I am having a trouble to share a resource with three threads...
public class Subject{
int i;
boolean valueSet1 = false;
boolean valueSet2 = true;
boolean valueSet3 = true;
void put(int i){
while(valueSet1){
try{
wait();
}catch(InterruptedException e){
System.out.println("Producer thread interrupted");
}
}
System.out.println("Producer thread wakesup");
valueSet1=true;
valueSet2=false;
this.i=i;
System.out.println("Put: "+i);
notify();
}
void makesquare(){
int a;
while(valueSet2){
try{
System.out.println("Convertor thread goin to sleep");
wait();
}catch(InterruptedException e){
System.out.println("Convertor thread interrupted");
}
}
System.out.println("Convertor thread wakesup");
valueSet2 = true;
valueSet3=false;
a = this.i;
this.i = a*a;
System.out.println("Made: "+i);
notify();
}
void get(){
while(valueSet3){
try{
System.out.println("Consumer thread goin to sleep");
wait();
}catch(InterruptedException e){
System.out.println("Consumer thread interrupted");
}
}
System.out.println("Consumer thread wakesup");
valueSet3 = true;
valueSet1 = false;
System.out.println("Got: "+i);
notify();
}
}
class Producer implements Runnable{
Subject q;
Thread t;
String msg;
Producer(Subject q, String msg){
this.q=q;
this.msg = msg;
t = new Thread(this, this.msg);
}
#Override
public void run(){
int i=2;
while(true){
synchronized(q){
q.put(i++);
}
}
}
}
class Consumer implements Runnable{
Subject q;
Thread t;
String msg;
Consumer(Subject q,String msg){
this.q = q;
this.msg = msg;
t = new Thread(this, this.msg);
}
#Override
public void run(){
while(true){
synchronized(q){
q.get();
}
}
}
}
class Convertor implements Runnable{
Subject q;
Thread t;
String msg;
Convertor(Subject q, String msg){
this.q=q;
this.msg = msg;
t = new Thread(this, this.msg);
}
#Override
public void run(){
while(true){
synchronized(q){
q.makesquare();
}
}
}
}
There are three threads in the program. One thread produces while another makes square of the produced quantity. While the last thread consumes the squared product.They all share the same object.
public class Thread3way {
public static void main(String[] args) {
Subject q = new Subject();
Producer P = new Producer(q, "producer");
Convertor Cv = new Convertor(q, "convertor");
Consumer Cs = new Consumer(q, "consumer");
P.t.start();
Cv.t.start();
Cs.t.start();
}
}
Your wait loop is not quite correct, you need to keep looping until your condition is true before you do any modifications. I'd do it like this (using a State enum to make things clearer):
public class Subject {
static enum State { EMPTY, WAITING_TO_SQUARE, WAITING_TO_GET };
State state;
int value;
public synchronized void put(int i) {
while (state != State.EMPTY) {
try {
wait();
}catch(InterruptedException e) {
System.out.println("Put interrupted");
}
}
value = i;
state = State.WAITING_TO_SQUARE;
}
}
As a commenter pointed out, you don't need to synchronize twice, either synchronized(q) around the Subject calls, or declare your methods synchronized. You don't need both.
No need to use Runnables explicitly, just make Producer, Convertor, and Consumer Threads directly. Then you don't need the t fields, and you can start the threads like P.start().
Sounds to me like you're doing some kind of pipeline with 3 stages. Why not enjoy the benefits of ExecutorService:
class ConvertorTask implements Runnable {
private int number;
private static ExecutorService consumer = Executors.newSingleThreadExecutor();
public ConvertorTask(int number) {
this.number = number;
}
public void run() {
consumer.submit(new ConsumerTask(number * number));
}
}
class ConsumerTask implements Runnable {
private int number;
public ConsumerTask(int number) {
this.number = number;
}
public void run() {
System.out.println(number);
}
}
class Producer implements Runnable {
private ExecutorService convertor = Executors.newSingleThreadExecutor();
public void run() {
int i = 0;
while(true) {
convertor.submit(new ConvertorTask(i++));
}
}
}
public class Test {
public static void main(String[] args) throws InterruptedException {
Thread t = new Thread(new Producer());
t.start();
}
}
Related
I run the code in IntelliJ IDEA and it would not stop. I use exec.shutdownNow() instead and it stops successfully. So, I think is the problem of generator, but I can't figure out what problem it is.
