w3resource

Java String Exercises: Counts occurrences of a certain character in a given string

Java String: Exercise-107 with Solution

Write a Java program to counts occurrences of a certain character in a given string.

Sample Solution:

Main.java Code:

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

public class Main {

    private static final String TEXT = "My high school, the Illinois Mathematics and Science Academy, "
            + "showed me that anything is possible and that you're never too young to think big. "
            + "At 15, I worked as a computer programmer at the Fermi National Accelerator Laboratory, "
            + "or Fermilab. After graduating, I attended Stanford for a degree in economics and "
            + "computer science.";
    private static final char CHAR_TO_COUNT = 'u';

    private static final String TEXT_CP = "😍 I love 💕 you Ӝ so much 💕 😍";
    private static final String CHAR_TO_COUNT_CP = "Ӝ";   // Unicode: \u04DC, Code Point: 1244
    private static final String CHAR_TO_COUNT_CPS = "💕"; // Unicode: \uD83D\uDC95, Code Point: 128149

    public static void main(String[] args) {
        System.out.println("Input text: \n" + TEXT + "\n");

        System.out.println("\n\nASCII or 16 bits Unicode characters (less than 65,535 (0xFFFF)) examples:\n");

        System.out.println("replace() based solution:");
        long startTimeV1 = System.nanoTime();

        int countV1 = Strings
                .countOccurrencesOfACertainCharacterV1(TEXT, CHAR_TO_COUNT);

        displayExecutionTime(System.nanoTime() - startTimeV1);
        System.out.println("Character '" + CHAR_TO_COUNT + "' occured " + countV1 + " time(s)");

        System.out.println();
        System.out.println("charAt() based solution:");
        long startTimeV2 = System.nanoTime();

        int countV2 = Strings
                .countOccurrencesOfACertainCharacterV2(TEXT, CHAR_TO_COUNT);

        displayExecutionTime(System.nanoTime() - startTimeV2);
        System.out.println("Character '" + CHAR_TO_COUNT + "' occured " + countV2 + " time(s)");

        System.out.println();
        System.out.println("Java 8, functional-style solution:");
        long startTimeV3 = System.nanoTime();

        long countV3 = Strings
                .countOccurrencesOfACertainCharacterV3(TEXT, CHAR_TO_COUNT);

        displayExecutionTime(System.nanoTime() - startTimeV3);
        System.out.println("Character '" + CHAR_TO_COUNT + "' occured " + countV3 + " time(s)");

        System.out.println("\n--------------------------------------\n");
        
        System.out.println("Input text: \n" + TEXT_CP + "\n");
        
        System.out.println("\n\nIncluding Unicode surrogate pairs examples:\n");

        System.out.println("replace() based solution:");
        long startTimeV4 = System.nanoTime();

        int countV4 = Strings
                .countOccurrencesOfACertainCharacterVCP1(TEXT_CP, CHAR_TO_COUNT_CP);

        displayExecutionTime(System.nanoTime() - startTimeV4);
        System.out.println("Character '" + CHAR_TO_COUNT_CP + "' occured " + countV4 + " time(s)");

        System.out.println();
        System.out.println("replace() based solution:");
        long startTimeV5 = System.nanoTime();

        int countV5 = Strings
                .countOccurrencesOfACertainCharacterVCP1(TEXT_CP, CHAR_TO_COUNT_CPS);

        displayExecutionTime(System.nanoTime() - startTimeV5);
        System.out.println("Character '" + CHAR_TO_COUNT_CPS + "' occured " + countV5 + " time(s)");

        System.out.println();
        System.out.println("codePointAt() based solution:");
        long startTimeV6 = System.nanoTime();

        int countV6 = Strings
                .countOccurrencesOfACertainCharacterVCP2(TEXT_CP, CHAR_TO_COUNT_CP);

        displayExecutionTime(System.nanoTime() - startTimeV6);
        System.out.println("Character '" + CHAR_TO_COUNT_CP + "' occured " + countV6 + " time(s)");

        System.out.println();
        System.out.println("codePointAt() based solution:");
        long startTimeV7 = System.nanoTime();

        int countV7 = Strings
                .countOccurrencesOfACertainCharacterVCP2(TEXT_CP, CHAR_TO_COUNT_CPS);

        displayExecutionTime(System.nanoTime() - startTimeV7);
        System.out.println("Character '" + CHAR_TO_COUNT_CPS + "' occured " + countV7 + " time(s)");

        System.out.println();
        System.out.println("Java 8, functional-style solution:");
        long startTimeV8 = System.nanoTime();

        long countV8 = Strings
                .countOccurrencesOfACertainCharacterVCP3(TEXT_CP, CHAR_TO_COUNT_CP);

        displayExecutionTime(System.nanoTime() - startTimeV8);
        System.out.println("Character '" + CHAR_TO_COUNT_CP + "' occured " + countV8 + " time(s)");

        System.out.println();
        System.out.println("Java 8, functional-style solution:");
        long startTimeV9 = System.nanoTime();

        long countV9 = Strings
                .countOccurrencesOfACertainCharacterVCP3(TEXT_CP, CHAR_TO_COUNT_CPS);

        displayExecutionTime(System.nanoTime() - startTimeV9);
        System.out.println("Character '" + CHAR_TO_COUNT_CPS + "' occured " + countV9 + " time(s)");
    }

