I have this class:
class S2Es2SharedState {
public int x;
public int y;
}
that is used to share an s2Es2SharedState object in every thread inside in the loop:
class S2Es2Worker implements Runnable {
private final int id;
private S2Es2SharedState state;
private final int delay;
private int numErrors = 0;
public S2Es2Worker(final int id, final S2Es2SharedState state,
final int delay) {
this.state = state;
this.delay = delay;
this.id = id;
this.numErrors = 0;
System.out.println("Worker " + id + ": created with " + delay
+ " ms delay");
}
#Override
public void run() {
boolean check = true;
System.out.println("Worker " + id + ": started");
for (int i = 0; i < 150; i++) {
state.x++;
try {
Thread.sleep(delay);
} catch (final InterruptedException e) {
// do nothing
}
state.y++;
}
}
}
in this program working 10 threads where each of them enter in the loop and increment 150 times the value of x and y of the shared object, so the final result in the x and y of the shared object is 1500.If i don't change anything this program have a problem of race condition, so for fix this problem i used the lock (and it works). But now i want to fix the problem of the race condition using the immutable object instead of the locks. so i changed the class of the shared object in this way:
final class S2Es2SharedState {
private final int x;
private final int y;
public S2Es2SharedState(final int x, final int y) {
this.x = x;
this.y = y;
}
public int getX() {
return x;
}
public int getY() {
return y;
}
public S2Es2SharedState incrementX() {
return new S2Es2SharedState(x+1, y);
}
public S2Es2SharedState incrementY() {
return new S2Es2SharedState(x, y+1);
}
}
and in the run() method i changed the state variable in this way:
class S2Es2Worker implements Runnable {
....
private volatile S2Es2SharedState state;
....
and in the for loop i did this edit:
for (int i = 0; i < 150; i++) {
state = state.incrementX();
try {
Thread.sleep(delay);
} catch (final InterruptedException e) {
// do nothing
}
state = state.incrementY();
}
}
But i don't know why the final result is that the shared object has instead of x=1500 and y=1500 150 in both variableHow can i fix the problem with the race of condition using the immutable object?
I'm going to assume you create a S2Es2SharedState (use simpler names for asking questions) as
S2Es2SharedState state = new S2Es2SharedState(0, 0)
and passing it to the S2Es2Worker instances something like
for (int i = 0; i < 10;i++) {
S2Es2Worker worker = new S2Es2Worker(i, state, 5);
new Thread(worker).start();
}
In java, everything is passed by value. So the using state as a method argument, a copy of the value of the reference is passed.
So the field
private volatile S2Es2SharedState state;
is also referencing the same object. However, if you change the reference that the field state is holding, for example with
state = state.incrementX();
that doesn't affect any other references.
Related
My singleton class:
public class XandY {
private double x, y;
private static XandY xy;
//Constructor sets an x and y location
private XandY() {
x = 210.0;
y = 100.0;
}
public static XandY getXandY() {
if (xy == null)
xy = new XandY();
return xy;
}
public void updateXandY() {
x += 10;
y += 5;
}
}
Other class that changes singleton values and tries to reinitialize. My question is if I call changeXandY a few times then want to call resetXandY how do I make it reset back to the original x and y?
public class GameWorld {
private List<GameObject> objects;
public void initialize() {
objects = new ArrayList<GameObject>();
objects.add(XandY.getXandY());
...add other objects that are not singletons
}
public void changeXandY {
for (int i=0; i<gameObject.size(); i++) {
if (gameObject.get(i) instanceof XandY)
((XandY)gameObject.get(i)).updateXandY();
}
public void resetXandY {
initialize();
}
}
For this use case, you could simply store them as default values. Such as
private double x, y;
private static XandY xy;
private static final double default_x = 210.0;
private static final double default_y = 100.0;
That way when you reset, just:
public void resetXandY {
this.x = default_x;
this.y = default_y;
}
That being said, you may want to change your default constructor to look the same way.
If you can make the XandY reference protected, you can use a static initializer in an anonymous subclass:
// I need to reset the singleton!
new XandY(){
{ xy = null; }
};
But really, if you need to be able to (re)initialize the singleton, you should put a method to that effect into its signature. Obscure solutions are, at best, still obscure...
