feat: implement Practice 2.1

pom.xml

@@ -20,6 +20,7 @@
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-jar-plugin</artifactId>
+                <version>3.3.0</version>
                 <configuration>
                     <archive>
                         <manifest>

src/main/java/me/alex4386/gachon/sw14462/Main.java

@@ -2,32 +2,124 @@
 
 import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
 
 public class Main {
-    public static String currentTarget = "day02";
+    public static String currentTarget = "day03";
 
-    public static void main(String[] args) {
-        String target = currentTarget;
-        Class<?> klass = null;
+    public static Map<String, Class<?>> getAvailableTargetClassNames() {
+        Map<String, Class<?>> classes = new HashMap<>();
+
+        for (int i = 1; i <= 99; i++) {
+            String className = String.format(Main.class.getPackage().getName() + ".day%02d.Main", i);
+
+            try {
+                Class<?> klass = Class.forName(className);
+                classes.put(String.format("day%02d", i), klass);
+            } catch(ClassNotFoundException e) {
+                continue;
+            }
+        }
+
+        return classes;
+    }
+
+    public static void launchCurrentTarget(String[] args) {
+        Map<String, Class<?>> classes = Main.getAvailableTargetClassNames();
+
+        if (classes.containsKey(currentTarget)) {
+            Class<?> klass = classes.get(currentTarget);
+            if (klass != null) {
+                Main.launchGivenClass(klass, args);
+            }
+        }
+    }
+
+    public static void launchGivenClass(Class<?> klass, String[] args) {
+        Method mainMethod;
 
         try {
-            klass = Class.forName(Main.class.getPackage().getName()+"."+target+".Main");
-        } catch(ClassNotFoundException e) {
-            System.err.println("Failed to fetch class");
-            System.exit(1);
+            mainMethod = klass.getMethod("main", String[].class);
+        } catch (NoSuchMethodException e) {
+            System.out.println("Main method not found in the target class.");
+            return;
         }
 
-        Method mainMethod = null;
         try {
-            mainMethod = klass.getDeclaredMethod("main", String[].class);
-        } catch(NoSuchMethodException ignored) {}
+            mainMethod.invoke(null, (Object) args);
+        } catch (InvocationTargetException e) {
+            System.out.println("Failed to invoke main method.");
+            return;
+        } catch (IllegalAccessException e) {
+            e.printStackTrace();
+            return;
+        }
+    }
 
-        if (mainMethod != null) {
-            try {
-                Object[] invokeArgs = { args };
-                mainMethod.invoke(klass, invokeArgs);
-            } catch(IllegalAccessException | InvocationTargetException e) {
-                System.exit(1);
+    public static void main(String[] args) {
+        Class<?> klass;
+        if (currentTarget.length() > 0) {
+            Main.launchCurrentTarget(args);
+            return;
+        }
+
+        if (args.length >= 1) {
+            Map<String, Class<?>> classes = Main.getAvailableTargetClassNames();
+            currentTarget = args[0];
+
+            if (classes.containsKey(currentTarget)) {
+                klass = classes.get(currentTarget);
+                List<String> newArgs = new ArrayList<>();
+                if (args.length >= 1) {
+                    for (int i = 1; i < args.length; i++) {
+                        newArgs.add(args[i]);
+                    }
+                }
+
+                String[] newArgsArray = (String[]) newArgs.toArray();
+                Main.launchGivenClass(klass, newArgsArray);
+                return;
+            }
+        }
+
+        if (System.console() != null) {
+            askUserForLaunch(args);
+        } else {
+            System.out.println("Console not available for loading specific class. Exiting.");
+            System.exit(1);
+        }
+    }
+
+    public static void askUserForLaunch(String[] args) {
+        Map<String, Class<?>> classes = Main.getAvailableTargetClassNames();
+
+        // list all classes and make user select one
+        System.out.println("Available classes:");
+        for (String className : classes.keySet()) {
+            System.out.println("* " + className);
+        }
+
+        while (true) {
+            System.out.println("");
+            System.out.print("Class name to launch (Enter \"exit\" to exit): ");
+            String input = System.console().readLine();
+
+            if (input.equalsIgnoreCase("exit")) {
+                break;
+            }
+
+            if (classes.containsKey(input)) {
+                System.out.println("Launching " + input + "...");
+                System.out.println("");
+
+                currentTarget = input;
+                Main.launchCurrentTarget(args);
+                break;
+            } else {
+                System.out.println("Invalid class name.");
             }
         }
     }