The main method:
public static void main(String[] args) {
PrimeProducer generator = new PrimeProducer(new ArrayBlockingQueue<>(10));
ExecutorService exec = Executors.newFixedThreadPool(1);
exec.execute(generator);
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
throw new RuntimeException(e);
} finally {
generator.cancel();
}
//generator.get();
exec.shutdown();
}
And the PrimeProducer class:
public class PrimeProducer extends Thread{
private final BlockingQueue<BigInteger> queue;
PrimeProducer(BlockingQueue<BigInteger> queue){
this.queue = queue;
}
#Override
public void run() {
try {
BigInteger p = BigInteger.ONE;
while(!Thread.currentThread().isInterrupted()){
queue.put(p = p.nextProbablePrime());
}
} catch (InterruptedException e) {
}
}
public void cancel(){interrupt();}
public void get(){
for(BigInteger i : queue){
System.out.println(i.toString());
}
}
}
The latest version that still can't work:
public class PrimeProducer implements Runnable {
private final BlockingQueue<BigInteger> queue;
PrimeProducer(BlockingQueue<BigInteger> queue){
this.queue = queue;
}
#Override
public void run() {
try {
BigInteger p = BigInteger.ONE;
while(!Thread.currentThread().isInterrupted()){
queue.put(p = p.nextProbablePrime());
}
} catch (InterruptedException e) {
}
}
public void cancel(){Thread.currentThread().interrupt();}
public synchronized void get(){
for(BigInteger i : queue){
System.out.println(i.toString());
}
}
}
You have PrimeProducer extend Thread, but you don't actually use it as a thread, you only use it as a Runnable, which is executed on a thread of the thread pool. The cancel has no effect, because it calls the interrupt() of a different thread than the one that is actually running. When you then call exec.shutdown(), the thread pool will interrupt its threads to signal them they should end, when then means PrimeProducer.run will stop.
Instead, use a volatile boolean to end your run (and implement Runnable instead of extending Thread):
public class PrimeProducer implements Runnable {
private final BlockingQueue<BigInteger> queue;
private volatile boolean cancelled;
PrimeProducer(BlockingQueue<BigInteger> queue) {
this.queue = queue;
}
#Override
public void run() {
try {
BigInteger p = BigInteger.ONE;
while (!cancelled) {
queue.put(p = p.nextProbablePrime());
}
} catch (InterruptedException e) {
}
}
public void cancel() {
cancelled = true;
}
public void get() {
for (BigInteger i : queue) {
System.out.println(i.toString());
}
}
}
```
package programs;
public class TestThreads {
public static void main(String[] args) {
ThreadOne t1 = new ThreadOne();
ThreadTwo t2 = new ThreadTwo();
Thread one = new Thread(t1);
Thread two = new Thread(t2);
one.start();
two.start();
}
}
class Accum{
private static Accum a = new Accum();
private int counter = 0;
private Accum() {
}
public static Accum getAccum() {
return a;
}
public void updateCounter(int add) {
counter +=add;
}
public int getCount() {
return counter;
}
}
class ThreadOne implements Runnable{
Accum a = Accum.getAccum();
#Override
public void run() {
for(int x=0;x<98;x++) {
a.updateCounter(1000);
try {
Thread.sleep(50);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println("one "+ a.getCount());
}
}
class ThreadTwo implements Runnable{
Accum a = Accum.getAccum();
#Override
public void run() {
for(int x=0;x<99;x++) {
a.updateCounter(1);
try {
Thread.sleep(50);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println("two "+ a.getCount());
}
}
```
The expected output should be as below
One 98098
Two 98099
But I am getting values same for both One and Two.
Is this expected or both should result in different values?
When it comes to thread priorities eventhough the jvm scheduler takes the role to choose the turn of which thread should execute first, what about the results of this program where two void run programs of for loop with 98 and 99 which should result in two different values else the same
This is because shared data is NOT synchronized.
