or at a coarser frequency if you prefer, since it changes state infrequently. Call this periodically, say once per frame update, NextWaveSpawnTime = System.nanoTime() + 1000000000 That might look something like this: // Call this at the start of your level. Instead, what you really want to do is call this function periodically, and each time check if it's time to spawn a new wave, rather than hold the CPU hostage until it's time. ![]() So, don't put a wait loop inside your game loop. i is still less than 3, so we're stil not allowing the CPU to move on yet! Back to step 3 we go, and burn another second doing nothing. We hit the end of the while loop and check the condition again. You've effectively stalled your game here.įinally, an eternity of wasted CPU time later, we've managed to burn a second of time and our inner if condition finally passes. This is called a " busy-wait" or "spinning" - the processor is kept busy checking the clock then looping then re-checking the clock, so it's never allowed to exit this loop, return from this function, update the rest of your game state, process player input, or draw a frame. We keep doing this BILLIONS of times, because we do so little work inside the loop each time, nanoTime isn't much bigger each time we loop around. i is still less than 3, so we re-enter the loop and go back to step 3. We hit the end of the while loop, and check the condition again. We check if the current time is 1 second later than begin time yet, but of course it isn't! We've barely executed two instructions since begin time. We check the while loop's condition, and i is indeed less than 3, so we enter the loop. If( Greenfoot.Let's walk through this code, pretending that nanoTime = 0 at the start for simplicity. While you are there, make sure you are minimally familiar with all the tutorials provided on. Go to for easy-to-follow instructions on downloading and installing Greenfoot. Then, we will layer on additional functionality, such as scoring, introduction and game-over screens, and the notion of levels.Īs mentioned in the preface, we'll assume you have downloaded Greenfoot and have it installed. We will first build the basic components of the Avoider game, including the initial scenario, the game environment, the enemies, and the hero. We will pause frequently to consider best practices and good programming practice. As with Michael James Williams' tutorial, we will start small and slowly layer on functionality. We will build the same game in Greenfoot, instead of Flash and ActionScript. The longer you avoid them, the higher your score. The goal for the player is to avoid these enemies. In that tutorial, you build a game that creates smiley-faced enemies that rain down from the top of the screen. ![]() This tutorial is heavily based on AS3 Avoider Game Tutorial by Michael James Williams ( ). This chapter provides the framework to create Greenfoot applications that we will continue to use, and refer to, in later chapters. Many of the chapters in this book are independent however, most are dependent on this chapter. In other words, follow the advice of Confucius, quoted in the first line of this chapter. Experiment with the code and try new things-you'll be glad you did. Be sure to look at the simple tutorials and documentation at when needed. Similarly, this book assumes a minimal understanding of Greenfoot. Java is a well-established programming language, and there are endless online resources you can consult. If you are new to Java, or it's been a while since you've programmed in Java, be sure to take the time to look up things that may be confusing to you. As you proceed, think about the concepts presented and how you would use them in your own projects. Throughout this chapter, we'll learn basic programming concepts and gain familiarity with the Greenfoot development environment. Use the mouse to control the movement of an actorĭynamically spawn enemies and remove them when appropriate Create introduction and game-over screens
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