Add Java inheritance and polymorphism module

Author: lucasbellesi

CONCEPT_INDEX.md

@@ -39,7 +39,7 @@ Use this page to browse the same concept across every active language track.
 | structs-and-classes | [C++](languages/cpp/03-advanced/structs-and-classes/README.md) | [C#](languages/csharp/03-advanced/structs-and-classes/README.md) | [Go](languages/go/03-advanced/structs-and-classes/README.md) | [Java](languages/java/03-advanced/structs-and-classes/README.md) | [Python](languages/python/03-advanced/structs-and-classes/README.md) | [TypeScript](languages/typescript/03-advanced/structs-and-classes/README.md) |
 | constructors-and-invariants | [C++](languages/cpp/03-advanced/constructors-and-invariants/README.md) | [C#](languages/csharp/03-advanced/constructors-and-invariants/README.md) | [Go](languages/go/03-advanced/constructors-and-invariants/README.md) | [Java](languages/java/03-advanced/constructors-and-invariants/README.md) | [Python](languages/python/03-advanced/constructors-and-invariants/README.md) | [TypeScript](languages/typescript/03-advanced/constructors-and-invariants/README.md) |
 | copy-and-move-semantics | [C++](languages/cpp/03-advanced/copy-and-move-semantics/README.md) | [C#](languages/csharp/03-advanced/copy-and-move-semantics/README.md) | [Go](languages/go/03-advanced/copy-and-move-semantics/README.md) | Planned | [Python](languages/python/03-advanced/copy-and-move-semantics/README.md) | [TypeScript](languages/typescript/03-advanced/copy-and-move-semantics/README.md) |
-| inheritance-and-polymorphism | [C++](languages/cpp/03-advanced/inheritance-and-polymorphism/README.md) | [C#](languages/csharp/03-advanced/inheritance-and-polymorphism/README.md) | [Go](languages/go/03-advanced/inheritance-and-polymorphism/README.md) | Planned | [Python](languages/python/03-advanced/inheritance-and-polymorphism/README.md) | [TypeScript](languages/typescript/03-advanced/inheritance-and-polymorphism/README.md) |
+| inheritance-and-polymorphism | [C++](languages/cpp/03-advanced/inheritance-and-polymorphism/README.md) | [C#](languages/csharp/03-advanced/inheritance-and-polymorphism/README.md) | [Go](languages/go/03-advanced/inheritance-and-polymorphism/README.md) | [Java](languages/java/03-advanced/inheritance-and-polymorphism/README.md) | [Python](languages/python/03-advanced/inheritance-and-polymorphism/README.md) | [TypeScript](languages/typescript/03-advanced/inheritance-and-polymorphism/README.md) |
 | templates-basics | [C++](languages/cpp/03-advanced/templates-basics/README.md) | [C#](languages/csharp/03-advanced/templates-basics/README.md) | [Go](languages/go/03-advanced/templates-basics/README.md) | Planned | [Python](languages/python/03-advanced/templates-basics/README.md) | [TypeScript](languages/typescript/03-advanced/templates-basics/README.md) |
 
 ### 04-expert

LANGUAGE_PARITY_MATRIX.md

@@ -47,7 +47,7 @@ Current parity progress in non-C++ tracks:
 
 - C#: `5/5` modules complete in `03-advanced`, `5/5` in `04-expert`
 - Go: `5/5` modules complete in `03-advanced`, `5/5` in `04-expert`
-- Java: `2/5` modules complete in `03-advanced`, `0/5` in `04-expert`
+- Java: `3/5` modules complete in `03-advanced`, `0/5` in `04-expert`
 - Python: `5/5` modules complete in `03-advanced`, `5/5` in `04-expert`
 - TypeScript: `5/5` modules complete in `03-advanced`, `5/5` in `04-expert`
 
@@ -58,7 +58,7 @@ Current parity progress in non-C++ tracks:
 | 1 | structs-and-classes | Done | Done | Done | Done | Done | Done |
 | 2 | constructors-and-invariants | Done | Done | Done | Done | Done | Done |
 | 3 | copy-and-move-semantics | Done | Done | Done | Planned | Done | Done |
-| 4 | inheritance-and-polymorphism | Done | Done | Done | Planned | Done | Done |
+| 4 | inheritance-and-polymorphism | Done | Done | Done | Done | Done | Done |
 | 5 | templates-basics | Done | Done | Done | Planned | Done | Done |
 
 `04-expert` projects and assessments are now implemented across all active non-Java tracks.