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

Strings.java Code:

//MIT License: https://bit.ly/35gZLa3
public final class Strings {

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

    public static int countOccurrencesOfACertainCharacterV1(String str, char ch) {

        if (str == null || str.isEmpty()) {
            // or throw IllegalArgumentException            
            return -1;
        }

        return str.length() - str.replace(String.valueOf(ch), "").length();
    }

    public static int countOccurrencesOfACertainCharacterVCP1(String str, String ch) {

        if (str == null || ch == null || str.isEmpty() || ch.isEmpty()) {
            // or throw IllegalArgumentException            
            return -1;
        }

        if (ch.codePointCount(0, ch.length()) > 1) {
            return -1; // there is more than 1 Unicode character in the given String
        }

        int result = str.length() - str.replace(ch, "").length();

        // if ch.length() return 2 then this is a Unicode surrogate pair
        return ch.length() == 2 ? result / 2 : result;
    }

    public static int countOccurrencesOfACertainCharacterV2(String str, char ch) {

        if (str == null || str.isEmpty()) {
            // or throw IllegalArgumentException
            return -1;
        }

        int count = 0;
        for (int i = 0; i < str.length(); i++) {
            if (str.charAt(i) == ch) {
                count++;
            }
        }
        return count;
    }

    public static int countOccurrencesOfACertainCharacterVCP2(String str, String ch) {

        if (str == null || ch == null || str.isEmpty() || ch.isEmpty()) {
            // or throw IllegalArgumentException            
            return -1;
        }

        if (ch.codePointCount(0, ch.length()) > 1) {
            return -1; // there is more than 1 Unicode character in the given String
        }

        int count = 0;
        int codePoint = ch.codePointAt(0);
        for (int i = 0; i < str.length(); i++) {
            if (str.codePointAt(i) == codePoint) {
                count++;
            }
        }

        return count;
    }

    public static long countOccurrencesOfACertainCharacterV3(String str, char ch) {

        if (str == null || str.isEmpty()) {
            // or throw IllegalArgumentException
            return -1;
        }

        return str.chars()
                .filter(c -> c == ch)
                .count();
    }
    
    public static long countOccurrencesOfACertainCharacterVCP3(String str, String ch) {

        if (str == null || ch == null || str.isEmpty() || ch.isEmpty()) {
            // or throw IllegalArgumentException            
            return -1;
        }

        if (ch.codePointCount(0, ch.length()) > 1) {
            return -1; // there is more than 1 Unicode character in the given String
        }

        int codePoint = ch.codePointAt(0);        
        
        return str.codePoints()
                .filter(c -> c == codePoint)
                .count();
    }
}

Sample Output:

Input text: 
My high school, the Illinois Mathematics and Science Academy, showed me that anything is possible and that you're never too young to think big. At 15, I worked as a computer programmer at the Fermi National Accelerator Laboratory, or Fermilab. After graduating, I attended Stanford for a degree in economics and computer science.



ASCII or 16 bits Unicode characters (less than 65,535 (0xFFFF)) examples:

replace() based solution:
Execution time: 2451480 ns (2 ms)
Character 'u' occured 5 time(s)

charAt() based solution:
Execution time: 40343 ns (0 ms)
Character 'u' occured 5 time(s)

Java 8, functional-style solution:
Execution time: 156518104 ns (156 ms)
Character 'u' occured 5 time(s)

--------------------------------------

Input text: 
😍 I love 💕 you Ӝ so much 💕 😍



Including Unicode surrogate pairs examples:

replace() based solution:
Execution time: 97920 ns (0 ms)
Character 'Ӝ' occured 1 time(s)

replace() based solution:
Execution time: 403313 ns (0 ms)
Character '💕' occured 2 time(s)

codePointAt() based solution:
Execution time: 31224 ns (0 ms)
Character 'Ӝ' occured 1 time(s)

codePointAt() based solution:
Execution time: 25059 ns (0 ms)
Character '💕' occured 2 time(s)

Java 8, functional-style solution:
Execution time: 1735086 ns (1 ms)
Character 'Ӝ' occured 1 time(s)

Java 8, functional-style solution:
Execution time: 41650 ns (0 ms)
Character '💕' occured 2 time(s)

Flowchart:

Flowchart: Java String Exercises - Count Occurrences Of Character
Flowchart: Java String Exercises - Count Occurrences Of Character

Java Code Editor:

Improve this sample solution and post your code through Disqus

Previous: Write a Java program to count the occurrences of a given string in another given string.
Next: Java Date, Time and Calendar Exercises

What is the difficulty level of this exercise?



Java: Tips of the Day

Directory Content:

Java allows you to get the names of all subdirectories and files in a folder as an array, which can then be sequentially viewed.

import java.io.*;
  
public class ListContents {
  public static void main(String[] args) {
    File file = new File("//home//user//Documents/");
    String[] files = file.list();
  
    System.out.println("Listing contents of " + file.getPath());
    for(int i=0 ; i < files.length ; i++)
    {
      System.out.println(files[i]);
    }
  }
}