How to count the number of times a string appears in an array without using maps?

Question

I am trying to count the number of times a string appears in an array, but I cannot use any type of Map. From my research, I tried the following expecting it to work:

int count = Collections.frequency(strings, search);

However, when I got an error when running the program. The error was a Null Pointer Exception error and I'm not sure how to fix that or why it happened. Any ideas on whats wrong with my code?

EDIT: Here's my full code. I can't find where the issue lies. Maybe someone else can find it import java.util.*;

public class Lab9 {

 public static void main(String[] args){
     Scanner kb =  new Scanner (System.in);
     String name = null;
     List<String> strings =null;
     System.out.println("How many strings do you want to enter?:");
     int size = kb.nextInt();

     List<String> String = getStrings(size,name);
     System.out.println(String);
     System.out.println("What string would you like to search for?:");
     String search = kb.next();

     int numofTimes = countValues (search,strings);
     System.out.print("That word appears "+numofTimes+" times in the array.");


 }
 public static List<String> getStrings(int size,String name) {
     Scanner kb = new Scanner (System.in);
     int count = 0;
     List<String> strings = new ArrayList<String>();
     while (count != size) {
         System.out.println("Enter a string:");
         name = kb.nextLine();
         strings.add(name);
         count = count + 1;
          }

return strings;

 }
 public static int countValues (String search, List<String> strings){

     int count = Collections.frequency(strings , search);



     return count;
 }



 }

Answer 1

You can do a linear search through the array

    String strings[] = {"A","B",null,"C","A",null}; // the array contains nulls
    String search = "A";
    int count = 0;
    for (int i = 0;i< strings.length ;i++ ) 
    {
        if(strings[i] != null)
        {
            if(strings[i].equals(search))
            {
                count++;
            }
        }                   
    }
    System.out.println(count);  

Answer 2

Do a null and size check on strings before using it:

if (strings != null && strings.size() > 0) {
    int count = Collections.frequency(strings, search);
}

If size() returns a number larger than 0 that means your strings has something in it and you can perform frequency() on it.

Answer 3

Check out http://www.tutorialspoint.com/java/util/collections_frequency.htm

There's a good example of the use of frequency(Collection c, Object o);

The nullpointerexception can occur if one of the values in your collection is null. Are you sure that the excpetion is at the line of your frequency?

Answer 4

This is the code for Collections.frequency() :

public static int frequency(Collection<?> c, Object o) {
    int result = 0;
    if (o == null) {
        for (Object e : c)
            if (e == null)
                result++;
    } else {
        for (Object e : c)
            if (o.equals(e))
                result++;
    }
    return result;
}

As you can see, it is able to deal with o == null and with c containing null elements. The only case it will throw a NullPointerException is when c == null .

Also, from the documentation:

frequency

public static int frequency(Collection c, Object o)

Throws:

• NullPointerException - if c is null