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?

Code :

---

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;
	}
}

---

Code :

---

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:

Quote:

---

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

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

Quote:

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Originally Posted by pbrockway2

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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String reverseWords = reverseArray.toString();

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:

Code :

---

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.

Quote:

---

Originally Posted by pbrockway2

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:

Code :

---

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]

Code :

---

	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;
	}

---

Quote:

---

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:

Code :

---

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");

---

How about just using Collections.reverse(list_parameters)? That saves a lot of your time