Lecture 6: Do I HAVE TO Test IT? Why ?

CSC 213 – Large Scale Programming Lecture 6:Do I HAVE TO Test IT? Why?

Why Do We Test?

But the Problem Is…

Can We Get Best Of Both?

Testing To The Rescue • Before we start coding, write solid set of tests • Best, worst, & average casesexamined in tests • Includes tests for all possible boundary conditions • Tests drive the code & set pace of the coding • Code only written to be good enough to pass tests • Debugging simplified using the simple failed cases • Once system can pass all tests, we’re golden

Oops… • Design we created has lots of side effects • Results printed out at the end of void methods • Other methods just update values of object’s fields • Add special methods that allow field access • Not a big deal & needed to test affect on fields' values

OOPS… • Stuck with BIG issue: how to check I/O correct? • How to test screen to verify warning are printed? • How to insure file has correct values before & after?

OOPS… • Stuck with BIG issue: how to check I/O correct? • How to test screen to verify warning are printed? • How to insure file has correct values before & after? • Cannot skip tests since correctness important • Always want to be correct, but cannot test online

OOPS… • Stuck with BIG issue: how to check I/O correct? • How to test screen to verify warning are printed? • How to insure file has correct values before & after? • Cannot skip tests since correctness important • Always want to be correct, but cannot test online And it monitors a nuclear reactor

OOPS…

Laziness Depends on Design • Well-planned design makes testable methods • Testable methods leads to good tests • Good tests leads to easy coding & debugging • Easy coding & debugging leads to free time • Free time leads to laziness

Design Methods For Testing • Start by defining method for each calculation • Method returns result once it is calculated • As needed, create other methods to output results • Easy to test & debug methods with observable results

Design Methods For Testing • Start by defining method for each calculation • Method returns result once it is calculated • As needed, create other methods to output results • Easy to test & debug methods with observable results versus

Mock Objects To Win • Some things just cannot be tested with JUnit • Failed nuclear plant tests a bit of a problem • Some times its impossible to access or use classes • File or network I/O required, but cannot test offline • These situations leave limited options • Whine about it being unfair • Leave code untested and say "oops" a lot • Prove correctness mathematically • Create stand-ins called mock objects

Getting Ready for Mocks • Use interfaces in place of hard-to-use classes • Interface defines minimum number of methods • Update old classto implement this interface • Must modify code when hard-to-use class alloc'd • Pass instance to method & eliminate new command • If instance was field, set using constructor's parameter

Original Example public class ThreeMileIsland {private NuclearReactor reactor;public ThreeMileIsland() {reactor= new NuclearReactor("SNPP");}public booleancheckForBreach() { if (!reactor.withinLimits()) {return reactor.alarmSounding(); } return false;} }

New Interface public interface NRInterface {public booleanwithinLimits();public booleanalarmSounding(); } public class ThreeMileIsland {// More code went herepublic booleancheckForBreach() { if (!reactor.withinLimits()) {return reactor.alarmSounding(); } return false;} }

Rewrite From This… public class ThreeMileIsland {private NuclearReactor reactor;public ThreeMileIsland() {reactor= new NuclearReactor("SNPP");}public booleancheckForBreach() { if (!reactor.withinLimits()) {return reactor.alarmSounding(); } return false;} }

Rewrite From This… …To This public class ThreeMileIsland {private NRInterface reactor;public ThreeMileIsland(NRInterface nm) {reactor= nm;}public booleancheckForBreach() { if (!reactor.withinLimits()) {return reactor.alarmSounding(); } return false;} }

Why We Write Mock Classes! Testing ThreeMileIslandnotNuclearReactor

Create Mock Class • Write classes needed to test code fully • Testing ThreeMileIsland notNuclearReactor • ONLY purpose is testing other classes code • May hard-code values; write as simply as possible • Store parameters in Sequences to check calls correct • Do not use files or network, mock to avoid these needs • Duct tape & mock classes are similar • Useful & pretty, but not the real thing

Writing Mock Class public interface NRInterface {public booleanwithinLimits();public booleanalarmSounding(); } public class MeltdownMock implements NRInterface{public booleanwithinLimits() {return false; }public booleanalarmSounding() {return true; } }

Using Mock Class • Now run original method in two different ways • Provide and use mock objects when testing code • Actual classes during execution so still works • Remember: Testing ThreeMileIslandnot mock class

Writing Mock Class public class MeltdownMock implements NRInterface {public booleanwithinLimits() {return false; }public booleanalarmSounding() {return true; } } @Test public void testTMI() {NRInterfacemltdn= new MeltdownMock();ThreeMileIslandtmi = new ThreeMileIsland(mltdn);assertTrue(tmi.checkForBreach()); }

Testing Printing To Screen • Java already thought of this & created solution • Only need to set static field in Systemclass • System.setOut(PrintStream)does this for us • Instance passed as parameter will be sent all output • No new classes required for this to work • Can add code to method run before each test • @Beforemethod needed to contain this code

Writing Code to Test Output import java.io.PrintStream; public class TestCode {private OutputStreamos;String nl = System.getProperty("line.separator"); @Beforepublic void setUp() {os= new ByteArrayOutputStream();System.setOut(new PrintStream(os));}@Testpublic void testPrintln() {callMethodWith_println_Hi();assertEquals(os.toString(), "Hi" +nl);}

For Next Lecture • Next weekly assignment available online • Due as usual tomorrow at 5PM • Use virtual extensions to delay this due date • Reading on event-driven programming • How to code if we do not know what will happen? • What error is found most starting GUI programs? • How can I make my code finish within my lifetime?

Lecture 6: Do I HAVE TO Test IT? Why ?