Develop Reference

Start objects faster and update Textview faster

I want to create multiple timers.
I initialize this in an ArrayList in the MainActivity. These are then added in a ReyclerView so that they are also displayed. To start, I go through a for loop and execute the startTimer () method for each individual object.
Unfortunately, the times are not correct for every timer. That means one timer is faster, the second is slower and so on. So each timer starts at different times or the text changes at different times.
My question now is, are there other approaches to start the timers and change the texts in the TextView so that the GUI is updated faster and the program itself runs faster?
The goal should be that all timers are equally fast and there is no delay. Thanks in advance. Looking forward to answer!
MainActivity
timerList= new ArrayList<>();
for (int i = 1; i <= 10; i++) {
timerList.add(new Timer(i, 600000 * i))); //
Each timer has a different time
}
recyclerView.setLayoutManager(recyclerViewLayoutManager);
adapter = new TimerAdapter(this, timerList);
recyclerView.setAdapter(adapter);
context = this;
button_start.setOnClickListener(v -> {
for (Timer timer: timerList) {
timer.startTimer();
});
#Override
public void updateMyText(int index, long time) {
timerList.get(index-1).setTime(time);
adapter.notifyDataSetChanged();
}
Timer
public interface MyCallback {
public void updateMyText(int index, long time);
}
public Timer(int index, long startTimeMilliseconds) {
this.index = index;
this.time = startTimeMilliseconds;
mTimeLeftInMillis = startTimeMilliseconds;
startTime = startTimeMilliseconds;
}
public void startTimer() {
mCountDownTimer = new CountDownTimer(mTimeLeftInMillis, 1000) {
#Override
public void onTick(long millisUntilFinished) {
mTimeLeftInMillis = millisUntilFinished;
updateCountDownText();
}
#Override
public void onFinish() {
mTimerRunning = false;
}
}.start();
mTimerRunning = true;
}
public void resetTimer() {
mCountDownTimer.cancel();
mTimerRunning = false;
mTimeLeftInMillis = startTime;
timeLeftFormatted = formattedTime(startTime);
changeText(index-1);
}
public void updateCountDownText() {
//MainActivity.timerList.get(getIndex()-1).setTime(mTimeLeftInMillis);
//MainActivity.adapter.notifyDataSetChanged();
if(myCallback != null) {
myCallback.updateMyText(getIndex(), mTimeLeftInMillis);
}
}

Use Live Data and Binding or MVVM design pattern. You don't have to worry about faster update operation of views.

The reason the code you wrote works slowly (and time isnt updated on all views together), is because you are creating a separate CountDownTimer for each of your timers.
You dont have to do that. All you need to do is have a single CountDownTimer, that runs every second and updates all of your views simultaneously.
Make Timer class only store the start time of the timer, and a string with the time it should show, dont create a CountDownTimer inside it.
Instead create a method inside it - "updateTimer()" for updating the string, based on
the current time and the start time of the counter.
Using a single CountDownTimer in your Activity, loop through the list of timers and call updateTimer() on each of your timers, and call adapter.notifyDataSetChanged() when the loop is done.
This will make all your views update together, remove the inefficiency of using multiple CountDownTimers, and only call adapter.notifyDataSetChanged() once every second (as opposed to the amount of timers you have - every second)

i think the main issue is the sequential executing of instructions, and adding Threads concept may resolve the issue ,what i think is Threads and Multi threading will allow timers to start at the same time and any updates or other instructions of code execution will not effect the already running code .
Multi threading refers to two or more tasks executing concurrently within a single program. Tutorial
now To start the threads at exactly the same time (at least as good as possible),we can use a CyclicBarrier:
// We want to start just 10 threads at the same time, but let's control that
// timing from the button_start click thread. That's why we have 11 "parties" instead
//of 10.
final CyclicBarrier gate = new CyclicBarrier(11);
button_start.setOnClickListener(v - > {
for (Timer timer: timerList) {
new Thread() {
public void run() {
gate.await(); // waiting for the gate to open
timer.startTimer();
}
// At this point, all the 10 timers are blocked on the gate.
// Since we gave "11" as the argument, gate is not opened yet.
// Now if we block on the gate from the main thread, it will open
// and all threads will start to do stuff!
gate.await(); // this is the extra 11th which will open the gate
};
This still can't make sure that they are started exactly at the same time on standard JVMs, but you can get pretty close.
replicate the same solution if it works , for the update part

