public String reverse(String word) {
if ((word == null) || (word.length() <= 1)) {
return word;
}
return reverse(word.substring(1)) + word.charAt(0);
}
I have this code that professor sent me but I don't get it. I know what recursion is but I'm still a newbie at Java Programming so if anybody would care to explain to me the part return reverse(word.substring(1)) + word.charAt(0);
what does the subString(1) does and the chartAt(0)?
This is recursion. Here are documentation for subString() and charAt(). Coming to how this works:
public static String reverse(String word) {
if ((word == null) || (word.length() <= 1)) {
return word;
}
return reverse(word.substring(1)) + word.charAt(0);
}
Pass1: reverse("user") : return reverse("ser")+'u';
Pass2: reverse("ser")+'u' : return reverse("er")+'s'+'u';
Pass3: reverse("er")+'s'+'u' : return reverse("r")+'e'+'s'+'u';
Pass4: reverse("r")+'e'+'s'+'u' : return 'r'+'e'+'s'+'u'; // because here "r".length()==1
The way the recursive part of this works is that to reverse a string, you remove the first character, reverse what's left, and then append the first character to the result. That's what the prof's code is doing.
word.substring(1) returns the substring starting at index 1 and going to the endword.charAt(0) returns the character at index 0There's a bit more going on when the two pieces are appended using +. The issue is that word.charAt(0) has a return type of char. Since the left-hand part of the + is a String, the Java language rules say that the right-hand side must be converted to a String if it isn't one. So the char value is first converted to a Character and then the toString() method of the Character class is called. This returns a String consisting of the single character.
It might have been more efficient code to write that line like:
return reverse(word.substring(1)) + word.substring(0, 1);
The two-argument version of substring returns the substring between the two indexes. That would eliminate the autoboxing and conversion to String.
return reverse(word.substring(1)) + word.charAt(0);
you should read it this way:
wordif you assume this function reverses the strings of length N, you can easily see that it must reverse the strings of length N+1. If you realize that the word with at most one letter is the same if reversed (the first three lines of code), you have a complete very simple proof using Mathematical Induction that this function really reverses the string.
