In Java, would all these methods be considered pure functions?

Question

This program determines whether the string the user has input is a palindrome or not.

import acm.program.ConsoleProgram;

public class PurePalindrome extends ConsoleProgram {

public void run() {
    String originalString;
    String reversedString;
    boolean isPalindrome;

    originalString = readLine("? ");
    reversedString = reverseString(originalString);
    isPalindrome = checkPalindrome(originalString, reversedString);

    println("The word you entered " + determineWord(isPalindrome)
            + " a palindrome. " + originalString + " reversed is: "
            + reversedString + ".");
}

private boolean checkPalindrome(String word, String revWord) {
    if (revWord.equals(word)) {
        return true;
    } else {
        return false;
    }
}

private String reverseString(String wordToReverse) {
    String reversedWord = "";
    for (int i = 0; i < wordToReverse.length(); i++) {
        reversedWord = wordToReverse.charAt(i) + reversedWord;
    }
    return reversedWord;
}

private String determineWord(boolean palindrome) {
    if (palindrome) {
        return "is";
    } else {
        return "is not";
    }

}

}

Would all these methods be considered pure functions? If not, why not? I'm having a bit of trouble determining whether a method is a pure function or not.

score 2 · Accepted Answer · answered Nov 30 '13 at 22:46

2

A method is a pure function if its returned value depends exclusively on its arguments, and not on anything else, and if it doesn't have any side effect.

So the last three methods are pure functions, whereas the first one is not: it doesn't return anything, depends on the user input, and has the side effect of printing on the screen.

Side note:

if (revWord.equals(word)) {
    return true;
} else {
    return false;
}

should be replaced by

return revWord.equals(word);

answered Nov 30 '13 at 22:46

JB Nizet

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Ok, thanks. The only thing is, unless you are counting the run method, I only see 3 methods. checkPalindrome, reverseString, and determineWord. I'm going to assume that you are referring to run as the 1st method. So the run method is not a pure function because there is user input and the user input is not in the methods arguments, say, as a string, so it is not pure. Correct? As for the side note, it is useful to know you could do that! I completely forgot. – user3053240 Nov 30 '13 at 22:52
Yes, run() is a method, and it's the first one to appear in your code, so that's indeed what I called "the first method". The run() one would be a pure function if, as the other ones, it took an argument, computed a result based on this argument, and returned that result. Without getting any information from elsewhere (like the user input), and without having any side effect (like changing what is displayed on the screen). – JB Nizet Nov 30 '13 at 23:09
So, if I were to call the reverseString() method from inside the checkPalindrome() method and store it in a local variable and change the checkPalindrome() arguments to only require the original word, then the checkPalindrome() method would no longer be a pure function because it is getting information from something other than its arguments? – user3053240 Nov 30 '13 at 23:20

In Java, would all these methods be considered pure functions?

1 Answers1