Create a resetXandY() method to set default value:
public class XandY {
private double x, y;
private static XandY xy;
//Constructor sets an x and y location
private XandY() {
x = 210.0;
y = 100.0;
}
//reset x=0 and y=0
public void resetXandY() {
x = 0;
y = 0;
}
public static XandY getXandY() {
if (xy == null)
xy = new XandY();
return xy;
}
public void updateXandY() {
x += 10;
y += 5;
}
}
Now I'm struggling with the task from the title. I create X threads, each of them prints Y equal digits (getting from constructor, for example "11111", "222222" etc) for Z times in cycle. So the result looks like:
111111111
222222222
333333333
111111111
222222222
333333333
for X = 3, Y = 9 and Z = 2.
Firstly I've solved this issue using sleep, interrupt and passing "next" thread to the constructor of previous one. One interrupts another etc. Next step is to get the same output using wait/notify instead sleep and interrupt. As far as I can see, it's neccesary to create the shared monitor object, to invoke wait after every printing and in a some moment " I should invoke notifyAll.
Current code is:
public class PrinterController {
private static final int THREADS_NUMBER = 5;
public static void main(String[] args) {
Printer[] printers = new Printer[THREADS_NUMBER];
for (int i = 0; i < THREADS_NUMBER; i++) {
printers[i] = new Printer(i);
printers[i].start();
}
}
}
public class Printer extends Thread {
private static int portion = 10;
private static int totalNumber = 100;
private int digit;
private static final Object monitor = new Object();
public Printer(int digit) {
this.digit = digit;
}
#Override
public void run() {
synchronized (monitor) {
int portionsNumber = totalNumber / portion;
for (int i = 0; i < portionsNumber; i++) {
printLine();
try {
monitor.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
private void printLine() {
for (int j = 0; j < portion; j++) {
System.out.print(digit);
}
System.out.println();
}
}
Could you help to improve it? I found similar tasks but they don't contain appropriate answers. Thanks.
Final solution based on the Nadir's answer:
public class Printer extends Thread {
private static int portion = 10;
private static int totalNumber = 100;
private int digit;
static Object monitor = new Object();
static Integer counter = 0;
public Printer(int digit) {
this.digit = digit;
}
#Override
public void run() {
int portionsNumber = totalNumber / portion;
for (int i = 0; i < portionsNumber; i++) {
synchronized (monitor) {
while (digit != counter) {
try {
monitor.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
printLine();
monitor.notifyAll();
}
}
}
private void printLine() {
for (int j = 0; j < portion; j++) {
System.out.print(digit);
}
System.out.println();
counter = (counter + 1) % PrinterController.THREADS_NUMBER;
}
}
It can be accomplished with a class used to synchronize the threads (and even make sure they are orderer). All threads would share the same instance.
public class Synchronizer
{
private int nextThread;
private int maxNumThreads;
public Synchronizer(int numThreads)
{
maxNumThreads = numThreads;
nextThread = 0;
}
public void doSync(int threadId) throws Exception
{
synchronized(this)
{
while(nextThread != threadId)
{
wait();
}
}
}
public void threadDone(int threadId) throws Exception
{
synchronized(this)
{
nextThread = (threadId + 1) % maxNumThreads;
notifyAll();
}
}
}
On your thread's run(), you would call doSync() before printing anything. Then you would put the code for printing, and afterwards, you would call threadDone(), allowing the next thread to be released. The id is used to enforce an order.
I don't get what's going on here, but the final method
s.castable()
that overrides the motherclass's namesake abstract method doesn't get called.
