How to Reverse a String in Java

1. Overview

In this quick tutorial, we’re going to see how we can reverse a String in Java.

We’ll start to do this processing using plain Java solutions. Next, we’ll have a look at the options that third-party libraries like Apache Commons provide.

Furthermore, we’ll demonstrate how to reverse the order of words in a sentence.

2. A Traditional for Loop

We know that strings are immutable in Java. An immutable object is an object whose internal state remains constant after it has been entirely created.

Therefore, we cannot reverse a String by modifying it. We need to create another String for this reason.

First, let’s see a basic example using a for loop. We’re going to iterate over the String input from the last to the first element and concatenate every character into a new String:

public String reverse(String input) {

    if (input == null) {
        return input;
    }

    String output = "";

    for (int i = input.length() - 1; i >= 0; i--) {
        output = output + input.charAt(i);
    }

    return output;
}

As we can see, we need to be careful with the corner cases and treat them separately.

To better understand the example, we can build a unit test:

@Test
public void whenReverseIsCalled_ThenCorrectStringIsReturned() {
    String reversed = ReverseStringExamples.reverse("cat");
    String reversedNull = ReverseStringExamples.reverse(null);
    String reversedEmpty = ReverseStringExamples.reverse(StringUtils.EMPTY);

    assertEquals("tac", reversed);
    assertNull(reversedNull);
    assertEquals(StringUtils.EMPTY, reversedEmpty);
}

3. A StringBuilder

Java also offers some mechanisms like StringBuilder and StringBuffer that create a mutable sequence of characters. These objects have a reverse() method that helps us achieve the desired result.

Here, we need to create a StringBuilder from the String input and then call the reverse() method:

public String reverseUsingStringBuilder(String input) {
    if (input == null) {
        return null;
    }

    StringBuilder output = new StringBuilder(input).reverse();
    return output.toString();
}

4. Stream API

The Stream API provides multiple convenient methods to reverse a given string.

Let’s take a close look at each option.

4.1. Using IntStream.range()

Typically, we can use the IntStream.range() method to reverse a particular string in a more functional style.

So, let’s see it in action:

public static String reverseUsingIntStreamRangeMethod(String str) {
    if (str == null) {
        return null;
    }

    char[] charArray = str.toCharArray();
    return IntStream.range(0, str.length())
      .mapToObj(i -> charArray[str.length() - i - 1])
      .collect(StringBuilder::new, StringBuilder::append, StringBuilder::append)
      .toString();
}

As we can see, we used the range() method to return a sequentially ordered IntStream between 0 and the string’s length.

Furthermore, we chained with mapToObj() to get each character starting from the end. Then, we used StringBuilder to build the reversed string.

Now, let’s add a test case to confirm that everything works as expected:

@Test
public void whenReverseStringUsingStreamRangeMethod_ThenCorrectStringIsReturned() {
    String reversed = ReverseStringExamples.reverseUsingStreamRangeMethod(STRING_INPUT);
    String reversedNull = ReverseStringExamples.reverseUsingStreamRangeMethod(null);
    String reversedEmpty = ReverseStringExamples.reverseUsingStreamRangeMethod(StringUtils.EMPTY);

    assertEquals(STRING_INPUT_REVERSED, reversed);
    assertNull(reversedNull);
    assertEquals(StringUtils.EMPTY, reversedEmpty);
}

As we can see, our method works as expected for non-null, null, and empty strings.

4.2. Using Stream.of()

Alternatively, we can use the Stream.of() method to achieve the same objective:

public static String reverseUsingStreamOfMethod(String str) {
    if (str == null) {
        return null;
    }

    return Stream.of(str)
      .map(string -> new StringBuilder(string).reverse())
      .collect(Collectors.joining());
}

As shown above, we used StringBuilder.reverse() as a mapper to invert the specified string.  Then, we concatenated the result into a new String object using the joining() collector.

