Recursion: Concepts, Components, and Practical Applications — Java

This article explains the concept of recursion in programming. It describes its key components: the base case and the recursive case. Using a Java example, it illustrates how recursion is implemented and emphasizes safeguards to prevent infinite loops and stack overflow errors.

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In computer science, understanding the concept of recursion is essential as it is often the base of more complex algorithms, and in programming, it is a tool used to solve problems by breaking them down into smaller, more manageable subproblems. This post explores the components of a recursive method — the base case and the recursive case — using the programming language Java.

Recursive Method Explanation

A recursive algorithm or method solves complex problems by calling itself and by breaking the problems into smaller, more manageable subproblems.

The basic components to create a recursive method are a base case and a recursive case.

• A base case is a condition that when met stops the recursion, usually in an if statement.
• A recursive case is a set of code lines or functionalities that are computed ‘if’ the base case condition is not met, always followed by the recursive method calling itself usually with a modified input. Typically, the code lines and the recursive call are found in an ‘else’ statement following the ‘if’ statement checking if the base condition is met. However, If the ‘if’ statement contains a ‘return’ statement, the code lines and the recursive call are found right after the ‘if’ statement.

Note that a recursive method that calls itself with an unmodified input, or a recursive method that does not take an input, will not create an infinitely recursive loop if and only if the base case condition is based on external factors that change independently of the method’s input.

To avoid creating an infinitely recursive method, the method needs to contain at least one base case that will eventually be reached. Note that a recursive method can have more than one base case. For example, the recursive method can contain a base case that checks a specific condition, and others can act as safeguards. If the first base case condition is never reached, a safeguard such as a counter can limit the number of recursions based on the available computing memory, preventing a stack overflow error.

On a side note: the Python programming language has a built-in mechanism that limits the number of recursions a program can perform. If needed, this limit can be modified, either decreased or increased, by using the Python system (sys) library.

Here is an example of a recursion method:

import java.util.Random;

public class AreWeThereYet {
    private static final Random randomGenerateMiles = new Random();

    public static void askAreWeThereYet(int totalMilesDriven, int tripTotalMiles) {

        // ---- Base case ---- We've arrived!
        if (totalMilesDriven >= tripTotalMiles) {
            System.out.println("We're here! Finally!");
            return;
        }

        // ---- Recursive case ----
        // Miles driven
        int milesDriven = randomGenerateMiles.nextInt(50) + 1; // Drive 1-50 miles

        // Keep asking and driving
        System.out.println("Are we there yet?");
        System.out.println("Not yet, we've traveled " + totalMilesDriven + "miles.");

        if (milesDriven + totalMilesDriven >= tripTotalMiles) {
            milesDriven = tripTotalMiles - totalMilesDriven;
        }

        System.out.println("--- Drives " + milesDriven + " miles ---");
        totalMilesDriven += milesDriven;

        // ---- Recursive call ----
        askAreWeThereYet(totalMilesDriven, tripTotalMiles);
    }

    public static void main(String[] args) {
        int tripTotalMiles = 100; // Total trip distance
        System.out.println("Trip total miles: " + tripTotalMiles);
        askAreWeThereYet(0, tripTotalMiles);
    }
}

Output

import java.util.Random;

public class AreWeThereYet {
    private static final Random randomGenerateMiles = new Random();

    public static void askAreWeThereYet(int totalMilesDriven, int tripTotalMiles) {

        // ---- Base case ---- We've arrived!
        if (totalMilesDriven >= tripTotalMiles) {
            System.out.println("We're here! Finally!");
            return;
        }

        // ---- Recursive case ----
        // Miles driven
        int milesDriven = randomGenerateMiles.nextInt(50) + 1; // Drive 1-50 miles

        // Keep asking and driving
        System.out.println("Are we there yet?");
        System.out.println("Not yet, we've traveled " + totalMilesDriven + "miles.");

        if (milesDriven + totalMilesDriven >= tripTotalMiles) {
            milesDriven = tripTotalMiles - totalMilesDriven;
        }

        System.out.println("--- Drives " + milesDriven + " miles ---");
        totalMilesDriven += milesDriven;

        // ---- Recursive call ----
        askAreWeThereYet(totalMilesDriven, tripTotalMiles);
    }

    public static void main(String[] args) {
        int tripTotalMiles = 100; // Total trip distance
        System.out.println("Trip total miles: " + tripTotalMiles);
        askAreWeThereYet(0, tripTotalMiles);
    }
}

To summarize, recursion is an elegant and powerful approach to solving complex problems. By defining a base case and a recursive case, developers can create algorithms that effectively manage problem complexity. However, it is important to ensure that recursion stops appropriately to prevent infinite loops or stack overflow errors. The provided Java example, “AreWeThereYet,” illustrates these principles in action, showing how recursion can be used dynamically to solve a problem while maintaining clarity and functionality. As we continue to explore programming techniques, recursion remains an invaluable skill that underscores the importance of thoughtful problem decomposition and method design.

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Originally published at Alex.omegapy on Medium by Level UP Coding on November 8, 2024.

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