To resolve this problem, use synchronized on methods those touch the shared data, to make Accum class Thread-Safe.
example:
class Accum{
private static Accum a = new Accum();
private int counter = 0;
private Accum() {
}
public static Accum getAccum() {
return a;
}
public synchronized void updateCounter(int add) {
counter +=add;
}
public synchronized int getCount() {
return counter;
}
}
Maybe make the methods and fields in the Accum class static, because then it is saved in a general place and not an instance...
Code:
class Accum{
private static Accum a = new Accum();
private static int counter = 0;
private Accum() {
}
public static Accum getAccum() {
return a;
}
public static void updateCounter(int add) {
counter +=add;
}
public static int getCount() {
return counter;
}
}
class ThreadTwo implements Runnable{
#Override
public void run() {
for(int x=0;x<99;x++) {
Accum.updateCounter(1);
try {
Thread.sleep(50);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println("two "+ Accum.getCount());
}
}
class ThreadOne implements Runnable{
#Override
public void run() {
for(int x=0;x<98;x++) {
Accum.updateCounter(1000);
try {
Thread.sleep(50);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println("one "+ Accum.getCount());
}
}
I am not very experienced in Java yet, so this might not work...
[EDIT] I tested it and I believe it works
I have this code:
public class UsbDrive extends HardDrive {
private Date lastUpdate;
private void updateDate() {
lastUpdate = new Date();
}
public synchronized void cutAndPaste(UsbDrive other, int originAddress, int destAddress) {
byte[] data = read(originAddress);
boolean success = other.write(data, destAddress);
if (success) {
erase(originAddress);
updateDate();
} else {
throw new RuntimeException("Write failed!");
}
}
}
class HardDrive {
...
public synchronized byte[] read(int address) {...}
public synchronized boolean write(byte[] data, int address) {...}
public synchronized void erase(int address) {...}
...
I'm trying to simulate a deadlock
public class Main {
private static UsbDrive usb1 = new UsbDrive();
private static UsbDrive usb2 = new UsbDrive();
public static void main(String[] args) throws Exception {
Thread thread1 = new Thread(new ThreadA());
Thread thread2 = new Thread(new ThreadB());
thread1.start();
thread2.start();
thread1.join();
thread2.join();
}
static class ThreadA implements Runnable {
#Override
public void run() {
try {
Thread.sleep(5000);
}
catch (Exception e) {
e.printStackTrace();
}
synchronized (usb1) {
usb1.cutAndPaste(usb2, 1, 2);
}
}
}
static class ThreadB implements Runnable {
#Override
public void run() {
synchronized (usb2) {
usb2.cutAndPaste(usb1, 1, 2);
}
}
}
}
However deadlock doesn't happen - why? ThreadA calls method cutAndPaste() locked on usb1, while inside that method write() is called which is locked on usb2
Shouldn't deadlock occur?
How should I change the code to trigger deadlock?
I get this output:
reading data
Erasing data
reading data
Erasing data
It is a race condition that decides whether the deadlock happens or not as far as I can see, one good option to make the deadlock more likely to happen is a loop, but an easier option might be to insert Thread.sleep(5000); between byte[] data = read(originAddress); and boolean success = other.write(data, destAddress); in UsbDrive::cutAndPaste. EDIT: And remove the existing Thread.sleep(5000);.
EDIT: Clarified answer.
EDIT2: I just ran the code with the changes, and it indeed induces a deadlock now:
import java.util.Date;
public class Main {
private static UsbDrive usb1 = new UsbDrive();
private static UsbDrive usb2 = new UsbDrive();
public static void main(String[] args) throws Exception {
Thread thread1 = new Thread(new ThreadA());
Thread thread2 = new Thread(new ThreadB());
thread1.start();
thread2.start();
thread1.join();
thread2.join();
}
static class ThreadA implements Runnable {
#Override
public void run() {
synchronized (usb1) {
usb1.cutAndPaste(usb2, 1, 2);
}
}
}
static class ThreadB implements Runnable {
#Override
public void run() {
synchronized (usb2) {
usb2.cutAndPaste(usb1, 1, 2);
}
}
}
}
class UsbDrive extends HardDrive {
private Date lastUpdate;
private void updateDate() {
lastUpdate = new Date();
}
public synchronized void cutAndPaste(UsbDrive other, int originAddress, int destAddress) {
byte[] data = read(originAddress);
try {
Thread.sleep(5000);
}
catch (Exception e) {
e.printStackTrace();
}
boolean success = other.write(data, destAddress);
if (success) {
erase(originAddress);
updateDate();
} else {
throw new RuntimeException("Write failed!");
}
}
}
class HardDrive {
public synchronized byte[] read(int address) {return new byte[]{};}
public synchronized boolean write(byte[] data, int address) {return true;}
public synchronized void erase(int address) {}
}
DefaultRunners are producers
and OrderTaker is a consumer
They both share a OrderQueue.
