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Thread: Reverse the order of Strings in an ArrayList?

  1. #1
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    Default Reverse the order of Strings in an ArrayList?

    Hey guys. I'm trying to make a class called "purse" where I make an array list and store String in it (quarters, dimes, etc), and then I print out the names of each coin I stored in the purse. I got the printing out part just fine. Then I have to make a method where I reverse the order of the names and prints them out as well. I think I got this one right until I print them out and notice that I get an error in the output. This is the "reverse" method. Could anyone tell me what I'm doing wrong here?

    import java.util.ArrayList;
     
    public class Purse
    {
    	private ArrayList<String> coins;
    	private ArrayList<String> reverseArray;
     
    	public Purse()
    	{
    		ArrayList<String> coin = new ArrayList<String>();
    		coins = coin;
    	}
     
    	public void addCoin(String coinName)
    	{
    		coins.add(coinName);
    	}
     
    	public String reverse()
    	{
    		int n = coins.size();
    		for(int i = n; i == 0; i--)
    		{
    			String s = coins.get(n);
    			reverseArray.add(s);
    		}
    		String reverseWords = reverseArray.toString();
    		return reverseWords;
    	}
     
     
    	public String toString()
    	{
    		String words = coins.toString();
    		return words;
    	}
    }

    public class PurseTester 
    {
    	public static void main(String[] args)
    	{
    		Purse jean = new Purse();
    		jean.addCoin("Quarter");
    		jean.addCoin("Dime");
    		jean.addCoin("Nickel");
    		jean.addCoin("Dime");
     
    		System.out.println(jean.toString());
    		System.out.println(jean.reverse());
    	}
     
    }

    Output:
    [Quarter, Dime, Nickel, Dime]
    Exception in thread "main" java.lang.NullPointerException
    at Purse.reverse(Purse.java:27)
    at PurseTester.main(PurseTester.java:12)


  2. #2
    Super Moderator pbrockway2's Avatar
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    Default Re: Reverse the order of Strings in an ArrayList?

    Which is line 27 of Purse.java? The runtime exception is saying that you are using a variable here that is null.

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    Default Re: Reverse the order of Strings in an ArrayList?

    Quote Originally Posted by pbrockway2 View Post
    Which is line 27 of Purse.java? The runtime exception is saying that you are using a variable here that is null.
    String reverseWords = reverseArray.toString();

  4. #4
    Super Moderator pbrockway2's Avatar
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    Default Re: Reverse the order of Strings in an ArrayList?

    OK. So reverseArray is null - that is it has not been assigned a value yet. And you cannot call a method like toString() on something that doesn't exist.

    You can test this claim (that reverseArray is null) easily enough:

    public String reverse()
    {
        int n = coins.size();
        for(int i = n; i == 0; i--)
        {
            String s = coins.get(n);
            reverseArray.add(s);
        }
        System.out.println("About to call toString() with reverseArray = " + reverseArray);
        String reverseWords = reverseArray.toString();
        return reverseWords;
    }

    If it turns out that reverseArray is null, go back to where you thought you had assigned it a value and figure out why that didn't happen. If you simply forgot to assign it any value, then do so and see what happens.

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    Default Re: Reverse the order of Strings in an ArrayList?

    Quote Originally Posted by pbrockway2 View Post
    OK. So reverseArray is null - that is it has not been assigned a value yet. And you cannot call a method like toString() on something that doesn't exist.

    You can test this claim (that reverseArray is null) easily enough:

    public String reverse()
    {
        int n = coins.size();
        for(int i = n; i == 0; i--)
        {
            String s = coins.get(n);
            reverseArray.add(s);
        }
        System.out.println("About to call toString() with reverseArray = " + reverseArray);
        String reverseWords = reverseArray.toString();
        return reverseWords;
    }

    If it turns out that reverseArray is null, go back to where you thought you had assigned it a value and figure out why that didn't happen. If you simply forgot to assign it any value, then do so and see what happens.
    Ok, so I stated a value to the array and effectively it fixed the Exception. However, now when I print it I just get an empty array instead of the expected words in reverse order. What is wrong with my loop?

    [CODE]import java.util.ArrayList;

    public class Purse
    {
    private ArrayList<String> coins;
    private ArrayList<String> reverseArray;
    private String reverseWords;

    public Purse()
    {
    ArrayList<String> coin = new ArrayList<String>();
    coins = coin;
    ArrayList<String> reversed = new ArrayList<String>();
    reverseArray = reversed;
    }

    public void addCoin(String coinName)
    {
    coins.add(coinName);
    }

    public String reverse()
    {
    int n = coins.size();
    String reverseWords;

    for(int i = n; i == 0; i--)
    {
    String s = coins.get(n);
    reverseArray.add(s);
    }

    reverseWords = reverseArray.toString();
    return reverseWords;
    }


    public String toString()
    {
    String words = coins.toString();
    return words;
    }
    }
    [CODE]

  6. #6
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    Default Re: Reverse the order of Strings in an ArrayList?

    	public String reverse()
    	{
    		int n = coins.size();
    		String reverseWords;
     
    		for(int i = n; i == 0; i--)
    		{
    			String s = coins.get(n);
    			reverseArray.add(s);
    		}
     
    		reverseWords = reverseArray.toString();
    		return  reverseWords;
    	}

  7. #7
    Super Moderator pbrockway2's Avatar
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    Default Re: Reverse the order of Strings in an ArrayList?

    now when I print it I just get an empty array instead of the expected words in reverse order. What is wrong with my loop?
    System.out.println() - the poor man's debugger - to the rescue again...

    Have a look at what your loop is doing:

    System.out.println("About to start loop");
    for(int i = n; i == 0; i--)
    {
        System.out.println("In loop, i = " + i);
        String s = coins.get(n);
        reverseArray.add(s);
    }
    System.out.println("Loop finished");

  8. #8
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    Default Re: Reverse the order of Strings in an ArrayList?

    How about just using Collections.reverse(list_parameters)? That saves a lot of your time
    to do or not to do is a question

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