Lander
An Lunar Example of Interacting Classes
Apr 10, 2009
LunarLanderGame
This class contains the publicstaticvoidmain(String[]args) method. In this method, you should (1) create a LunarLander object, (2) create an IOFrame object, and (3) send messages to these two objects in order to get the burn rate from the user, tell the lander what to do, find out the lander status, and report the status back to the user. When the lander's altitude is less than or equal to zero, the game ends (one way or the other). When the game finishes, you should ask the user whether he/she would like to play again. Allow as many games as the user wants before quitting the program.
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Additional constraints
At the time this program was assigned, we had:
Classes and methods Declarations, with and without initialization Assignment statements while loops if and ifelse statements System.out.print and System.out.println
The first version of this program reflects these constraints
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publicstaticvoidmain(String[]args)
Heres the outline of the mainmethod: publicstaticvoidmain(String[]args){ IOFrameframe=newIOFrame("LunarLander"); LunarLanderlander; doublesafeLandingSpeed=10.0; Stringagain="yes";
//MainloopOnegameisoncethroughtheloop
//Innerloop:PlayuntiltheLandercrashesorlands
frame.exit(); }
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Main loop
Recall that we had a Stringagain="yes"; declaration Heres the main loop:
while("yes".equals(again)){ //setup lander=newLunarLander(100,0,15); frame.clear(); frame.displayLine("Altitude:"+lander.getAltitude()); frame.displayLine("Velocity:"+lander.getVelocity()); frame.displayLine("Fuelleft:"+lander.getFuelRemaining()); //innerlooptoplaythegame //telluserhowgameended again=frame.getString("Wouldyouliketoplayagain?"); }
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Inner loop to play game
//innerlooptoplayonegame
while(lander.getAltitude()>0){ doubleamount=frame.getDouble("HowmuchfuelshouldIburn?"); lander.burn(amount); frame.displayLine(""); frame.displayLine("Altitude:"+lander.getAltitude()); frame.displayLine("Velocity:"+lander.getVelocity()); frame.displayLine("Fuelleft:"+lander.getFuelRemaining()); }
//telluserhowgameended
doublefinalVelocity=lander.getVelocity(); if(finalVelocity>safeLandingSpeed){ frame.display("CRASH!"); } else{ frame.display("Congratulations!Asafelanding!"); }
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LunarLander instance variables
publicclassLunarLander{ doublealtitude; doublevelocity; doublefuel; finaldoubleacceleration=1.64; finaldoublefuelFactor=1.0; finaldoublefuelLimit=2.0;
Where:
acceleration is the Moons gravity (meters/second/second) fuelFactor is how much to reduce the velocity per unit of fuel burned fuelLimit is the maximum amount of fuel that can be burned at once
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The LunarLander constructor
LunarLander(doublealtitude,doublevelocity, doublefuel){ this.altitude=altitude; this.velocity=velocity; this.fuel=fuel; }
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The burn(double) method
This is the method that does all the work of the LunarLanderit recomputes velocity, altitude, and fuel remaining
voidburn(doubleamount){ if(amount>fuel)amount=fuel; if(amount>fuelLimit)amount=fuelLimit; velocity=velocity+accelerationamount*fuelFactor; fuel=fuelamount; altitude=altitudevelocity; }
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The getter methods
doublegetFuelRemaining(){ returnfuel; } doublegetAltitude(){ returnaltitude; } doublegetVelocity(){ returnvelocity; } Thats all... ...but the program could be improved
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Part II
Improving the Lunar Lander program
Refactoring
Refactoring is the process of rearranging and modifying code to improve it (without changing what it does) The following lines occur twice in the code:
frame.displayLine("Altitude:"+lander.getAltitude()); frame.displayLine("Velocity:"+lander.getVelocity()); frame.displayLine("Fuelleft:"+lander.getFuelRemaining())
Duplicated code is bad because, if you have to change it, you have to change it everyplace it is used One way to avoid code repetition is to put the offending code into a method
This particular refactoring is called Method Extract It isnt necessarily a single-step method Ill do it informally
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Extract Method IdisplayStatus()
I create the following method:
voiddisplayStatus(){ frame.displayLine("Altitude:"+lander.getAltitude()); frame.displayLine("Velocity:"+lander.getVelocity()); frame.displayLine("Fuelleft:"+lander.getFuelRemaining()); } displayStatus();
I call our new method this way (in two places):
This gives me syntax errors, because both frame and lander are local variables in main I have three choices:
Pass in frame and lander as parameters Make frame and lander instance variables of LunarLanderGame Make frame and lander class variables of LunarLanderGame
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Extract Method IIframe
I only ever want one IOFrame, so it makes sense to make frame a static variable I move this line IOFrameframe=newIOFrame("LunarLander"); out of the main method, put the word static in front of it, and make it a field of the LunarLanderGameclass publicclassLunarLanderGame{ staticIOFrameframe=newIOFrame("LunarLander"); This solves the syntax error for frame
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Extract Method III--lander
It seemed to me that each LunarLanderGameshould have its ownLunarLander
Besides, I have no easy way to reset LunarLander for a new game I therefore decided to make LunarLander an instance variable Hence, this code: publicclassLunarLanderGame{ publicstaticvoidmain(String[]args){ LunarLanderlander; ... Becomes this code: publicclassLunarLanderGame{ LunarLanderlander; publicstaticvoidmain(String[]args){ ...
This does not fix the syntax errorI cant access LunarLander (an instance variable) from within main (a static method)
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Extract Method III--run
We have a syntax error because:
Every LunarLanderGame has its own LunarLander We have no LunarLanderGame objects
Solution: Make a LunarLanderGame object in the main method, and let it do the work
publicstaticvoidmain(String[]args){ LunarLanderGamegame=newLunarLanderGame(); game.run(); } voidrun(){...uselanderhere...}
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Minor improvements I
It seems unnatural to set the variable again just so we can go through the loop the first time:
Stringagain="yes"; while("yes".equalsIgnoreCase(again)){ .... again=frame.getString("Wouldyouliketoplayagain?"); } Stringagain; do{ ... again=frame.getString("Wouldyouliketoplayagain?"); }while("yes".equals(again)); Note: It doesnt work to define again within the loop
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With thedo...whileloop, we can revise this to:
Minor improvements II
As written, any response other than yes will be taken to mean no
This is a real nuisance, especially if you make a typing error do{ ...mainbodyofloop... do{ again=frame.getString("Wouldyouliketoplayagain?"); }while(!"yes".equals(again)&&!"no".equals(again)); }while("yes".equals(again));
Instead of equals, we can use equalsIgnoreCase
This allows responses such as YES as well as yes
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How is this improved?
Weve moved our status display code into a method, so any corrections or improvements in it need only be done in one place
This also made the main loop smaller, so its easier to read and understand
Weve used the right kind of loop to avoid a rather silly-looking initialization Weve made the yes/no question much more robust
Come to think of it, this would be a good candidate for a methodgetYesOrNo("Wouldyouliketoplayagain?")
The code ends up being eight lines longer (you cant win them all!)
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Summary: Refactoring
Refactoring is the process of rearranging and modifying code to improve it (without changing what it does)
You improve code by making it:
Easier to read and understand Easier to debug Easier to modify
Real programs are constantly being read and modified Hence, refactoring is important and useful
Conventional wisdom says: If it aint broke, dont fix it!
This is because its easy to introduce errors Refactoring is best done if you have a suite of tests you can run to make sure you didnt break anything JUnit is one of the best ways to develop a suite of tests
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The End
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