrayList;
import java.util.Collections;
import static java.lang.System. * ;

public class MadLib {
    private ArrayList<String> verbs = new ArrayList<String>();
    private ArrayList<String> nouns = new ArrayList<String>();
    private ArrayList<String> adjectives = new ArrayList<String>();

    public MadLib() {}

    public MadLib(String fileName) {
        //load stuff
        try {
            Scanner file = new Scanner(new File(fileName));
        }
        catch(Exception e) {
            out.println("Houston we have a problem!");
        }
    }

    public void loadNouns() {
        nouns = new ArrayList < String > ();
        try {
            Scanner chopper = new Scanner("nouns.dat");
            while (chopper.hasNext()) {
                nouns.add(chopper.next());
            }
            chopper.close();
            out.println(nouns);
        }
        catch(Exception e) {
            out.println("Will");
        }
    }

    public void loadVerbs() {
        verbs = new ArrayList < String > ();
        try {
            Scanner chopper = new Scanner("verbs.dat");
            while (chopper.hasNext()) {
                verbs.add(chopper.next());
            }
            chopper.close();
        }
        catch(Exception e) {
            out.println("run");
        }
    }

    public void loadAdjectives() {
        adjectives = new ArrayList < String > ();
        try {
            Scanner chopper = new Scanner("adjectives.dat");
            while (chopper.hasNext()) {
                adjectives.add(chopper.next());
            }
            chopper.close();
        }
        catch(Exception e) {}
    }

    public String getRandomVerb() {
        String verb = "";
        int num = 0;
        num = (int)(Math.random() * verbs.size());
        verb = verbs.get(num);
        return verb;
    }

    public String getRandomNoun() {
        String noun = "";
        int num = 0;
        num = (int)(Math.random() * nouns.size());
        noun = nouns.get(num);
        return noun;
    }

    public String getRandomAdjective() {
        String adj = "";
        int num = 0;
        num = (int)(Math.random() * adjectives.size());
        adj = adjectives.get(num);
        return adj;
    }

    public String toString() {
        String output = "The " + getRandomNoun() + getRandomVerb() + " after the " + getRandomAdjective() + getRandomAdjective() + getRandomNoun() + " while the " + getRandomNoun() + getRandomVerb() + " the " + getRandomNoun();
        return output;
    }
}

Eclipse is pointing to the issue occurring at the linenum = (int)(Math.random()*nouns.size()); but this seems to not make much sense to me.

I have the private ArrayList<String> initialized at the method loadNouns. I origianlly had ArrayList<String> nouns initialized at getRandomNoun(), but that threw a different error, so I was advised to move the initialization statement to the loadNouns method.

Runner Class:

import static java.lang.System.*;

public class Lab16d 
public static void main( String args[] ) {
     //make a new MadLib
     MadLib fun = new MadLib();
     out.println(fun);
}

Update:

The real issue appears to be that ArrayList<String> nouns never is “loaded up” with the separate strings which are supposed to be scanned in from the nouns.dat file

Update 2:

Java:

import java.io.File;
import java.io.IOException;
import java.util.Scanner;
import java.util.ArrayList;
import java.util.Collections;
import static java.lang.System. * ;

public class MadLib {
    private ArrayList<String> verbs = new ArrayList<String>();
    private ArrayList<String> nouns = new ArrayList<String>();
    private ArrayList<String> adjectives = new ArrayList<String>();

    public MadLib() {
        loadNouns();
        loadVerbs();
        loadAdjectives();
        out.println(nouns);
    }

    public MadLib(String fileName) {
        //load stuff
        loadNouns();
        loadVerbs();
        loadAdjectives();
        try {
            Scanner file = new Scanner(new File(fileName));
        }
        catch(Exception e) {
            out.println("Houston we have a problem!");
        }
    }

    public void loadNouns() {
        nouns = new ArrayList < String > ();
        try {
            //nouns = new ArrayList<String>();
            String nou = "";
            Scanner chopper = new Scanner(new File("nouns.dat"));

