Add 'Retirement Calculator' solution

README.md

@@ -9,7 +9,7 @@ Exercises for Programmers: 57 Challenges to Develop Your Coding Skills by Brian
 - Exercise 3. [Printing Quotes](https://github.com/durimkryeziu/exercises-for-programmers-java/tree/main/printing-quotes)
 - Exercise 4. [Mad Lib](https://github.com/durimkryeziu/exercises-for-programmers-java/tree/main/mad-lib)
 - Exercise 5. [Simple Math](https://github.com/durimkryeziu/exercises-for-programmers-java/tree/main/simple-math)
-- Exercise 6. Retirement Calculator
+- Exercise 6. [Retirement Calculator](https://github.com/durimkryeziu/exercises-for-programmers-java/tree/main/retirement-calculator)
 
 **Calculations**
 - Exercise 7. Area of a Rectangular Room

retirement-calculator/build.gradle

@@ -0,0 +1,8 @@
+plugins {
+    id 'com.craftsmanshipinsoftware.common-conventions'
+}
+
+dependencies {
+    testImplementation(libs.assertj.core)
+    testImplementation(libs.junit.jupiter)
+}

retirement-calculator/src/main/java/com/craftsmanshipinsoftware/retirement/Main.java

@@ -0,0 +1,11 @@
+package com.craftsmanshipinsoftware.retirement;
+
+import java.time.Year;
+
+public class Main {
+
+    public static void main(String[] args) {
+        RetirementCalculator calculator = new RetirementCalculator(System.in, System.out, Year::now);
+        calculator.printYearsLeft();
+    }
+}

retirement-calculator/src/main/java/com/craftsmanshipinsoftware/retirement/RetirementCalculator.java

@@ -0,0 +1,60 @@
+package com.craftsmanshipinsoftware.retirement;
+
+import java.io.InputStream;
+import java.io.PrintStream;
+import java.time.Year;
+import java.util.Objects;
+import java.util.Scanner;
+
+class RetirementCalculator {
+
+    private final InputStream inputStream;
+    private final PrintStream printStream;
+    private final YearProvider yearProvider;
+
+    RetirementCalculator(InputStream inputStream, PrintStream printStream, YearProvider yearProvider) {
+        Objects.requireNonNull(inputStream, "inputStream must not be null");
+        Objects.requireNonNull(printStream, "printStream must not be null");
+        Objects.requireNonNull(yearProvider, "yearProvider must not be null");
+        this.inputStream = inputStream;
+        this.printStream = printStream;
+        this.yearProvider = yearProvider;
+    }
+
+    void printYearsLeft() {
+        try (Scanner scanner = new Scanner(inputStream)) {
+            int currentAge = toInt(promptForInput("What is your current age? ", scanner));
+            int retirementAge = toInt(promptForInput("At what age would you like to retire? ", scanner));
+            int yearsLeft = retirementAge - currentAge;
+            if (yearsLeft <= 0) {
+                this.printStream.println("You can already retire.");
+            } else {
+                this.printStream.printf("You have %d years left until you can retire.%n", yearsLeft);
+            }
+            Year currentYear = this.yearProvider.currentYear();
+            this.printStream.printf("It's %s, so you can retire in %s.%n", currentYear,
+                    currentYear.plusYears(yearsLeft));
+        }
+    }
+
+    private String promptForInput(String prompt, Scanner scanner) {
+        this.printStream.print(prompt);
+        String input = scanner.hasNext() ? scanner.nextLine() : null;
+        if (input == null || input.isBlank()) {
+            throw new IllegalArgumentException("Input must not be empty!");
+        }
+        return input;
+    }
+
+    private int toInt(String value) {
+        try {
+            int number = Integer.parseInt(value);
+            if (number < 0) {
+                throw new IllegalArgumentException("Please enter a positive number! Input: %s".formatted(value));
+            }
+            return number;
+        } catch (NumberFormatException e) {
+            throw new IllegalArgumentException("Please enter a valid number! Input: %s".formatted(value), e);
+        }
+    }
+}

retirement-calculator/src/main/java/com/craftsmanshipinsoftware/retirement/YearProvider.java

