How to reverse a String in Java ?

• 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 a 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 at the corner cases and treat them separately.

• Using 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();
}
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 in order 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();
}