Java Timer For Game for food system

I need a java timer to recognize when it hits a certain time for the game I am making. I am new to timers and I was wondering if this code works somehow. I need the if statement to work.
Timer timer = new Timer(whatevergoeshere,this);
public void farmingTimer()
{
timer.start();
if(timer == 1000)
{
food++;
timer.restart();
timer.start();
{
}

I found a good example page.
But I don't think you should be using timers for this kind of thing. You are probably better off updating things as often as you can, and keeping track of the time between updates. Then in the object that has then timer in, can just keep track of the amount of time passed.
public class FoodTimer
{
public int food = 0;
private double timer = 0;
public void update(double dt)
{
timer += dt;
if(timer > 1000)
{
food++;
timer = 0;
}
}
}

I suggest you read this: How to setup a timer (Java 7).
What you basically want to do is create an instance of the class Timer eg:
Timer timer = new Timer();
In your question above, your constructor call used two parameters. If you read the docs, you will find out those two parameters accept a string (name) and a boolean (isDaemon) eg
Timer(String name, boolean isDaemon)
If you want to learn more about daemon threads, I suggest you read this article: Daemon Threads in Java.
Now, once you have your instance ready, do this:
timer.scheduleAtFixedRate(new TimerTask() { //scheduleAtFixedRate means execution will repeat
#Override
public void run() {
//your code here eg food++;
}
}, 0, 60*1000); // the two values at the end are delay and period - delay is 0 seconds and period states that the block will be executed every minute (both values are in milliseconds)

Java Design Issue: Enforce method call sequence

There is a question which was recently asked to me in an interview.
Problem: There is a class meant to profile the execution time of the code. The class is like:
Class StopWatch {
long startTime;
long stopTime;
void start() {// set startTime}
void stop() { // set stopTime}
long getTime() {// return difference}
}
The client is expected to create an instance of the StopWatch and call methods accordingly. User code can mess up the use of the methods leading to unexpected results. Ex, start(), stop() and getTime() calls should be in order.
This class has to be "reconfigured" so that user can be prevented from messing up the sequence.
I proposed use of custom exception if stop() is called before start(), or doing some if/else checks, but interviewer was not satisfied.
Is there a design pattern to handle these kind of situations?
Edit: The class members and method implementations can be modified.

First there is the fact that implementing an own Java profiler is a waste of time, since good ones are available (maybe that was the intention behind the question).
If you want to enforce the correct method order at compile time, you have to return something with each method in the chain:
start() has to return a WatchStopper with the stop method.
Then WatchStopper.stop() has to return a WatchResult with the getResult() method.
External Construction of those helper classes as well as other ways of accessing their methods have to be prevented of course.

With minor changes to the interface, you can make the method sequence the only one that can be called - even at compile time!
public class Stopwatch {
public static RunningStopwatch createRunning() {
return new RunningStopwatch();
}
}
public class RunningStopwatch {
private final long startTime;
RunningStopwatch() {
startTime = System.nanoTime();
}
public FinishedStopwatch stop() {
return new FinishedStopwatch(startTime);
}
}
public class FinishedStopwatch {
private final long elapsedTime;
FinishedStopwatch(long startTime) {
elapsedTime = System.nanoTime() - startTime;
}
public long getElapsedNanos() {
return elapsedTime;
}
}
The usage is straightforward - every method returns a different class which only has the currently applicable methods. Basically, the state of the stopwatch is encapsuled in the type system.
In comments, it was pointed out that even with the above design, you can call stop() twice. While I consider that to be added value, it is theoretically possible to screw oneself over. Then, the only way I can think of would be something like this:
class Stopwatch {
public static Stopwatch createRunning() {
return new Stopwatch();
}
private final long startTime;
private Stopwatch() {
startTime = System.nanoTime();
}
public long getElapsedNanos() {
return System.nanoTime() - startTime;
}
}
That differs from the assignment by omitting the stop() method, but that's potentially good design, too. All would then depend on the precise requirements...

We commonly use StopWatch from Apache Commons StopWatch check the pattern how they've provided.
IllegalStateException is thrown when the stop watch state is wrong.
stop
public void stop()
Stop the stopwatch.
This method ends a new timing session, allowing the time to be retrieved.
Throws:
IllegalStateException - if the StopWatch is not running.
Straight forward.