@@ -0,0 +1,9 @@
+package com.craftsmanshipinsoftware.retirement;
+
+import java.time.Year;
+
+@FunctionalInterface
+interface YearProvider {
+
+    Year currentYear();
+}

retirement-calculator/src/test/java/com/craftsmanshipinsoftware/retirement/RetirementCalculatorTest.java

@@ -0,0 +1,102 @@
+package com.craftsmanshipinsoftware.retirement;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
+
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+import java.io.PrintStream;
+import java.time.Year;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
+
+class RetirementCalculatorTest {
+
+    @Test
+    void asksForCurrentAge() {
+        ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
+        RetirementCalculator calculator = new RetirementCalculator(new ByteArrayInputStream("25\n65".getBytes()),
+                new PrintStream(outputStream), Year::now);
+
+        calculator.printYearsLeft();
+
+        assertThat(outputStream.toString()).contains("What is your current age?");
+    }
+
+    @Test
+    void asksForRetirementAge() {
+        ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
+        RetirementCalculator calculator = new RetirementCalculator(new ByteArrayInputStream("25\n65".getBytes()),
+                new PrintStream(outputStream), Year::now);
+
+        calculator.printYearsLeft();
+
+        assertThat(outputStream.toString()).contains("At what age would you like to retire?");
+    }
+
+    @Test
+    void displaysYearsLeftUntilRetirement() {
+        ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
+        RetirementCalculator calculator = new RetirementCalculator(new ByteArrayInputStream("25\n65".getBytes()),
+                new PrintStream(outputStream), new FakeYearProvider());
+
+        calculator.printYearsLeft();
+
+        assertThat(outputStream.toString()).containsSequence("You have 40 years left until you can retire.\n",
+                "It's 2015, so you can retire in 2055.\n");
+    }
+
+    @ParameterizedTest
+    @ValueSource(strings = {"65\n65", "75\n65"})
+    void displaysYouCanAlreadyRetireWhenCurrentAgeIsRetirementAgeOrMore(String input) {
+        ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
+        RetirementCalculator calculator = new RetirementCalculator(new ByteArrayInputStream(input.getBytes()),
+                new PrintStream(outputStream), Year::now);
+
+        calculator.printYearsLeft();
+
+        assertThat(outputStream.toString()).contains("You can already retire.");
+    }
+
+    @ParameterizedTest
+    @ValueSource(strings = {"\n65", "25\n"})
+    void inputIsRequired(String input) {
+        RetirementCalculator calculator = new RetirementCalculator(new ByteArrayInputStream(input.getBytes()),
+                new PrintStream(new ByteArrayOutputStream()), Year::now);
+
+        assertThatIllegalArgumentException()
+                .isThrownBy(calculator::printYearsLeft)
+                .withMessage("Input must not be empty!");
+    }
+
+    @ParameterizedTest
+    @ValueSource(strings = {"abc\n65", "25\nabc"})
+    void inputMustBeANumber(String input) {
+        RetirementCalculator calculator = new RetirementCalculator(new ByteArrayInputStream(input.getBytes()),
+                new PrintStream(new ByteArrayOutputStream()), Year::now);
+
+        assertThatIllegalArgumentException()
+                .isThrownBy(calculator::printYearsLeft)
+                .withMessage("Please enter a valid number! Input: abc");
+    }
+
+    @ParameterizedTest
+    @ValueSource(strings = {"-1\n65", "25\n-1"})
+    void inputMustBeAPositiveNumber(String input) {
+        RetirementCalculator calculator = new RetirementCalculator(new ByteArrayInputStream(input.getBytes()),
+                new PrintStream(new ByteArrayOutputStream()), Year::now);
+
+        assertThatIllegalArgumentException()
+                .isThrownBy(calculator::printYearsLeft)
+                .withMessage("Please enter a positive number! Input: -1");
+    }
+
+    static final class FakeYearProvider implements YearProvider {
+
+        @Override
+        public Year currentYear() {
+            return Year.of(2015);
+        }
+    }
+}

settings.gradle

@@ -1,2 +1,9 @@
 rootProject.name = 'exercises-for-programmers-java'
-include('saying-hello', 'characters-count', 'printing-quotes', 'mad-lib', 'simple-math')
+include(
+        'saying-hello',
+        'characters-count',
+        'printing-quotes',
+        'mad-lib',
+        'simple-math',
+        'retirement-calculator'
+)