Question

This code is inside the main function:

Scanner input = new Scanner(System.in);

System.out.println("Type a sentence");
String sentence = input.next();

Stack<Character> stk = new Stack<Character>();
int i = 0;

while (i < sentence.length())
{
    while (sentence.charAt(i) != ' ' && i < sentence.length() - 1)
    {
        stk.push(sentence.charAt(i));
        i++;
    }
    stk.empty();
    i++;
}

And this is the empty() function:

public void empty()
{
    while (this.first != null)
        System.out.print(this.pop());
}

It doesn’t work properly, as by typing example sentence I am getting this output: lpmaxe. The first letter is missing and the loop stops instead of counting past the space to the next part of the sentence.

I am trying to achieve this:

This is a sentence —> sihT si a ecnetnes

Answer 1

Per modifications to the original post, where the OP is now indicating that his goal is to reverse the letter order of the words within a sentence, but to leave the words in their initial positions.

The simplest way to do this, I think, is to make use of the String split function, iterate through the words, and reverse their orders.

String[] words = sentence.split(" "); // splits on the space between words

for (int i = 0; i < words.length; i++) {
    String word = words[i];
    System.out.print(reverseWord(word));

    if (i < words.length-1) {
        System.out.print(" "); // space after all words but the last
    }
}

Where the method reverseWord is defined as:

public String reverseWord(String word) {
    for( int i = 0; i < word.length(); i++) {
        stk.push(word.charAt(i));
    }
    return stk.empty();
}

And where the empty method has been changed to:

public String empty() {
    String stackWord = "";
    while (this.first != null)
        stackWord += this.pop();
    return stackWord;
}

Original response

The original question indicated that the OP wanted to completely reverse the sentence.

You’ve got a double-looping construct where you don’t really need it.

Consider this logic:

1. Read each character from the input string and push that character to the stack
2. When the input string is empty, pop each character from the stack and print it to screen.

So:

for( int i = 0; i < sentence.length(); i++) {
    stk.push(sentence.charAt(i));
}
stk.empty();