Currently, I use the variable isDone to indicate if a game is finished.
Once each round is done, I want to make it repeat again and again.
However, in my current implementation it will only run once.
How could I solve it?
public class OrderQueue {
public synchronized void pushOrder(Order order) throws InterruptedException {
if (isDone) {
wait();
} else {
runnersQueue.addLast(order);
notifyAll();
}
}
public void pullOrder() {
try {
if (runnersQueue.size() == 0) {
} else if (isDone) {
wait();
} else {
handleOrder(runnersQueue.pop());
}
} catch (InterruptedException e) {
}
}
In my main class
while(true){
enterYesToStart();
DefaultRunners dfltRunner = new DefaultRunners(queue);
OrderTaker taker = new OrderTaker(queue);
taker.run();
System.out.println("This round is finished"); # never reach to this line
}
Here's the full source code for the example
https://gist.github.com/poc7667/d98e3bf5b3b470fcb51e00d9a0d80931
I've taken a look at your code snippets and the problem is fairly obvious.
The main thread runs the OrderTaker runnable. The main thread is stuck in an eternal loop as the while statement cannot complete unless it throws an exception. (Note that the same is true for your ThreadRunner runnable.)
This means that the main thread i still pulling orders while the race is already done.
The OrderTaker should exit it's while loop while once the race is done. I guess that there are multiple ways achieve this, but one way is use a shared variable.
I took your code and adapted it into a working example.
import java.util.*;
import java.util.concurrent.ConcurrentLinkedDeque;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;
public class RaceApp {
public static void main(String[] args) throws InterruptedException {
final RaceUpdateManager queue = new RaceUpdateManager();
for (int i = 0; i < 3; i++) {
queue.reset();
List<Thread> threads = Arrays.asList(
new Thread(new Runner("Tortoise", 0, 10, queue)),
new Thread(new Runner("Hare", 90, 100, queue))
);
for (Thread thread : threads) {
thread.start();
}
RaceUpdatesProcessor processor = new RaceUpdatesProcessor(queue);
processor.run();
System.out.println("Game finished");
}
}
private static class RaceUpdateManager {
private static final int TOTAL_DISTANCE = 300;
//thread-safe implementation for queue so no external syncrhonization is required when adding/removing updates
private final Deque<RaceUpdate> runnersQueue = new ConcurrentLinkedDeque<>();
//lock used to sync changes to runnersRecords and done variables
private final ReadWriteLock raceStatusLock = new ReentrantReadWriteLock();
private final Map<String, Integer> runnersRecords = new HashMap<>();
private volatile boolean raceDone = false;//volatile keyword guarantees visibility of changes to variables across threads
public boolean isRaceDone() {
return raceDone;
}
//updates can by added simultaneously (read lock)
public void register(RaceUpdate raceUpdate) throws InterruptedException {
Lock readLock = raceStatusLock.readLock();
readLock.lock();
try {
if (!raceDone) {
runnersQueue.addLast(raceUpdate);
}//ignore updates when the race is done
} finally {
readLock.unlock();
}
}
//but they need to be processed in order (exclusive write lock)
public void processOldestUpdate() {
Lock writeLock = raceStatusLock.writeLock();
writeLock.lock();
try {
RaceUpdate raceUpdate = runnersQueue.poll();
if (raceUpdate != null) {
handleUpdate(raceUpdate);
}
} finally {
writeLock.unlock();
}
}
private void handleUpdate(RaceUpdate raceUpdate) {
Integer distanceRun = runnersRecords.merge(
raceUpdate.runner, raceUpdate.distanceRunSinceLastUpdate, (total, increment) -> total + increment
);
System.out.printf("%s: %d\n", raceUpdate.runner, distanceRun);
if (distanceRun >= TOTAL_DISTANCE) {
raceDone = true;
System.out.printf("Winner %s\n", raceUpdate.runner);
}
}
public void reset() {
Lock writeLock = raceStatusLock.writeLock();
writeLock.lock();
try {
runnersQueue.clear();
runnersRecords.clear();
raceDone = false;
} finally {
writeLock.unlock();
}
}
}
public static class Runner implements Runnable {
private final String name;
private final int rest;
