Exception in thread "main" java.lang.NullPointerException

[Asha Dore]

import java.util.ArrayList;
public class Weekdays
{
	private static final int DAYS_IN_WEEK = 7;
	
	private String[] dayNamesA;
	private ArrayList<String>dayNamesAL;
	
	/**
		Creates an instance with the seven day names
	*/	
	public Weekdays()
	{
		// Create space in dayNamesA for DAYS_IN_WEEK Strings
		dayNamesA = new String[DAYS_IN_WEEK];
		// Create a new ArrayList<String> for dayNamesAL
		ArrayList<String>dayNamesAL = new ArrayList<String>();
		
		// Fill the DAYS_IN_WEEK names into dayNamesA, one by one
		dayNamesA[0] = "Sunday";
		dayNamesA[1] = "Monday";
		dayNamesA[2] = "Tuesday";
		dayNamesA[3] = "Wednesday";
		dayNamesA[4] = "Thursday";
		dayNamesA[5] = "Friday";
		dayNamesA[6] = "Saturday";

	// Copy the names from dayNamesA into dayNamesAL with a for loop
		for(int i = 0; i < dayNamesA.length; i++) 

		{
			dayNamesAL.add(dayNamesA[i]);
		}		
	
	}

	/**
		Searches for the index of name in the list
		@param name the name of the week day to look up
		@return the index (0 - 6) or -1 if not found
	*/
	public int findDayA(String name)
	{
		// Returns the index of "name" in dayNamesA
		for (int i = 0; i < dayNamesA.length; i++)
      {
       if (dayNamesA[i].equals(name))
        {
         return(i);
        }
      }
  		return(-1);
		
		// Returns -1 if not found
	}
	
	/**
		Searches for the index of name in the list
		@param name the name of the week day to look up
		@return the index (0 - 6) or -1 if not found
	*/
	public int findDayAL(String name)
	{
		// Returns the index of "name" in dayNamesAL
		
		for(String names: dayNamesAL)
		{
			if(names.equals(name))
				System.out.println(name);
			
  				
		}
		return(-1);
			

      
		
		// Returns -1 if not found
	}
}

This compiles fine, but when I run the tester:

import java.util.Scanner;
public class WeekdaysTester
{
	public static void main(String[] args)
	{
		Scanner in = new Scanner(System.in);
		System.out.print("The name of a day please: ");
		Weekdays list = new Weekdays();
		String name = in.nextLine();
		System.out.println("Found in array: " + list.findDayA(name));
		System.out.println("Found in array list: " + list.findDayAL(name));
	}
}

I get these errors:at Weekdays.findDayAL(Weekdays.java:66)
at WeekdaysTester.main(WeekdaysTester.java:11)

[Zachary Pepin]

Hey, Asha. This problem is one of the reasons I suggest using an editor with syntax highlighting as it makes mistakes like this slightly more obvious. What has happened here is this:

You have created a field in the Weekdays class called dayNamesAL on line 7, but you have also created a local variable called dayNamesAL on line 17 by placing a type in front of the variables name. Now, Java does allow this, but unfortunately, the rest of the code in your constructor is initializing this local variable and not the field, so when findDayAL is called, the dayNamesAL field is still uninitialized resulting in the NullPointerException.

To correct this, simply remove the type information from in front of the variable name on line 17. That line should look like this.

dayNamesAL = new ArrayList<String>();

[Asha Dore]

Thank you so much!

[Luigi Plinge]

OP, what you're doing here is really simple... don't you feel a little bit bad taking 80 lines of code? Think of someone trying to maintain it!

You can start by eliminating your constructor and initializing the fields in place, then check out the indexOf method on ArrayList, so you'll end up with something like:

import java.util.*;

public class Weekdays {
    private String[] dayNamesA = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"};
    private ArrayList<String> dayNamesAL = new ArrayList<String>(Arrays.asList(dayNamesA));
    public int findDayA (String name) { return Arrays.asList(dayNamesA).indexOf(name); }
    public int findDayAL(String name) { return dayNamesAL.indexOf(name); }
}

True, in theory the findDayA method isn't quite as efficient as yours since it'll create a new ArrayList each time, but you should write the simplest code that will work, rather than trying to solve a performance problem that doesn't actually exist ("premature optimization").

[Campbell Ritchie]

A lot shorter, but don't encourage beginners to dispense with the constructor.

[Asha Dore]

Thanks for the tip! Part of the requirements for this project was to have the constructor, but any information is very useful in the long run!