A Programmer[ap]s Guide to Java Certification

Finding primes using for - loops .

// Filename: ForPrimes.java public class ForPrimes { final static int MAX = 100; public static void main(String[] args) { numbers: for (int num = 1; num < MAX; num++) { int divLim = (int) Math.sqrt(num); for (int div = 2; div <= divLim; div++) if ((num % div) == 0) continue numbers; System.out.println(num); } } }

Finding primes using while -loops.

// Filename: WhilePrimes.java public class WhilePrimes { final static int MAX = 100; public static void main(String[] args) { int num = 1; numbers: while (num < MAX) { int number = num++; int divLim = (int) Math.sqrt(number); int div = 2; while (div <= divLim) if ((number % div++) == 0) continue numbers; System.out.println(number); } } }

/** A PowerPlant with a reactor core. */ public class PowerPlant { /** Each power plant has a reactor core. This has package accessibility so that the Control class which is defined in the same package can access it. */ Reactor core; /** Create and initialize the PowerPlant with a reactor core. */ PowerPlant() { core = new Reactor(); } /** Sound the alarm to evacuate the power plant. */ public void soundEvacuateAlarm() { // ... implementation unspecified ... } /** Get the level of reactor output that is most desirable at this time. (Units are unspecified.) */ public int getOptimalThroughput() { // ... implementation unspecified ... return 0; } /** The main entry point of the program. Will set up a PowerPlant object and a Control object and let the Control object run the PowerPlant. */ public static void main(String[] args) { PowerPlant plant = new PowerPlant(); Control ctrl = new Control(plant); ctrl.runSystem(); } } /** A reactor core that has a throughput that can be either decreased or increased. */ class Reactor { /** Get the current throughput of the reactor. (Units are unspecified.) */ public int getThroughput() { // ... implementation unspecified ... return 0; } /** @returns true if the reactor status is critical, false otherwise. */ public boolean isCritical() { // ... implementation unspecified ... return false; } /** Ask the reactor to increase throughput. */ void increaseThroughput() throws ReactorCritical { // ... implementation unspecified ... } /** Ask the reactor to decrease throughput. */ void decreaseThroughput() { // ... implementation unspecified ... } } /** This exception class should be used to report that the reactor is critical. */ class ReactorCritical extends Exception {} /** A controller that will manage the power plant to make sure that the reactor runs with optimal throughput. */ class Control { PowerPlant thePlant; public Control(PowerPlant p) { thePlant = p; } /** Run the power plant by continuously monitoring the optimalThroughput and the actual throughput of the reactor. If the throughputs differ by more than 10 units, adjust reactor throughput. If the reactor goes critical, the evacuate alarm is sounded and the reactor is shut down. <p>The runSystem() method does handle the reactor core directly but calls methods needAdjustment(), adjustThroughput(), and shutdown instead. */ public void runSystem() { try { while (true) { // infinite loop if (needAdjustment()) adjustThroughput(thePlant.getOptimalThroughput()); } } catch (ReactorCritical rc) { thePlant.soundEvacuateAlarm(); } finally { shutdown(); } } /** Reports whether the throughput of the reactor needs adjusting. This method should also monitor and report if the reactor goes critical. @returns true if the optimal and actual throughput values differ by more than 10 units. */ public boolean needAdjustment() throws ReactorCritical { /* We added the throws clause to the method declaration so that the method can throw a ReactorCritical exception if the reactor goes critical. */ if (thePlant.core.isCritical()) throw new ReactorCritical(); return (Math.abs(thePlant.getOptimalThroughput() - thePlant.core.getThroughput())) > 10; } /** Adjust the throughput of the reactor by calling increaseThroughput() and decreaseThroughput() until the actual throughput is within 10 units of the target throughput. */ public void adjustThroughput(int target) throws ReactorCritical { /* We added the throws clause to the method declaration so that the method can pass on ReactorCritical exceptions thrown by increaseThroughput(). We do this because the adjustThroughput does not want to handle the exception. */ while (needAdjustment()) { if (thePlant.getOptimalThroughput() > thePlant.core.getThroughput()) thePlant.core.increaseThroughput(); else thePlant.core.decreaseThroughput(); } } /** Shut down the reactor by lowering the throughput to 0. */ public void shutdown() { while (thePlant.core.getThroughput() != 0) { thePlant.core.decreaseThroughput(); } } }