            //chopper.nextLine();
            while (chopper.hasNext()) {
                nou = chopper.next();
                out.println(nou);
                nouns.add(nou);
                //chopper.nextLine();
            }
            //chopper.close();
            out.println(nouns.size());
        }
        catch(Exception e) {
            out.println("Will");
        }
    }

    public void loadVerbs() {
        verbs = new ArrayList < String > ();
        try {
            Scanner chopper = new Scanner(new File("verbs.dat"));
            while (chopper.hasNext()) {
                verbs.add(chopper.next());
                chopper.nextLine();
            }
            chopper.close();
        }
        catch(Exception e) {
            out.println("run");
        }
    }

    public void loadAdjectives() {
        adjectives = new ArrayList < String > ();
        try {
            Scanner chopper = new Scanner(new File("adjectives.dat"));
            while (chopper.hasNext()) {
                adjectives.add(chopper.next());
                chopper.nextLine();
            }
            chopper.close();
        }
        catch(Exception e) {}
    }

    public String getRandomVerb() {

        String verb = "";
        int num = 0;
        num = (int)(Math.random() * (verbs.size() - 1));
        verb = verbs.get(num);
        return verb;
    }

    public String getRandomNoun() {
        String noun = "";
        int num = 0;
        if (nouns == null) {
            loadNouns();
        }
        double rand = (Math.random());
        num = (int)(rand * (nouns.size() - 1));
        out.println(num);
        noun = nouns.get((int) num);
        out.print(noun);
        return noun;
    }

    public String getRandomAdjective() {
        String adj = "";
        int num = 0;
        num = (int)(Math.random() * (adjectives.size() - 1));
        adj = adjectives.get(num);
        return adj;
    }

    public String toString() {
        String output = "The " + getRandomNoun() + getRandomVerb() + " after the " + getRandomAdjective() + getRandomAdjective() + getRandomNoun() + " while the " + getRandomNoun() + getRandomVerb() + " the " + getRandomNoun();
        return output;
    }
}

Answer 1

You are creating an instance of MadLib, then printing the object in a println in your Runner class…

     //make a new MadLib
     MadLib fun = new MadLib();
     out.println(fun);

The out.println calls the toString() method you overrode in MadLib…

    String output = "The " + getRandomNoun() + getRandomVerb() + " after the " + getRandomAdjective() + getRandomAdjective() + getRandomNoun() + " while the " + getRandomNoun() + getRandomVerb() + " the " + getRandomNoun();
    return output;

Your MadLib object has 3 ArrayLists you never initialized, so they are null…

private ArrayList<String> verbs;
private ArrayList<String> nouns;
private ArrayList<String> adjectives

The easiest way to fix the NullPointerException is to initialize the variables….

private ArrayList<String> verbs = new ArrayList<String>();
private ArrayList<String> nouns = new ArrayList<String>();
private ArrayList<String> adjectives = new ArrayList<String>();

However, what I really think you want to do is load all the nouns, verbs and adjectives when your object is constructed so your toString actually prints something useful. I’d add this to your constructor as well…

public MadLib() {
  loadNouns();
  loadVerbs();
  loadAdjectives();
}

Edit: Your getRandom methods need to check if the list is empty to avoid the IndexOutOfBounds exception as well…

public String getRandomVerb() {
    String verb = "";

    if (!verbs.isEmpty()) {
        int num = (int) (Math.random() * verbs.size() - 1);
        verb = verbs.get(num);
    }

    return verb;
}

public String getRandomNoun() {
    String noun = "";

    if (!nouns.isEmpty()) {
        int num = (int) (Math.random() * nouns.size() - 1);
        noun = nouns.get(num);
    }

    return noun;
}

public String getRandomAdjective() {
    String adj = "";

    if (!adjectives.isEmpty()) {
        int num = (int) (Math.random() * adjectives.size());
        adj = adjectives.get(num);
    }

    return adj;
}

Hope that helps