Maybe he expected this 'reconfiguration' and question was not about method sequence at all:
class StopWatch {
public static long runWithProfiling(Runnable action) {
startTime = now;
action.run();
return now - startTime;
}
}

Once given more thought
In hindsight it sounds like they were looking for the execute around pattern. They're usually used to do things like enforce closing of streams. This is also more relevant due to this line:
Is there a design pattern to handle these kind of situations?
The idea is you give the thing that does the "executing around" some class to do somethings with. You'll probably use Runnable but it's not necessary. (Runnable makes the most sense and you'll see why soon.) In your StopWatch class add some method like this
public long measureAction(Runnable r) {
start();
r.run();
stop();
return getTime();
}
You would then call it like this
StopWatch stopWatch = new StopWatch();
Runnable r = new Runnable() {
#Override
public void run() {
// Put some tasks here you want to measure.
}
};
long time = stopWatch.measureAction(r);
This makes it fool proof. You don't have to worry about handling stop before start or people forgetting to call one and not the other, etc. The reason Runnable is nice is because
Standard java class, not your own or third party
End users can put whatever they need in the Runnable to be done.
(If you were using it to enforce stream closing then you could put the actions that need to be done with a database connection inside so the end user doesn't need to worry about how to open and close it and you simultaneously force them to close it properly.)
If you wanted, you could make some StopWatchWrapper instead leave StopWatch unmodified. You could also make measureAction(Runnable) not return a time and make getTime() public instead.
The Java 8 way to calling it is even simpler
StopWatch stopWatch = new StopWatch();
long time = stopWatch.measureAction(() - > {/* Measure stuff here */});
A third (hopefully final) thought: it seems what the interviewer was looking for and what is being upvoted the most is throwing exceptions based on state (e.g., if stop() is called before start() or start() after stop()). This is a fine practice and in fact, depending on the methods in StopWatch having a visibility other than private/protected, it's probably better to have than not have. My one issue with this is that throwing exceptions alone will not enforce a method call sequence.
For example, consider this:
class StopWatch {
boolean started = false;
boolean stopped = false;
// ...
public void start() {
if (started) {
throw new IllegalStateException("Already started!");
}
started = true;
// ...
}
public void stop() {
if (!started) {
throw new IllegalStateException("Not yet started!");
}
if (stopped) {
throw new IllegalStateException("Already stopped!");
}
stopped = true;
// ...
}
public long getTime() {
if (!started) {
throw new IllegalStateException("Not yet started!");
}
if (!stopped) {
throw new IllegalStateException("Not yet stopped!");
}
stopped = true;
// ...
}
}
Just because it's throwing IllegalStateException doesn't mean that the proper sequence is enforced, it just means improper sequences are denied (and I think we can all agree exceptions are annoying, luckily this is not a checked exception).
The only way I know to truly enforce that the methods are called correctly is to do it yourself with the execute around pattern or the other suggestions that do things like return RunningStopWatch and StoppedStopWatch that I presume have only one method, but this seems overly complex (and OP mentioned that the interface couldn't be changed, admittedly the non-wrapper suggestion I made does this though). So to the best of my knowledge there's no way to enforce the proper order without modifying the interface or adding more classes.
I guess it really depends on what people define "enforce a method call sequence" to mean. If only the exceptions are thrown then the below compiles
StopWatch stopWatch = new StopWatch();
stopWatch.getTime();
stopWatch.stop();
stopWatch.start();
True it won't run, but it just seems so much simpler to hand in a Runnable and make those methods private, let the other one relax and handle the pesky details yourself. Then there's no guess work. With this class it's obvious the order, but if there were more methods or the names weren't so obvious it can begin to be a headache.
Original answer
More hindsight edit: OP mentions in a comment,
"The three methods should remain intact and are only interface to the programmer. The class members and method implementation can change."
So the below is wrong because it removes something from the interface. (Technically, you could implement it as an empty method but that seems to be like a dumb thing to do and too confusing.) I kind of like this answer if the restriction wasn't there and it does seem to be another "fool proof" way to do it so I will leave it.
To me something like this seems to be good.
class StopWatch {
private final long startTime;
public StopWatch() {
startTime = ...
}
public long stop() {
currentTime = ...
return currentTime - startTime;
}
}
The reason I believe this to be good is the recording is during object creation so it can't be forgotten or done out of order (can't call stop() method if it doesn't exist).
One flaw is probably the naming of stop(). At first I thought maybe lap() but that usually implies a restarting or some sort (or at least recording since last lap/start). Perhaps read() would be better? This mimics the action of looking at the time on a stop watch. I chose stop() to keep it similar to the original class.
The only thing I'm not 100% sure about is how to get the time. To be honest that seems to be a more minor detail. As long as both ... in the above code obtain current time the same way it should be fine.

