Thread: 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)

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

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.

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:

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.

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:

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]

	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;
	}

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");