Now, let’s confirm this solution using another test:

@Test
public void whenReverseStringUsingStreamOfMethod_ThenCorrectStringIsReturned() {
    String reversed = ReverseStringExamples.reverseUsingStreamOfMethod(STRING_INPUT);
    String reversedNull = ReverseStringExamples.reverseUsingStreamOfMethod(null);
    String reversedEmpty = ReverseStringExamples.reverseUsingStreamOfMethod(StringUtils.EMPTY);

    assertEquals(STRING_INPUT_REVERSED, reversed);
    assertNull(reversedNull);
    assertEquals(StringUtils.EMPTY, reversedEmpty);
}

4.3. Using String.chars()

Another solution would be using the chars() method from the String class.

This method is introduced in Java 9 and returns an IntStream containing the code points of the string’s characters.

Simply put, a code point is a decimal value associated with a character in the Unicode standard.

Let’s exemplify the use of the chars() method:

public static String reverseUsingCharsMethod(String str) {
    if (str == null) {
        return null;
    }

    return str.chars()
      .mapToObj(c -> (char) c)
      .reduce("", (a, b) -> b + a, (a2, b2) -> b2 + a2);
}

In a nutshell, we used mapToObj() to convert each code point into a character. Then, we accumulated the characters in reverse order using the reduce() method.

As always, let’s create a test case to test our method:

@Test
public void whenReverseStringUsingCharsMethod_ThenCorrectStringIsReturned() {
    String reversed = ReverseStringExamples.reverseUsingCharsMethod(STRING_INPUT);
    String reversedNull = ReverseStringExamples.reverseUsingCharsMethod(null);
    String reversedEmpty = ReverseStringExamples.reverseUsingCharsMethod(StringUtils.EMPTY);

    assertEquals(STRING_INPUT_REVERSED, reversed);
    assertNull(reversedNull);
    assertEquals(StringUtils.EMPTY, reversedEmpty);
}

5. Apache Commons

Apache Commons is a popular Java library with a lot of utility classes including string manipulation.

As usual, to get started using Apache Commons, we first need to add the Maven dependency:

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-lang3</artifactId>
    <version>3.12.0</version>
</dependency>

The StringUtils class is what we need here because it provides the reverse() method similar to StringBuilder.

One advantage of using this library is that its utility methods perform null-safe operations. So, we don’t have to treat the edge cases separately.

Let’s create a method that fulfills our purpose and uses the StringUtils class:

public String reverseUsingApacheCommons(String input) {
    return StringUtils.reverse(input);
}

Now, looking at these three methods, we can certainly say that the third one is the simplest and the least error-prone way to reverse a String.

6. Reversing the Order of Words in a Sentence

Now, let’s assume we have a sentence with words separated by spaces and no punctuation marks. We need to reverse the order of words in this sentence.

We can solve this problem in two steps: splitting the sentence by the space delimiter and then concatenating the words in reverse order.

First, we’ll show a classic approach. We’re going to use the String.split() method to fulfill the first part of our problem. Next, we’ll iterate backward through the resulting array and concatenate the words using a StringBuilder. Of course, we also need to add a space between these words:

public String reverseTheOrderOfWords(String sentence) {
    if (sentence == null) {
        return null;
    }

    StringBuilder output = new StringBuilder();
    String[] words = sentence.split(" ");

    for (int i = words.length - 1; i >= 0; i--) {
        output.append(words[i]);
        output.append(" ");
    }

    return output.toString().trim();
}

Second, we’ll consider using the Apache Commons library. Once again, it helps us achieve a more readable and less error-prone code. We only need to call the StringUtils.reverseDelimited() method with the input sentence and the delimiter as arguments:

public String reverseTheOrderOfWordsUsingApacheCommons(String sentence) {
    return StringUtils.reverseDelimited(sentence, ' ');
}

7. Conclusion

In this tutorial, we’ve first looked at different ways of reversing a String in Java. We went through some examples using core Java and a popular third-party library like Apache Commons.

Next, we’ve seen how to reverse the order of words in a sentence in two steps. These steps can also help achieve other permutations of a sentence.