I suggest something like:
interface WatchFactory {
Watch startTimer();
}
interface Watch {
long stopTimer();
}
It will be used like this
Watch watch = watchFactory.startTimer();
// Do something you want to measure
long timeSpentInMillis = watch.stopTimer();
You can't invoke anything in wrong order. And if you invoke stopTimer twice you get meaningful result both time (maybe it is better rename it to measure and return actual time each time it invoked)

Throwing an exception when the methods are not called in the correct order is common. For example, Thread's start will throw an IllegalThreadStateException if called twice.
You should have probably explained better how the instance would know if the methods are called in the correct order. This can be done by introducing a state variable, and checking the state at the start of each method (and updating it when necessary).

This can also be done with Lambdas in Java 8. In this case you pass your function to the StopWatch class and then tell the StopWatch to execute that code.
Class StopWatch {
long startTime;
long stopTime;
private void start() {// set startTime}
private void stop() { // set stopTime}
void execute(Runnable r){
start();
r.run();
stop();
}
long getTime() {// return difference}
}

Presumably the reason for using a stopwatch is that the entity that's interested in the time is distinct from the entity that's responsible for starting and stopping the timing intervals. If that is not the case, patterns using immutable objects and allowing code to query a stop watch at any time to see how much time has elapsed to date would likely be better than those using a mutable stopwatch object.
If your purpose is to capture data about how much time is being spent doing various things, I would suggest that you might be best served by a class which builds a list of timing-related events. Such a class may provide a method to generate and add a new timing-related event, which would record a snapshot of its created time and provide a method to indicate its completion. The outer class would also provide a method to retrieve a list of all timing events registered to date.
If the code which creates a new timing event supplies a parameter indicating its purpose, code at the end which examines the list could ascertain whether all events that were initiated have been properly completed, and identify any that had not; it could also identify if any events were contained entirely within others or overlapped others but were not contained within them. Because each event would have its own independent status, failure to close one event need not interfere with any subsequent events or cause any loss or corruption of timing data related to them (as might occur if e.g. a stopwatch had been accidentally left running when it should have been stopped).
While it's certainly possible to have a mutable stopwatch class which uses start and stop methods, if the intention is that each "stop" action be associated with a particular "start" action, having the "start" action return an object which must be "stopped" will not only ensure such association, but it will allow sensible behavior to be achieved even if an action is started and abandoned.

I know this already has been answered but couldn't find an answer invoking builder with interfaces for the control flow so here is my solution :
(Name the interfaces in a better way than me :p)
public interface StartingStopWatch {
StoppingStopWatch start();
}
public interface StoppingStopWatch {
ResultStopWatch stop();
}
public interface ResultStopWatch {
long getTime();
}
public class StopWatch implements StartingStopWatch, StoppingStopWatch, ResultStopWatch {
long startTime;
long stopTime;
private StopWatch() {
//No instanciation this way
}
public static StoppingStopWatch createAndStart() {
return new StopWatch().start();
}
public static StartingStopWatch create() {
return new StopWatch();
}
#Override
public StoppingStopWatch start() {
startTime = System.currentTimeMillis();
return this;
}
#Override
public ResultStopWatch stop() {
stopTime = System.currentTimeMillis();
return this;
}
#Override
public long getTime() {
return stopTime - startTime;
}
}
Usage :
StoppingStopWatch sw = StopWatch.createAndStart();
//Do stuff
long time = sw.stop().getTime();

As Per Interview question ,It seems to like this
Class StopWatch {
long startTime;
long stopTime;
public StopWatch() {
start();
}
void start() {// set startTime}
void stop() { // set stopTime}
long getTime() {
stop();
// return difference
}
}
So now All user need to create object of StopWatch class at beginning and getTime() need to call at End
For e.g
StopWatch stopWatch=new StopWatch();
//do Some stuff
stopWatch.getTime()

