如何在Java中使用返回类型为void的递归方法？

Question

So I understand how to use a recursive method that has some other return type other than void. Typically I would call the same method again in the same method( inside the recursive case), while decremented or increment some value in the call to reach the base case. Then at some point the base case is reached and the problem is solved, so it starts returning the value from every call. Along those lines.

BUT
What if the method has the return type void, so you can't call the method as it won't/can't return anything? I'm trying to write a sentence backwards, which I've solved both with a for loop and a resucrive method that can return a string value, but I'm not sure how to approach it if it's void which is what the assignment is asking for.
Edit: I should also mention the sentence can only be passed in the parameter

Thank you everyone for the information and the help!

Answer 1

Recursion doesn't work only with methods/functions that return values. Recursion means only that the method/function calls itself.

You must guarantee that there is at least one stop condition but this does not require the function to return a value. This is commonly achieved by incrementally changing one or more arguments that you pass each time the function recursively calls itself. When that/those arguments satisfy a certain condition your function no longer calls itself and all pending operations are solved.

I am not fully aware of the task you are trying to do but here is an example of a recursive function that writes a string backwards. I use PSEUDO-functions with names that hopefully are self-explanatory.

public void writeBackwards(String str) {
    // This is the negation of the stop condition, so the stop condition
    // is when the string is empty, in which case this function will do
    // nothing:
    if (!str.isEmpty()) {
        char firstCharacter = str.getFirstCharacter();
        str = str.removeFirstCharacter();
        writeBackwards(str); // the recursive call
        // The following operation will be pending, waiting for the
        // recursive call to be resolved first:
        writeCharacter(firstCharacter);
    }
}

Answer 2

You can use any mutable Object as a parameter of the recursive function to store the result. For example, the backwards-sentence problem you mentioned could be written as:

public void stringReverse(String s, int index, StringBuilder sb) {
    if (index < 0)
        return;
    sb.append(s.charAt(index));
    stringReverse(s, index - 1, sb);
}

And called like this

StringBuilder sb = new StringBuilder();
stringReverse(mySentence, mySentence.length() - 1, sb);

Answer 3

Just like in C++ you can pass in pointers, here in Java you can simply pass in a class object to your function to hold the value generated from the recursive calls of the function. A simple example reflecting your question to compute fibonacci number is following.

public class ComputeFibonacci {
  static class Fibonacci {
    public int ith;
    public int value;
    Fibonacci(int a, int b) {
      ith = a;
      value = b;
    }
  }

  private static void fibonacci(Fibonacci result) {
    if (result.ith == 1 || result.ith == 2) {
      result.value = 1;
    } else {
      Fibonacci left = new Fibonacci(result.ith - 1, 0);
      Fibonacci right = new Fibonacci(result.ith - 2, 0);
      fibonacci(left);
      fibonacci(right);
      result.value = left.value + right.value;
    }
  }

  public static void main(String[] args) {
    // Here we compute the 10th fibonacci number
    Fibonacci f = new Fibonacci(10, 0);
    fibonacci(f);
    System.out.println("The result is " + f.value);
  }
}

Good luck.