Java counting string occurrences in ArrayList

Question

I have written a program where I have N strings and Q queries that are also strings. The goal is to determine how many times each query appears in the N strings.

This is my code:

import java.util.Scanner;
import java.util.ArrayList;

public class SparseArrays{


// count the number of occurances of a string in an array
int countStringOccurance(ArrayList<String> arr, String str){
    int count = 0;
    for (int i=0; i<arr.size(); i++) {
        if (str==arr.get(i)) {
            count += 1;
        }
    }
    return count;
}


void start(){
    // scanner object
    Scanner input = new Scanner(System.in);

    // ask for user to input num strings
    System.out.println("How many string would you like to enter?");
    // store the number of strings
    int numStrings = input.nextInt();
    // to get rid of extra space
    input.nextLine();

    // ask user to enter strings
    System.out.println("Enter the "+numStrings+" strings.");
    ArrayList<String> stringInputArray = new ArrayList<String>();
    for (int i=0; i<numStrings; i++) {
        stringInputArray.add(input.nextLine());
    } // all strings are in the stringInputArray

    // ask user to input num queries
    System.out.println("Enter number of queries.");
    int numQueries = input.nextInt();
    // to get rid of extra space
    input.nextLine();

    // ask user to enter string queries
    System.out.println("Enter the "+numQueries+" queries.");
    ArrayList<String> stringQueriesArray = new ArrayList<String>();
    for (int i=0; i<numQueries; i++) {
        stringQueriesArray.add(input.nextLine());
    } // all string queries are in the stringQueriesArray

    for (int i=0; i<stringQueriesArray.size(); i++) {
        int result = 
      countStringOccurance(stringInputArray,stringQueriesArray.get(i));
        System.out.println(result);
    }
}

void printArr(ArrayList<String> arr){
    for (int i=0; i<arr.size(); i++) {
        System.out.println(arr.get(i));
    }
    System.out.println(" ");
}

public static void main(String[] args) {
    SparseArrays obj = new SparseArrays();
    obj.start();
}
}

When I run my code and enter 4 strings such as {abc,abc,abc,def} and 3 queries such as {abc,def,ghi} I expect to get 3, 1 and 0 for my output since there are 3 "abc", 1 "def" and 0 "ghi". However, the output is zero for all the queries.

I am pretty sure the problem is from the method int countStringOccurance(ArrayList arr, String str) which is supposed to give me the number of times a string is repeated in an ArrayList of Strings.

Am I missing anything here?

Answer 1

Use String.equals() to compare strings, not ==. Instead of:

if (str1==str2)   //  WRONG!!!

write

if (str1.equals(str2))  // Correct

Answer 2

// count the number of occurances of a string in an array
int countStringOccurance(ArrayList<String> arr, String str){
    int count = 0;
    for (int i=0; i<arr.size(); i++) {
        if (str.equals(arr.get(i))) { //<-- change your condition to this
            count += 1;
        }
    }
    return count;
}

Read more on What is the difference between == vs equals() in Java?

Answer 3

Use .equals() between two objects, not ==. And then, when you have an error, try to put some log trace in the sensitive part, with simple System.out.println() for example, in order to check if each object and variables are well filled. Can help you to solve this kind of problem !

Answer 4

There's already a builtin method that handles this, Collections#frequency:

int occurrences = Collections.frequency(list, "String here");