﻿ Java - Counts occurrences of a certain character in a string

# Java: Counts occurrences of a certain character in a given string

## Java String: Exercise-107 with Solution

Write a Java program to count 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:

Java Code Editor:

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Previous Java Exercise: Concatenate a given string with itself of a given number of times.
Next Java Exercise: Check two consecutive, identical letters in a given string.

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