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