Waiting on a condition

[nephos]

What is the preferred way of waiting until a condition becomes true? Right now I have something like:

while(isTrue()){//wait}

Is there a better way of doing this please? Thanks!

[sunde887]

A loop requires the condition to be true already to enter the loop body. So if variable x is originally false, using it in the loop condition will make it so you never enter the loop. You can negate the condition to allow the loop to run until the condition is true.

 
boolean x = false;
while(x){//loop doesn't get entered since the condition is always false}
while(!x){//enters the loop as long as x is false, when x changes to true it will leave the loop}

[nephos]

Thanks sunde :) I was wondering is there is a way to wait on a condition without using an empty loop?

[sunde887]

Well if you have an empty loop statement you will create an finite loop, it will just enter the loop, do nothing and then repeat the loop. Each time through not doing anything, thus the condition will always be what it originally was.

Can you give more details as to what you want to do?

[dlorde]

It depends on exactly what you want to achieve, but in general just using an empty loop is not a good idea because the processor is occupied racing round the loop and has no time to handle anything else.

It is generally better to use a loop with a sleep for a certain time, to free up processor time for other threads. See Pausing Execution with Sleep.

[Fubarable]

Another way is to use an observer/observable and have the observe react when the observable notifies it of an event. Most GUI libraries (that I'm familiar with) are based on this paradigm.

[nephos]

Thanks guys, the observer pattern seems mostly what I need. :) Could anyone give me an example of it please? Lets say my code is:

class Class1{

boolean isTrue = false;

public boolean getIsTrue(){
return isTrue;
}
}

class Class2{


while(Class1.getIsTrue()){
//wait
}

//code to be executed after wait
}

How would I change this to become observable please?

[JosAH]

Thanks guys, the observer pattern seems mostly what I need. :) Could anyone give me an example of it please? Lets say my code is:

class Class1{

boolean isTrue = false;

public boolean getIsTrue(){
return isTrue;
}
}

class Class2{


while(Class1.getIsTrue()){
//wait
}

//code to be executed after wait
}

How would I change this to become observable please?

A Class1 object is the observable and Class2 objects are the observers. Observers should register itself at the observable and the observable should notify the observers when its state changes. Read the API documentation for the Observable/Observer classes/interfaces. If you're talking multiple threads your original wait() paradigm is also fine: as long as a condition is not met your object should wait(). If the condition changes somewhere the object responsible for the condition change should notifyAll() objects waiting on that condition. Read the API documentation for the Thread and Object classes. For a more solid approach read all classes APIs in the java.util.concurrent package.

kind regards,

Jos

[nephos]

Thanks Jos!

[JosAH]

Thanks Jos!

You're welcome of course; if you go for a single threaded approach also read the API documentation of the PropertyChangeSupport; it takes away the nitty gritty details of managing the observers and manages the groups of properties for which the observers should be notified. Also there are numerous Listeners for certain events, maybe you can use one of those.

kind regards,

Jos