Thread: Can't read into Array List.

Hi, First time poster here so sorry if I've done something wrong in relation to posting. I am creating a hangman game and I want to read in a list of words from a text file, but after the user inputs the name of the text file. I get 'Exception in thread "main" java.lang.NullPointerException'.

Here is the code, where I think the problems lie.

	public void runModel(){
		ArrayList<String> pirateWordsList = new ArrayList<String>();
		System.out.println("What is the name of the file you would like to load? (The file included is called  piratewords.txt'");
		Scanner in=new Scanner(System.in);
		String file=in.next();
		load(file);
		System.out.println(pirateWordsList);
 
	 public void load(String pirate){
	        Scanner infile;
			try {
				infile = new Scanner(new InputStreamReader
				(new FileInputStream(pirate)));
				 int num=infile.nextInt();infile.nextLine();
			        for (int i=0;i<num;i++){
			           String secretPirateWord=infile.nextLine();
			            pirateWordsList.add(secretPirateWord);
			        }
			        infile.close();
			} 
			catch (FileNotFoundException e) {
				System.out.println("File not found, are you sure you entered the files name correctly and is the file in the correct directory");
			}
 
}

The full error message is this:
Exception in thread "main" java.lang.NullPointerException
at uk.ac.aber.dcs.pirate_hangman.Model.load(Model.jav a:108)
at uk.ac.aber.dcs.pirate_hangman.Model.runModel(Model .java:45)
at uk.ac.aber.dcs.pirate_hangman.Main.main(Main.java: 6)

Please edit your post and wrap your code with code tags:
[code=java]
YOUR CODE HERE
[/code]
to get highlighting and preserve formatting.

java.lang.NullPointerException

Look at the line of code mentioned in the error message and find the variable with the null value. Then backtrack in the code to see why that variable has a null value.

Exception in thread "main" java.lang.NullPointerException
at uk.ac.aber.dcs.pirate_hangman.Model.load(Model.jav a:108)

Look at line 108. What variable on that line has a null value when it is executed?
Use a println() to print the values of all the variables used on line 108 if you can't tell which is null.

You can't be looking at the correct line if this is the line at 108:
for (int i=0;i<num;i++){

There aren't any object references on that line. You do not get the NullPointerException with a primitive like an int.

NOTE: the value of 0 in an primitive is NOT the same as the value of null in an object reference variable.

The error is happening at this line: pirateWordsList.add(secretPirateWord);

There are two variables on that line.
pirateWordsList
secretPirateWord
What are their values?

Null for both variables.

Why is the variable for the ArrayList null? Look at where that variable is given a value. Make sure that there is NOT another variable defined that is "shadowing" the class level variable.

Originally Posted by Norm

Why is the variable for the ArrayList null? Look at where that variable is given a value. Make sure that there is NOT another variable defined that is "shadowing" the class level variable.

Okay I will check now.

--- Update ---

The error definatly appears to happen when I add the words to the array.
I have added some test code as shown below, and I have put the output below that:

public void runModel(){
		ArrayList<String>pirateWordsList = new ArrayList<String>();
		System.out.println(pirateWordsList);
		System.out.println("What is the name of the file you would like to load? (The file included is called  piratewords.txt'");
		Scanner in=new Scanner(System.in);
		String file=in.next();
		System.out.println(file);
		load(file);
		System.out.println(pirateWordsList);
	    System.out.println(secretPirateWord);
	    in.close();
	}
 
public void load(String pirate){
	        Scanner infile;
			try {
				infile = new Scanner(new InputStreamReader
				(new FileInputStream(pirate)));
				 int num=infile.nextInt();infile.nextLine();
			        for (int i=0;i<num;i++){
			           String secretPirateWord=infile.nextLine();
			           System.out.println(secretPirateWord);
			           System.out.println(pirateWordsList);
			            pirateWordsList.add(secretPirateWord);
			            System.out.println(secretPirateWord);
			        }
			        infile.close();
 
			} 
			catch (FileNotFoundException e) {
				System.out.println("File not found, are you sure you entered the files name correctly and is the file in the correct directory");
			}
 
}

The output from in response to that code is:
[]
What is the name of the file you would like to load? (The file included is called piratewords.txt'
piratewords.txt
piratewords.txt
ahoy there
null
Exception in thread "main" java.lang.NullPointerException
at uk.ac.aber.dcs.pirate_hangman.Model.load(Model.jav a:116)
at uk.ac.aber.dcs.pirate_hangman.Model.runModel(Model .java:48)
at uk.ac.aber.dcs.pirate_hangman.Main.main(Main.java: 6)

Why is the variable: pirateWordsList null? Look at where that variable is given a value. Make sure that there is NOT another variable defined that is "shadowing" the class level variable.
Is pirateWordsList defined in more than one place?

It looks like the code NEVER assigns a value to the class instance variable. The class instance variable is what is used in the load() method. but its value is null.
The local variable defined in runModel() is a different variable from the class instance variable. It "shadows" or hides the class instance variable.

Originally Posted by Norm

It looks like the code NEVER assigns a value to the class instance variable. The class instance variable is what is used in the load() method. but its value is null.
The local variable defined in runModel() is a different variable from the class instance variable. It "shadows" or hides the class instance variable.

Okay well I tried to solve that my actually placing the load method in the runModel but have still had no joy. Do you have any suggestions please?

--- Update ---

I now altered it by actually initialising the Array List when it's being declared as an instance variable(I know that technically goes against the stanrads but I done it for test puproses) and I got these errors:
Exception in thread "main" java.lang.IllegalStateException: Scanner closed
at java.util.Scanner.ensureOpen(Scanner.java:1115)
at java.util.Scanner.findWithinHorizon(Scanner.java:1 713)
at java.util.Scanner.nextLine(Scanner.java:1583)
at uk.ac.aber.dcs.pirate_hangman.Model.load(Model.jav a:113)
at uk.ac.aber.dcs.pirate_hangman.Model.runModel(Model .java:47)
at uk.ac.aber.dcs.pirate_hangman.Main.main(Main.java: 6)

Do you have any suggestions

Define the variable in ONE place at the class level and
either assign it a value where it is defined
or assign it a value in a method.
Do NOT define variables with the same name in a method as is defined at the class level.

Exception in thread "main" java.lang.IllegalStateException: Scanner closed
at java.util.Scanner.ensureOpen(Scanner.java:1115)
at java.util.Scanner.findWithinHorizon(Scanner.java:1 713)
at java.util.Scanner.nextLine(Scanner.java:1583)
at uk.ac.aber.dcs.pirate_hangman.Model.load(Model.jav a:113)

The code at line 113 called the nextLine() method after the Scanner object has been closed.
Don't close the Scanner until the program is done using it.