Find indexes of all occurrences of character in a String in Java

This post will discuss how to find indexes of all occurrences of a character in a string in Java.

1. Using `indexOf()` and `lastIndexOf()` method

The String class provides an indexOf() method that returns the index of the first appearance of a character in a string.

public class Main

{

public static void main(String[] args)

{

String str = "C, C++, Java, C#, Kotlin";

char ch = ',';

int index = str.indexOf(ch);

System.out.println(index); // 1

}

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To get the indices of all occurrences of a character in a String, you can repeatedly call the indexOf() method within a loop. The following example demonstrates how to use the indexOf() method efficiently, where the search for the next index starts from the previous index.

public class Main

{

public static void main(String[] args)

{

String str = "C, C++, Java, C#, Kotlin";

char ch = ',';

int index = str.indexOf(ch);

while (index != -1) {

System.out.println(index);

index = str.indexOf(ch, index + 1);

}

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Output:

1
6
12
16

Or even shorter:

public class Main

{

public static void main(String[] args)

{

String str = "C, C++, Java, C#, Kotlin";

char ch = ',';

int index = -1;

while ((index = str.indexOf(ch, index + 1)) != -1) {

System.out.println(index);

}

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Output:

1
6
12
16

If you just need the index of the character’s last appearance in a string, use the lastIndexOf() method.

public class Main

{

public static void main(String[] args)

{

String str = "C, C++, Java, C#, Kotlin";

char ch = ',';

int lastIndex = str.lastIndexOf(ch);

System.out.println(lastIndex); // 16

}

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2. Using `IntStream.iterate()` method

With Java 9, you can use the IntStream.iterate(seed, hasNext, next) method which returns a sequential ordered IntStream produced by application of the next function to an initial element seed, conditioned on satisfying the hasNext predicate. Following is a simple example demonstrating usage of this:

import java.util.List;

import java.util.stream.Collectors;

import java.util.stream.IntStream;

public class Main

{

public static void main(String[] args)

{

String str = "C, C++, Java, C#, Kotlin";

char ch = ',';

List<Integer> indices = IntStream.iterate(str.indexOf(ch), i -> i != -1,

i -> str.indexOf(ch, i + 1))

.boxed().collect(Collectors.toList());

System.out.println(indices);

}

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Output:

[1, 6, 12, 16]

3. Using Regex

To get the indexes of all appearances of a character in a String, you can also use a regex. The idea is to create a matcher to match the string for the specified character. Here’s how the code would look like:

import java.util.regex.Matcher;

import java.util.regex.Pattern;

public class Main

{

public static void main(String[] args)

{

String str = "C, C++, Java, C#, Kotlin";

String ch = ",";

Matcher matcher = Pattern.compile(ch).matcher(str);

while (matcher.find()) {

System.out.println(matcher.start());

}

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Output:

1
6
12
16

That’s all about finding indexes of all occurrences of a character in a string in Java.

Find indexes of all occurrences of character in a String in Java

1. Using indexOf() and lastIndexOf() method

2. Using IntStream.iterate() method

3. Using Regex

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1. Using `indexOf()` and `lastIndexOf()` method

2. Using `IntStream.iterate()` method