I'm going to suggest that enforcing the method call sequence is solving the wrong problem; the real problem is a unfriendly interface where the user must be aware of the state of the stopwatch. The solution is to remove any requirement to know the state of the StopWatch.
public class StopWatch {
private Logger log = Logger.getLogger(StopWatch.class);
private boolean firstMark = true;
private long lastMarkTime;
private long thisMarkTime;
private String lastMarkMsg;
private String thisMarkMsg;
public TimingResult mark(String msg) {
lastMarkTime = thisMarkTime;
thisMarkTime = System.currentTimeMillis();
lastMarkMsg = thisMarkMsg;
thisMarkMsg = msg;
String timingMsg;
long elapsed;
if (firstMark) {
elapsed = 0;
timingMsg = "First mark: [" + thisMarkMsg + "] at time " + thisMarkTime;
} else {
elapsed = thisMarkTime - lastMarkTime;
timingMsg = "Mark: [" + thisMarkMsg + "] " + elapsed + "ms since mark [" + lastMarkMsg + "]";
}
TimingResult result = new TimingResult(timingMsg, elapsed);
log.debug(result.msg);
firstMark = false;
return result;
}
}
This allows a simple use of the mark method with a result returned and logging included.
StopWatch stopWatch = new StopWatch();
TimingResult r;
r = stopWatch.mark("before loop 1");
System.out.println(r);
for (int i=0; i<100; i++) {
slowThing();
}
r = stopWatch.mark("after loop 1");
System.out.println(r);
for (int i=0; i<100; i++) {
reallySlowThing();
}
r = stopWatch.mark("after loop 2");
System.out.println(r);
This gives the nice result of;
First mark: [before loop 1] at time 1436537674704
Mark: [after loop 1] 1037ms since mark [before loop 1]
Mark: [after loop 2] 2008ms since mark [after loop 1]

How to create a Java function which Once called, cannot be called Again unless there is some DELAY?

I'm trying to make a function which can ONLY be called again after there is some amount of time delay between the two calls, (Say 5 seconds).
I require this functionality for an android app I'm creating.
Since it is possible that the user would be calling that function too frequently within a few seconds, it would destroy his experience. Hence, I'm desperately looking for an answer on this.
public void doSomethin(){
//code here which makes sure that this function has not been called twice within the specified delay of 5 seconds
//Some code here
}
Any help would be awesome!
Adit

You could hold the time in milliseconds and check if the current time is greater than or equal to the previous time + 5 seconds. If it is, execute the method and replace the previous time with the current time.
See System.currentTimeMillis()
public class FiveSeconds {
private static Scanner scanner = new Scanner(System.in);
private static long lastTime = 0;
public static void main(String[] args) {
String input = scanner.nextLine();
while(!input.equalsIgnoreCase("quit")){
if(isValidAction()){
System.out.println(input);
lastTime = System.currentTimeMillis();
} else {
System.out.println("You are not allowed to do this yet");
}
input = scanner.nextLine();
}
}
private static boolean isValidAction(){
return(System.currentTimeMillis() > (lastTime + 5000));
}
}

If the code runs on your main thread, Thread.sleep(5000) is not an option. The easiest way to go then would be:
private long previous = 0;
public void doSomething() {
long now = Calendar.getInstance().getTimeInMillis();
if (now - previous < 5000)
return;
previous = now;
// do something
}

use a static field, which saves the datetime of last execution and before executing check the current datetime against this timestamp.

I won't write code for you but if you think a bit about this it's not that hard. You want to make sure it's not called before 5 seconds has passed since last function call. Let's split this into parts.
1) Get current time
2) Compare to stored time that keeps it value between calls.
3) If less than 5 seconds has passed don't do anything
4) Otherwise store the new value and do stuff.

I never did coding in Android. But if Android has threads (which is most likely, it does). Then within this function sleep the thread for 5 seconds, it means even though it is called, it won't be executed further until 5 seconds are passed
public void doSomething()
{
Thread.sleep(5000); // Please find the corresponding Android method
}

make doSomethin() as a private method.
Expose another function say exposedDoSomething().
public void exposeDoSomething(){
// Add to a queue
}
In a separate thread read the queue in every 5 mins and call doSomethin()

Maybe a TimerTask will help you, more details here:
TimerTask|Android Developers
Example:
private TimerTask tiemrTast= new TimerTask() {
#Override
public void run() {
}
};
And then use this method: public void scheduleAtFixedRate (TimerTask task, long delay, long period)