private final int speed;
private final RaceUpdateManager queue;
private final Random rand = new Random();
public Runner(String name, int rest, int speed, RaceUpdateManager queue) {
this.name = name;
this.rest = rest;
this.speed = speed;
this.queue = queue;
}
#Override
public void run() {
while (!queue.isRaceDone()) {
try {
if (!takeRest()) {
queue.register(new RaceUpdate(this.name, this.speed));
}
Thread.sleep(100);
} catch (InterruptedException e) {
//signal that thread was interrupted and exit method
Thread.currentThread().interrupt();
return;
}
}
}
private boolean takeRest() {
return rand.nextInt(100) < rest;
}
}
public static class RaceUpdatesProcessor implements Runnable {
private final RaceUpdateManager queue;
public RaceUpdatesProcessor(RaceUpdateManager queue) {
this.queue = queue;
}
#Override
public void run() {
while (!queue.isRaceDone()) {
try {
queue.processOldestUpdate();
Thread.sleep(50);
} catch (InterruptedException e) {
//signal that thread was interrupted and exit method
Thread.currentThread().interrupt();
return;
}
}
}
}
public static class RaceUpdate {
public final String runner;
public final int distanceRunSinceLastUpdate;
public RaceUpdate(String runner, int distanceRunSinceLastUpdate) {
this.runner = runner;
this.distanceRunSinceLastUpdate = distanceRunSinceLastUpdate;
}
}
}
I have two methods in two different classes, like this
public class ClassX implements Runnable {
public void methodAandB() {
for(int i=0;i<10;i++) {
System.out.println("This is A and B ");
}
}
#Override
public void run() {
methodAandB();
}
}
public class ClassY implements Runnable {
public void methodAorB() {
for(int i=0;i<10;i++) {
System.out.println("This is A or B");
}
}
#Override
public void run() {
methodAorB(a);
}
}
Thread t1 is calling methodAandB().
Thread t2 is calling methodAorB().
Can I switch between these two threads after each iteration of loop in methods?
I want to get output like this:
This is A and B
This is A or B
This is A and B
This is A or B
This is A and B
This is A or B
This is A and B
This is A or B
Best example of flip-flop between threads:
Given two int array (even and odd), 2 threads printing their numbers in natural order.
package com.rough;
public class ThreadsBehaviour {
static Object lock = new Object();
public static void main(String[] args) throws InterruptedException {
int a[] = {1,3,5,7,9};
int b[] = {2,4,6,8,10};
Thread odd = new Thread(new Looper(a, lock));
Thread even = new Thread(new Looper(b, lock));
odd.start();
even.start();
}
}
class Looper implements Runnable
{
int a[];
Object lock;
public Looper(int a[], Object lock)
{
this.a = a;
this.lock = lock;
}
#Override
public void run() {
for(int i = 0; i < a.length; i++)
{
synchronized(lock)
{
System.out.print(a[i]);
try
{
lock.notify();
if(i == (a.length - 1))
{
break;
}
lock.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
}
You can achieve this simply by using the shared variables. I have implemented and verified the problem. code is below
class X
public class ClassX implements Runnable {
public void methodAandB() {
for(int i=0;i<10;i++) {
while(GlobalClass.isClassXdone)
{}
System.out.println("This is A and B ");
GlobalClass.isClassXdone = true;
GlobalClass.isClassYdone = false;
}}
#Override
public void run() {
methodAandB(); } }
ClassY
public class ClassY implements Runnable {
public void methodAorB() {
for(int i=0;i<10;i++) {
while(GlobalClass.isClassYdone)
{}
System.out.println("This is A or B ");
GlobalClass.isClassYdone = true;
GlobalClass.isClassXdone = false;}}
#Override
public void run() {
methodAorB();}}
Definition of the shared variable
public class GlobalClass {
public static boolean isClassXdone = false ;
public static boolean isClassYdone = false ;
}
You can just start your thread using t1.start and t2.start to get the desired output
Thread t1 = new Thread(new ClassX());
Thread t2 = new Thread(new ClassY());
t1.start();
t2.start();
This is probably more than needed to solve the problem, but, as it seems to be an introduction to concurrent programming exercise, it should be along the lines of what you'll encounter.
