📓4.16: Recursion

Table of Contents

• Recursion
  • Recursive Call
  • Check Your Understanding
  • Is This Recursive?
  • Summary

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📖 This page is a condensed version of CSAwesome Topic 4.16

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Recursion

Recursion is when a method calls itself. It is a form of repetition.

Example:

public static void neverEnd() {
    System.out.println("This is the method that never ends!");
    neverEnd();
}

This method will print "This is the method that never ends!" and then call itself, which will print out the message again, and then call itself, and so on. This is called infinite recursion, which is recursion that never ends.

In practice, it will end — by crashing with a StackOverflowError because there is a limit to how many times a method can recursively call itself.

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Recursive Call

Well-constructed recursive methods contain:

• At least one base case, which halts the recursion.
• At least one recursive call, which calls the method again with different parameters.

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Check Your Understanding

Question: Which line in the method neverEnd (shown above) contains the recursive call (the call to the same method)?

Answer: Line 4 — neverEnd(); contains a call to the same method, which makes it a recursive method.

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Is This Recursive?

Consider:

public static int mystery() {
    int total = 0;
    for (int i = 10; i > 0; i--) {
        total = total + i;
    }
    return total;
}

Answer: No — this is not recursive. There is no call to the same method; it uses iteration instead.

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Summary

• Recursion is when a method calls itself.
• Infinite recursion occurs without a base case to stop it.

• Proper recursion requires:

  • Base case: stops recursion.
  • Recursive call: moves toward the base case.
• Without a base case, recursion causes a StackOverflowError.

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Acknowledgement

Content on this page is adapted from Runestone Academy - Barb Ericson, Beryl Hoffman, Peter Seibel.