src/main/java/me/alex4386/gachon/sw14462/day03/DigitParser.java

@@ -0,0 +1,25 @@
+package me.alex4386.gachon.sw14462.day03;
+
+public class DigitParser {
+    protected int integer;
+    protected int currentDigitPow;
+
+    public DigitParser(int integer) {
+        this.integer = integer;
+        this.currentDigitPow = DigitParser.digitPow(integer) - 1;
+    }
+
+    public static int digitPow(int integer) {
+        return String.valueOf(integer).length();
+    }
+
+    public int nextDigit() {
+        if (this.currentDigitPow < 0) {
+            return -1;
+        }
+
+        int digit = (int) ((this.integer / Math.pow(10, this.currentDigitPow)) % 10);
+        this.currentDigitPow--;
+        return digit;
+    }
+}

src/main/java/me/alex4386/gachon/sw14462/day03/EggBasket.java

@@ -0,0 +1,19 @@
+package me.alex4386.gachon.sw14462.day03;
+
+public class EggBasket {
+    // At the beginning of the section enclosed by curly braces `{}`
+    public static void main(String[] args) {
+        // A variable must be declared before it is used
+        int numberOfBaskets, eggsPerBasket, totalEggs; // variable declaration
+
+        numberOfBaskets = 20; // assignment statement
+        eggsPerBasket = 18;
+
+        totalEggs = numberOfBaskets * eggsPerBasket;
+
+        System.out.println("If you have");
+        System.out.println(eggsPerBasket + " eggs per basket and");
+        System.out.println(numberOfBaskets + " baskets, then");
+        System.out.println("the total number of eggs is " + totalEggs);
+    }
+}

src/main/java/me/alex4386/gachon/sw14462/day03/FahrenheitConverter.java

@@ -0,0 +1,7 @@
+package me.alex4386.gachon.sw14462.day03;
+
+public class FahrenheitConverter {
+    public static double toCelsius(double fahrenheit) {
+        return (fahrenheit - 32) * 5 / 9;
+    }
+}

src/main/java/me/alex4386/gachon/sw14462/day03/Main.java

@@ -0,0 +1,40 @@
+package me.alex4386.gachon.sw14462.day03;
+
+import java.util.Scanner;
+
+public class Main {
+    public static void main(String[] args) {
+        System.out.println("Exercise 2_1. Read a four-digit integer and display one digit per line");
+        Main.exercise2_1();
+
+        System.out.println("");
+        System.out.println("Exercise 2_2. Convert Fahrenheit to Celsius");
+        Main.exercise2_2();
+    }
+
+    public static void exercise2_1() {
+        Scanner scanner = new Scanner(System.in);
+
+        System.out.print("Enter a four-digit integer: ");
+        int integer = scanner.nextInt();
+
+        DigitParser parser = new DigitParser(integer);
+        while (true) {
+            int digit = parser.nextDigit();
+            if (digit == -1) {
+                break;
+            }
+            System.out.println(digit);
+        }
+    }
+
+    public static void exercise2_2() {
+        Scanner scanner = new Scanner(System.in);
+
+        System.out.print("Enter a temperature in Fahrenheit: ");
+        double fahrenheit = scanner.nextDouble();
+
+        double celsius = FahrenheitConverter.toCelsius(fahrenheit);
+        System.out.printf("The temperature in Celsius is %.02f°C.\n", celsius);
+    }
+}