Using CyclicBarrier for Alternating Thread Printing of ABC in Java

The problem presented is to have three threads alternately print the characters "A", "B", and "C" in sequence, a common interview question for assessing Java multithreading knowledge.

Analysis : Among various solutions, the author prefers using JUC's CyclicBarrier because it naturally supports repeated synchronization points across multiple threads. The barrier makes a group of threads wait until all have reached a common point before proceeding, and it can be reused for subsequent cycles.

An analogy is given: a shuttle bus waits until it is full before departing, similar to how a CyclicBarrier waits for all participating threads before allowing them to continue to the next round.

Implementation Code :

import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;

/**
 * 3 threads alternately print ABC
 */
public class ThreadLoopPrint {
    // shared counter
    private static int sharedCounter = 0;
    public static void main(String[] args) {
        String printString = "ABC";
        CyclicBarrier cyclicBarrier = new CyclicBarrier(3, () -> {
            // optional barrier action
        });
        Runnable runnable = new Runnable() {
            @Override
            public void run() {
                for (int i = 0; i < printString.length(); i++) {
                    synchronized (this) {
                        sharedCounter = sharedCounter > 2 ? 0 : sharedCounter; // cycle
                        System.out.println(printString.toCharArray()[sharedCounter++]);
                    }
                    try {
                        // wait for all 3 threads to finish one round
                        cyclicBarrier.await();
                    } catch (InterruptedException | BrokenBarrierException e) {
                        e.printStackTrace();
                    }
                }
            }
        };
        new Thread(runnable).start();
        new Thread(runnable).start();
        new Thread(runnable).start();
    }
}

The program's output shows the characters A, B, C printed in order repeatedly by the three threads, confirming the correct synchronization.

Conclusion : Implementing a cyclic multithreaded print task demonstrates a candidate's grasp of Java concurrency primitives; the time taken and code quality provide interviewers with a clear view of the applicant's fundamental coding skills.

Author Introduction : Wang Lei ("Lei Ge") has 13 years of frontline development experience, 3 years of teaching, and has held senior positions at companies like 360. He is a prolific technical blogger with millions of reads and multiple author recognitions.