Reverse a given sentence in Java

Question

Can anyone tell me how to write a Java program to reverse a given sentence?

For example, if the input is:

\"This is an interview question\"

Answer 1

public class ReverseString {

    public void reverse(String[] source) {

        String dest = "";
        for (int n = source.length - 1; n >= 0; n--) {
            dest += source[n] + " ";
        }
        System.out.println(dest);

    }

    public static void main(String args[]) {
        ReverseString rs = new ReverseString();
        String[] str = "What is going on".split(" ");
        rs.reverse(str);

    }

}

Answer 2

I also give it a try: Here's a version using a stack and a scanner:

String input = "this is interview question";
Scanner sc = new Scanner(input);
Stack<String> stack = new Stack<String>();

while(sc.hasNext()) {
    stack.push(sc.next());
}

StringBuilder output = new StringBuilder();

for(;;) { // forever
    output.append(stack.pop());

    if(stack.isEmpty()) {
        break; // end loop
    } else {
        output.append(" ");
    }
}

Answer 3

Bozho already gave a great Java-specific answer, but in the event you ever need to solve this problem without Java API methods:

To reverse, you can simply pop individual words onto a stack and pop them all back off when there are no words left.

(Just to be extra clear, Java does provide a Stack class, so it is possible to use this method in Java as well).

Answer 4

You split the string by the space then iterate over it backwards to assemble the reversed sentence.

String[] words =  "This is interview question".split(" ");

String rev = "";
for(int i = words.length - 1; i >= 0 ; i--)
{
   rev += words[i] + " ";
}

// rev = "question interview is This "

// can also use StringBuilder:
StringBuilder revb = new StringBuilder();
for(int i = words.length - 1; i >= 0 ; i--)
{
   revb.append(words[i]);
   revb.append(" ");
}
// revb.toString() = "question interview is This "

Answer 5

No one has mentioned a vanilla Java 8 based solution yet, which is the same as Bozho's, but without any third-party libraries. So here it is:

String input = "This is interview question";

List<String> list = Arrays.asList(input.split(" "));
Collections.reverse(list);
System.out.println(list.stream().collect(Collectors.joining(" ")));

Answer 6

Before StringTokenizer was declared legacy, many used StringTokenizer for this. Thought I would just leave it here.

String sentence = "This is interview question";
String reversed = "";
StringTokenizer tokens = new StringTokenizer(sentence); 

while (tokens.hasMoreTokens()) { // Loop through each token
    reversed =  tokens.nextToken() + ' ' + reversed; //add to start
}

System.out.println(reversed.trim());