Here is where I try to call s.castable():
public void cast(String[] request) {
System.out.println("cast called");
if (this.session.getPlayer()==this.game.getTurnPlayer()) {
System.out.println("first condition passed");
Spell s = this.session.getPlayer().getCharacter().getSpells().get(Integer.valueOf(request[1]));
ArrayList<String> usernames = new ArrayList();
System.out.println("Now printing spell: "+s);
for (int i = 6; i < request.length; i++) {
usernames.add(request[i]);
}
System.out.println("username create.d");
if (s.castable()) { //HERE
System.out.println("Second condition passed");
s.cast(Integer.valueOf(request[1]), Integer.valueOf(request[2]),request[3].charAt(0), request[4].charAt(0), usernames);
String str = "";
for (String st : usernames) {
str += st;
}
this.session.send("YOUSPELL "+request[1]+" "+request[2]+" "+request[3]+" "+request[4]+" "+str);
System.out.println("Done");
}
}
}
Here is the "Spell" MotherClass:
public abstract class Spell {
private int manaCost;
private int coolDown;
private int range;
private Player player;
public abstract void cast(int x, int y, char mode1, char mode2,ArrayList<String> usernames);
public abstract Boolean castable();
//Then all getters and setters.
}
And here is the final class "Velocity":
public final class Velocity extends Spell {
private final int manaCost;
private final Player player;
private final int coolDown;
private final int coolDownTime;
private final int additionalMovement;
private final int spellRef;
private final ArrayList<String> usernames = new ArrayList();
public Velocity(Player p) {
this.spellRef = 0;
this.additionalMovement = 5;
this.player = p;
this.manaCost = 5;
this.coolDownTime = 3;
this.coolDown = 0;
super.setCoolDown(coolDown);
super.setManaCost(manaCost);
super.setPlayer(p);
}
#Override
public final void cast(int x, int y, char mode1, char mode2,ArrayList<String> usernames) {
System.out.println("Velocity casted.");
player.setMovement(player.getMovement() + additionalMovement);
setCoolDown(coolDownTime);
}
#Override
public final Boolean castable() {
System.out.println(player.getMana());
System.out.println(manaCost);
System.out.println(getCoolDown());
if (player.getMana() >= manaCost && getCoolDown() >= 0) {
return true;
}
return false;
}
}
Finally, the console output:
cast called
first condition passed
Now printing spell: model.haraka.be.Velocity#739bb60f
username create.d.
As you can see the spell object is known.
Can you help me ?
Thank you
The only possible problem here can be that Abstract class Spell's variable s doesn't contain the reference to Velocity object.
hence the castable method of velocity class never gets called.
If the castable method is returning false as mentioned by many
people System.out.println() statements must be printed which is not
the case I think.
But to be sure this is the problem, Please explain:
Spell s = this.session.getPlayer().getCharacter().getSpells().get(Integer.valueOf(request[1]));
What are below methods return type ?
getPlayer()
getSpells()
get(Integer.valueOf(request[1])
This is too much to ask/comment in comment section hence posting as an answer.
I am working on a java project which contains 3 classes and an object array in one of the classes. This project is ultimately supposed to move 4 entity objects around on a board by using the coordinates of the entity objects. These entity objects are stored in an array in the world class. My problem is with the array initialization in the world class. I am not sure how to set each element of the array equal to an object from the entity class and then access that object's coordinates to move it around on the board. The coordinates for the entity objects are initially set at 20x30 in a default constructor. Here is my code:
public class entity {
private int xcoordinate;
private int ycoordinate;
private String name;
private char symbol;
public entity(){
xcoordinate = 20;
ycoordinate = 30;
}
private entity(int newxcoor, int newycoor, String newname, char newsymbol){
xcoordinate = newxcoor;
ycoordinate = newycoor;
name = newname;
symbol = newsymbol;
}
public int getXCoor(){
return xcoordinate;
}
public int getYCoor(){
return ycoordinate;
}
}
public class world {
private entity[] ObArray = new entity[4];
public world(){
world test = new world();
}
public void draw(){
for (int i = 0; i < 4; i++)
{
//int x = ObArray[i].getXLoc();
//int y = ObArray[i].getYLoc();
}
}
}
public class mainclass {
public static void main(String[] args){
world worldob = new world();
//entity a = new entity();
//entity b = new entity();
//entity c = new entity();
//entity d = new entity();
worldob.draw();
}
}
My draw function and main function are not finished. After the array is initialized I will be able to finish the draw method using the entity get functions.
Thanks for your help.
