Java 8 Possible Double Lambda Expression

Question

I am creating a simulator, but for simplicity sake, it's a 'game'. Thus it has a render() and an update() function. I've been trying to practice lambda expressions during summer but I cannot seem to be able to wrap my head around doing a lambda expression of another lambda expression. I am probably saying this wrong, but what I am trying to do is start two threads, one that loops rendering, and another that loops updating. I can get this far:

void render() {
    //draw entities, etc.
}
void update() {
    //update player/enemies, etc.
}

public GameFrame() {
    /* init stuff */
    Thread updateThread = new Thread(this::update);
    Thread renderThread = new Thread(this::render);
}

This is not what I want because this only runs update and draw once, thus this is just two threads, one that renders once, and one that updates once (no looping). I want to create a function that does something along the lines of:

public void loop(Supplier< /*?*/ > arg) {
    long startTime;
    while(running) {
        startTime = System.currentTimeMillis();
        supplier.get(arg) // <- not sure about this either
        try {
            long sleepTime = 1000/FPS - (System.currentTimeMillis() - startTime);
            if(sleepTime > 0)
                Thread.sleep(sleepTime);
        } catch(InterruptedException e) {
            e.printStackTrace();
        }
    }
}

Then I would call the function like this?

loop(this::update);

I believe this would cause the supplied function to loop, thus I tried:

Thread updateThread = new Thread(this::loop(this::update));

Or even something like this:

new Thread(() -> loop(this::update)).start();

I know I can just make my render function while loop, and my update function while loop by just copying the 'void loop(Supplier arg)' code into each part, but I wanted to see if I could do it this way anyways.

I don't need an exact answer, I would just like some guidance to what I'm not thinking about/what I'm thinking about wrong. I haven't been able to make much progress by reading up on lambda expressions on oracle.

Your last idea is OK, but the loop() method should take a Runnable as argument, not a Supplier, since you want to execute a function that takes no argument, and returns nothing, and that's what a Runnable is. And it should thus call run() on that runnable, at each iteration. — JB Nizet
– JB Nizet, Commented Aug 17, 2016 at 6:24
First thing, why are you thinking about a "sleep loop" instead of using a Timer? — RealSkeptic
– RealSkeptic, Commented Aug 17, 2016 at 6:25
As an alternative to using bare metal threads, you could as well use ExecutorServices and keep submitting jobs into them. That wont help with lambdas, but it will making testing a whole lot easier (because you can replace a multi-threaded executor service with a "do-it-in-the-current-thread" executor easily. — GhostCat
– GhostCat, Commented Aug 17, 2016 at 6:33
@RealSkeptic I will look into timers. I didn't know what they were though. — Michael Choi
– Michael Choi, Commented Aug 17, 2016 at 6:46
@JBNizet Ah! Thank you that makes so much sense! I always looped inside the run() but never looped outside of it so I was not used to the idea. — Michael Choi
– Michael Choi, Commented Aug 17, 2016 at 6:50

fabian · Accepted Answer · 2016-08-17 07:55:57Z

1

The Thread constructor takes Runnable and this is what you want to execute repeatedly. Therefore you don't need Supplier as parameter type for loop, but Runnable.

public void loop(Runnable arg) {
    long startTime;
    while(running) {
        startTime = System.currentTimeMillis();
        arg.run();
        try {
            long sleepTime = 1000/FPS - (System.currentTimeMillis() - startTime);
            if(sleepTime > 0)
                Thread.sleep(sleepTime);
        } catch(InterruptedException e) {
            e.printStackTrace();
        }
    }
}

Note that is isn't possible to specify parameters with a method reference. This is invalid syntax:

this::loop(this::update)

The statement you want the thread to execute is:

loop(this::update);

so you need to use a lambda expression with this statement as body as parameter of the Thread constructor:

Thread updateThread = new Thread(() -> loop(this::update));

Note that a ScheduledExecutorService provides the kind of scheduling you're implementing here.

answered Aug 17, 2016 at 7:55

fabian

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2 Comments

Michael Choi Over a year ago

would you happen to know why Java does not allow: this::loop(this::update)? I noticed that if a function has a parameter, I must reference it like () -> doFoo(arg). Why is this?

fabian Over a year ago

a) you need a target type for a method reference to work (for this::loop there isn't one) and b) even if you had a target type java doesn't support currying as a language feature, so you need to implement it differently.

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