Java: Count the occurrences of a given string in another given string

Last update on December 21 2024 09:43:24 (UTC/GMT +8 hours)

Write a Java program to count the occurrences of a given string in another given string.

Visual Presentation:

Sample Solution:

Java Code:

// Importing Arrays class from java.util package
import java.util.Arrays;

// Main class declaration
public class Main {    
    // Main method, entry point of the program
    public static void main(String[] args) {
        // Define main string and sub string
        String main_string = "abcd abc aabc baa abcaa";
        String sub_string = "aa";
        
        // Count occurrences of sub string in main string using method count_sub_str_in_main_str
        int countV1 = count_sub_str_in_main_str(main_string, sub_string);
        
        // Print the count of occurrences of sub string in main string
        System.out.println(sub_string + "' has occured " + countV1 + " times in '" + main_string + "'");            
    }
    
    // Method to count occurrences of sub string in main string
    public static int count_sub_str_in_main_str(String main_string, String sub_string) {
        // Check for null strings
        if (main_string == null || sub_string == null) {
            throw new IllegalArgumentException("The given strings cannot be null");
        }
        
        // Check for empty strings
        if (main_string.isEmpty() || sub_string.isEmpty()) {
            return 0;
        }
        
        // Initialize variables for position and counter
        int position = 0;
        int ctr = 0;
        int n = sub_string.length();
        
        // Loop to find occurrences of sub string in main string
        while ((position = main_string.indexOf(sub_string, position)) != -1) {
            position = position + n;
            ctr++;
        }
        return ctr;
    }
}

Sample Output:

aa' has occured 3 times in 'abcd abc aabc baa abcaa'

Flowchart:

Count string in another string.

Main.java Code:

//MIT License: https://bit.ly/35gZLa3
import java.time.Clock;
import java.util.concurrent.TimeUnit;

public class Main {

    private static final String STRING = "111111";
    private static final String SUBSTRING = "11";

    public static void main(String[] args) {

        Clock clock = Clock.systemUTC();
        long startTimeV1 = clock.millis();
        int countV1 = Strings.countStringInStringV1(STRING, SUBSTRING);
        displayExecutionTime(clock.millis() - startTimeV1);
        System.out.println("V1: '" + SUBSTRING + "' has occured " + countV1 + " times in '" + STRING + "'");

        long startTimeV2 = clock.millis();
        int countV2 = Strings.countStringInStringV2(STRING, SUBSTRING);
        displayExecutionTime(clock.millis() - startTimeV2);
        System.out.println("V2: '" + SUBSTRING + "' has occured " + countV2 + " times in '" + STRING + "'");

        long startTimeV3 = clock.millis();
        int countV3 = Strings.countStringInStringV3(STRING, SUBSTRING);
        displayExecutionTime(clock.millis() - startTimeV3);
        System.out.println("V3: '" + SUBSTRING + "' has occured " + countV3 + " times in '" + STRING + "'");
    }

    private static void displayExecutionTime(long time) {
        System.out.println("Execution time: " + time + " ms" + " ("
                + TimeUnit.SECONDS.convert(time, TimeUnit.MILLISECONDS) + " s)");
    }

}

Strings.java Code:

//MIT License: https://bit.ly/35gZLa3
import java.util.regex.Matcher;
import java.util.regex.Pattern;

public final class Strings {

    private Strings() {
        throw new AssertionError("Cannot be instantiated");
    }

    public static int countStringInStringV1(String string, String toFind) {

        if (string == null || toFind == null) {
            throw new IllegalArgumentException("The given strings cannot be null");
        }

        if (string.isEmpty() || toFind.isEmpty()) {
            return 0;
        }

        int position = 0;
        int count = 0;
        int n = toFind.length();

        while ((position = string.indexOf(toFind, position)) != -1) {
            position = position + n;
            count++;
        }

        return count;
    }

    public static int countStringInStringV2(String string, String toFind) {

        if (string == null || toFind == null) {
            throw new IllegalArgumentException("The given strings cannot be null");
        }

        if (string.isEmpty() || toFind.isEmpty()) {
            return 0;
        }

        return string.split(Pattern.quote(toFind), -1).length - 1;
    }

    public static int countStringInStringV3(String string, String toFind) {

        if (string == null || toFind == null) {
            throw new IllegalArgumentException("The given strings cannot be null");
        }

        if (string.isEmpty() || toFind.isEmpty()) {
            return 0;
        }

        Pattern pattern = Pattern.compile(Pattern.quote(toFind));
        Matcher matcher = pattern.matcher(string);

        int position = 0;
        int count = 0;
        while (matcher.find(position)) {           
            position = matcher.start() + 1;
            count++;
        }

        return count;
    }
}

Sample Output:

Execution time: 1 ms (0 s)
V1: '11' has occured 3 times in '111111'
Execution time: 2 ms (0 s)
V2: '11' has occured 3 times in '111111'
Execution time: 1 ms (0 s)
V3: '11' has occured 5 times in '111111'

Flowchart:

Java Code Editor:

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