You should probably have a shared object that both your threads know, so that they may synchronize through it. Like so:
public class MyMutex {
private int whoGoes;
private int howMany;
public MyMutex(int first, int max) {
whoGoes = first;
howMany = max;
}
public synchronized int getWhoGoes() { return whoGoes; }
public synchronized void switchTurns() {
whoGoes = (whoGoes + 1) % howMany;
notifyAll();
}
public synchronized void waitForMyTurn(int id) throws
InterruptedException {
while (whoGoes != id) { wait(); }
}
}
Now, your classes should receive their respective identifier, and this shared object.
public class ClassX implements Runnable {
private final int MY_ID;
private final MyMutex MUTEX;
public ClassX(int id, MyMutex mutex) {
MY_ID = id;
MUTEX = mutex;
}
public void methodAandB() {
for(int i = 0; i < 10; i++) {
try {
MUTEX.waitForMyTurn(MY_ID);
System.out.println("This is A and B ");
MUTEX.switchTurns();
} catch (InterruptedException ex) {
// Handle it...
}
}
}
#Override
public void run() { methodAandB(); }
}
ClassY should do the same. Wait for its turn, do its action, and then yield the turn to the other.
I know it's a little late to answer this. But it's yesterday only I have come across this question. So I guess it's never too late.. ;)
Solution, as #afsantos mentioned is having a shared object between the two threads and implementing mutual exclusion on the shared object. The shared object could be alternatively locked by the two threads. Two possible implementations are as follows. This is actually more like an extension of #afsantos solution. His work is hereby acknowledged.
Solution 1:
Blueprint of the object that will be shared is as follows.
public class MutEx {
public int whoGoes, howMany;
public MutEx(int whoGoes, int howMany) {
this.whoGoes = whoGoes;
this.howMany = howMany;
}
public synchronized void switchTurns(){
this.whoGoes = (this.whoGoes + 1) % 2;
notifyAll();
}
public synchronized void waitForTurn(int id) throws InterruptedException{
while(this.whoGoes != id)
wait();
}
}
Then, you could implement the ClassX as follows.
public class ClassX implements Runnable {
private final int MY_ID;
private final MutEx MUT_EX;
public ThreadOne(int MY_ID, MutEx MUT_EX) {
this.MY_ID = MY_ID;
this.MUT_EX = MUT_EX;
}
#Override
public void run(){
this.doTheWork();
}
public void doTheWork(){
for(int i = 0; i < 10; i++){
try {
MUT_EX.waitForMyTurn(MY_ID);
System.out.println("This is A and B");
MUT_EX.switchTurns();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
ClassY also will be the same, with whatever the differences you need to be there. Then, in the invocation (i.e. in the main method),
public static void main(String[] args) {
MutEx mutEx = new MutEx(0, 2);
Thread t1 = new Thread(new ClassX(0, mutEx);
Thread t2 = new Thread(new ClassY(1, mutEx));
t1.start();
t2.start();
}
Voila! You have two threads, alternating between each as you need.
Solution 2: Alternatively, you could implement the ClassX & ClassY as follows.
public class ClassX extends Thread{
Here, you are subclassing the java.lang.Thread to implement your requirement. For this to be invoked, change the main method as follows.
public static void main(String[] args) {
MutEx mutEx = new MutEx(0, 2);
ClassX t1 = new ClassX(0, mutEx);
ClassY t2 = new ClassY(1, mutEx);
t1.start();
t2.start();
}
Run this, and you have the same result.
If you don't need to use Thread try this code:
for (int i = 0; i < 20; i++) {
if (i % 2 == 0) {
methodAandB();
} else {
methodAorB();
}
}