That is one way of doing it. You can also define all of your entities inline like this:
private entity[] ObArray = {
new entity(0,0,"Entity1",'a'),
new entity(10,10,"Entity2",'b'),
new entity(20,20,"Entity3",'c'),
new entity(30,30,"Entity4",'d')
};
A better way may be to do an ArrayList instead of an array:
private List<entity> ObArray = new ArrayList<>();
ObArray.add(new entity(0,0,"Entity1",'a');
ObArray.add(new entity(10,10,"Entity2",'b');
ObArray.add(new entity(20,20,"Entity3",'c');
ObArray.add(new entity(30,30,"Entity4",'d');
To access each element you just need to get the element from the array and either get or set the properties you need:
ObArray[0].getXCoor();
ObArray[0].setXCoor(5);
Your problem is only creating new object of world inside world's constructor which throws stack overflow error, otherwise it is fine:
public world(){ world test = new world(); //REMOVE THIS LINE
}
You simply need to initialise the array. This can be done in the world constructor.
public world()
{
for (int i = 0; i < 4; i++)
{
ObArray[i] = new entity();
}
}
Then you can access the objects in your draw method, as you've shown:
public void draw()
{
for (int i = 0; i < 4; i++)
{
int x = ObArray[i].getXCoor();
int y = ObArray[i].getYCoor();
System.out.println("x" + x);
System.out.println("y" + y);
// Manipulate items in the array
// ObArray[i].setXCoor(10);
}
}
A more complete example, with the move functions added, and the class names capitalised:
public class Entity
{
private int xcoordinate;
private int ycoordinate;
private String name;
private char symbol;
public Entity()
{
xcoordinate = 20;
ycoordinate = 30;
}
private Entity(int newxcoor, int newycoor, String newname, char newsymbol)
{
xcoordinate = newxcoor;
ycoordinate = newycoor;
name = newname;
symbol = newsymbol;
}
public int getXCoor()
{
return xcoordinate;
}
public void setXCoor(int xcoordinate)
{
this.xcoordinate = xcoordinate;
}
public int getYCoor()
{
return ycoordinate;
}
public void setYcoor(int ycoordinate)
{
this.ycoordinate = ycoordinate;
}
public static void main(String[] args)
{
World worldob = new World();
worldob.draw();
worldob.move(0, 15, 30);
worldob.move(1, 45, 0);
worldob.move(2, 23, 27);
worldob.move(3, 72, 80);
worldob.draw();
}
}
class World
{
private final Entity[] ObArray;
public World()
{
this.ObArray = new Entity[4];
for (int i = 0; i < ObArray.length; i++)
{
ObArray[i] = new Entity();
}
}
public void move(int index, int xCoor, int yCoor)
{
if (index >= 0 && index < ObArray.length)
{
Entity e = ObArray[index];
e.setXCoor(xCoor);
e.setYcoor(yCoor);
}
}
public void draw()
{
for (Entity e : ObArray)
{
int x = e.getXCoor();
int y = e.getYCoor();
System.out.println("x" + x);
System.out.println("y" + y);
}
}
}
It's a little program written with a purpose of studying multithreading. I expected to get in main method different random numbers after run. About 4 numbers per second. But I got many thousands of zeros. Where is an error?
Main Class:
public class Main {
public static void main(String[] args) {
ExternalWorld externalWorld = new ExternalWorld();
externalWorld.start();
int x = 0;
while (true) {
while(!externalWorld.signal){
System.out.println("qqq");}
System.out.println(++x + ") " + externalWorld.getAnInt());
}
}
}
ExternalWorld Class:
import java.util.Random;
public class ExternalWorld extends Thread {
private int anInt = 0;
public boolean signal = false;
#Override
public void run() {
Random random = new Random(100);
while(true) {
anInt = random.nextInt(100);
signal = true;
try {
Thread.sleep(200);
signal = false;
Thread.sleep(50);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
public int getAnInt() {
if (!signal) {
int p = 1 / 0;
}
int result = anInt;
anInt = 0;
return result;
}
}
problem:
private int anInt = 0;
public boolean signal = false;
You are access those variables from one thread to another thus giving you 0 and false on the main thread
solution:
use volatile keyword to access those variables from multiple threads
sample:
private volatile int anInt = 0;
public volatile boolean signal = false;