One problem is that you'll have to have a session level timer for each and every user. This could be a fair amount of overhead depending on how many simultaneous users you have.
It's easy for one user: start a timer on request. It's distinguishing between users that's the problem. Your service is now stateful, which is usually a bad thing. You're giving away idempotence.
You could also maintain state in a database. Don't worry about calls; check persistent operations to make sure they don't happen too frequently.
It's not clear to me whether you mean to exclude all users for five seconds once the method is called by anyone, or if every user is prevented from calling the method again for five seconds once it's called the first time.
This sounds like a throughput killer. I'd reconsider this design.

you can use hander.postDalayed method to call method to run after specific delay
you can use your method something like this
int delay=5000;
int k;
int iDelayTimeInMiliSeconds=2000;
for( k=0;k<arrBatch.size();k++)
{
delay=delay+iDelayTimeInMiliSeconds;
callAllDocumentList(arrBatch.get(k),k,delay);
}
public void callAllDocumentList(final String strInvoiceId,final int count,int delay)
{
mHandler.postDelayed(new Runnable()
{
#Override
public void run()
{
// call your function here that you want to call after specific delay
//getOrderDetails(strInvoiceId,count,true);
}
}, delay);
}

Each thread should run for particular duration of time

I am working on a project in which I will be spawning multiple threads from a multithreaded code.
Suppose I am spawning 10 threads, then I need to make sure each thread should be running for particular duration of time.
For example, if I want each thread should run for 30 minutes, then in the config.properties file, I will be having TOTAL_RUNNING_TIME=30
So I came up with the below design to make sure each thread is running for 30 minutes.
private static long durationOfRun;
private static long sleepTime;
public static void main(String[] args) {
// create thread pool with given size
ExecutorService service = Executors.newFixedThreadPool(threads);
try {
readPropertyFile();
for (int i = 0; i < threads; i++) {
service.submit(new ReadTask(durationOfRun, sleepTime));
}
}
}
private static void readPropertyFile() throws IOException {
prop.load(Read.class.getClassLoader().getResourceAsStream("config.properties"));
threads = Integer.parseInt(prop.getProperty("NUMBER_OF_THREADS"));
durationOfRun = Long.parseLong(prop.getProperty("TOTAL_RUNNING_TIME"));
sleepTime = Long.parseLong(prop.getProperty("SLEEP_TIME"));
}
Below is my ReadTask class.
class ReadTask implements Runnable {
private long durationOfRun;
private long sleepTime;
public ReadTask(long durationOfRun, long sleepTime) {
this.durationOfRun = durationOfRun;
this.sleepTime = sleepTime;
}
#Override
public void run() {
long startTime = System.currentTimeMillis();
long endTime = startTime + (durationOfRun*60*1000);
//Each thread is running less than endTime
while(System.currentTimeMillis() <= endTime) {
//Do whatever you want to do
}
Thread.sleep(sleepTime);
}
}
If you take a look into my run method, I have a while loop which will check the time. So this approach of making each thread run for particular duration of time is correct or not? Or is there any better way also? Please ignore my ignorance if there are any other better approach or this will also serve the purpose?
Let me know if there are any thread safety issues here as well?
What I am looking for is each thread should run for 30 minutes and if the time for that thread has finished, then complete the task on which it is running currently and do not take anything else after that, just like we have shutdown for ExecutorService. If there is any better approach or better design than this. Please provide me some example so that I can learn that stuff just from my knowledge point of view. Thanks for the help.
UPDATE:-
If you take a look into my while loop in the run method, inside that while loop I will be trying to make a Select call to the database. So what I am looking is something like this- As soon as the time for that thread is finished, it will not make any other select call to the database and finished whatever it was doing previously. Just like shutdown works for ExecutorService.
And I don't want this scenario- as soon as the time for that thread is finished, it will timeout the thread as it might be possible, that particular thread was doing select to database in that period?

One of the concerns I have with this design, is what if the amount of work being done within the while-loop takes more to them the time our?
For example
while(System.currentTimeMillis() <= endTime) {
calaculateTheMeaningOfLife();
}
What happens now? What's stopping the thread, or encouraging it to check the timeout? The samething will occur if you have a blocking operation, such as a File or socket read/write
You could try to interrupt the thread, but there is no guareentee that this will help

You can set the time limit to the the executor. You need to cast the executor returned by Executors and set the time limit:
ThreadPoolExecutor service = (ThreadPoolExecutor) Executors.newFixedThreadPool(4);
service.setKeepAliveTime(1800, TimeUnit.SECONDS);
Taken from "Java Concurrency in Practice" by Brian Goetz.