README.md

@@ -56,7 +56,7 @@ If you only want to learn one track, follow that track README first because lang
 | C++ | 00-setup, 01-foundations, 02-core, 03-advanced, 04-expert | Foundations, Core, Advanced, Expert, projects, assessments | Most complete and primary track |
 | C# | 01-foundations, 02-core, 03-advanced, 04-expert | 8/8 foundations modules, 6/6 core modules, 5/5 advanced modules, 5/5 expert modules, 4/4 projects, 4/4 assessments | Module and checkpoint parity complete through expert |
 | Go | 01-foundations, 02-core, 03-advanced, 04-expert | 8/8 foundations modules, 6/6 core modules, 5/5 advanced modules, 5/5 expert modules, 4/4 projects, 4/4 assessments | Module and checkpoint parity complete through expert |
-| Java | 01-foundations, 02-core, 03-advanced partial | 8/8 foundations modules, 6/6 core modules, 2/5 advanced modules, 2/4 projects, 2/4 assessments | Java 21 MVP through core plus two advanced modules; remaining advanced, expert, and later checkpoints planned |
+| Java | 01-foundations, 02-core, 03-advanced partial | 8/8 foundations modules, 6/6 core modules, 3/5 advanced modules, 2/4 projects, 2/4 assessments | Java 21 MVP through core plus three advanced modules; remaining advanced, expert, and later checkpoints planned |
 | Python | 01-foundations, 02-core, 03-advanced, 04-expert | 8/8 foundations modules, 6/6 core modules, 5/5 advanced modules, 5/5 expert modules, 4/4 projects, 4/4 assessments | Module and checkpoint parity complete through expert |
 | TypeScript | 01-foundations, 02-core, 03-advanced, 04-expert | 8/8 foundations modules, 6/6 core modules, 5/5 advanced modules, 5/5 expert modules, 4/4 projects, 4/4 assessments | Module and checkpoint parity complete through expert |
 

languages/java/03-advanced/README.md

@@ -1,24 +1,26 @@
 # Java 03 Advanced
 
-This level starts the advanced Java track with object modeling and constructor-enforced invariants. It is currently a partial expansion.
+This level starts the advanced Java track with object modeling, constructor-enforced invariants, and polymorphism. It is currently a partial expansion.
 
 ## Learning Metadata
 
 - Difficulty: Intermediate.
-- Estimated Time: 1.5-2 hours for the current modules.
+- Estimated Time: 2-3 hours for the current modules.
 - Prerequisites: Completion of `01-foundations` and `02-core`.
 - Study Strategy: Focus on data plus behavior first, then compare Java classes with records, structs, and classes in other tracks.
 
 ## Module Order
 
 1. [structs-and-classes](./structs-and-classes/README.md)
 2. [constructors-and-invariants](./constructors-and-invariants/README.md)
+3. [inheritance-and-polymorphism](./inheritance-and-polymorphism/README.md)
 
 ## Level Outcomes
 
 - Model related data and behavior with Java records and classes.
 - Protect object state through constructors and methods.
 - Define invariants that remain true after construction and updates.
+- Treat different concrete classes through one shared Java interface.
 - Compare Java object modeling with C++, C#, Go, Python, and TypeScript.
 
 ## Done When
@@ -27,10 +29,13 @@ This level starts the advanced Java track with object modeling and constructor-e
 - [ ] I completed both `structs-and-classes` exercises.
 - [ ] I ran `constructors-and-invariants/example/Main.java`.
 - [ ] I completed both `constructors-and-invariants` exercises.
+- [ ] I ran `inheritance-and-polymorphism/example/Main.java`.
+- [ ] I completed both `inheritance-and-polymorphism` exercises.
 - [ ] I can explain when a Java record is enough and when a class is better.
 - [ ] I can keep state updates behind methods instead of editing fields directly.
 - [ ] I can explain how constructors protect invariants.
+- [ ] I can explain how interfaces enable polymorphic collections.
 
 ## Study Tip
 
-This level is intentionally partial. Finish these object-state modules before expanding into copy/reference behavior, inheritance, and generics.
+This level is intentionally partial. Finish these object-state and polymorphism modules before expanding into copy/reference behavior and generics.

languages/java/03-advanced/inheritance-and-polymorphism/README.md

@@ -0,0 +1,61 @@
+# Inheritance and Polymorphism (Java)
+
+This module models behavior variation with interfaces and overriding implementations.
+
+## Learning Metadata
+
+- Difficulty: Intermediate.
+- Estimated Time: 35-50 minutes.
+- Prerequisites: `03-advanced/structs-and-classes`, `03-advanced/constructors-and-invariants`.
+- Cross-Language Lens: Compare virtual dispatch, interfaces, and duck-typed behavior to see how polymorphism changes by language.
+
+## Quick Run
+
+~~~bash
+javac -d build/java languages/java/03-advanced/inheritance-and-polymorphism/example/Main.java
+java -cp build/java Main
+~~~
+
+## Topics Covered
+
+- Interfaces and required methods.
+- Runtime polymorphism through shared interface references.
+- Overriding concrete behavior in implementing classes.
+- Aggregating mixed implementations in one list.
+
+## Common Pitfalls
+
+- Forgetting to implement required interface methods.
+- Accessing implementation-only state from generic iteration code.
+- Replacing polymorphism with `instanceof` checks.
+
+## Cross-Language Notes
+
+- Java interfaces define named contracts that classes explicitly implement.
+- Compared with Go, Java uses nominal interface implementation; compared with Python, more mistakes are caught before execution.
+- The key lesson is that polymorphism is a design idea first, not just a language feature.
+
+## Exercise Focus
+
+- exercises/Exercise01.java: extend shape model with `Triangle`.
+- exercises/Exercise02.java: aggregate shapes in `List<Shape>`.
+
+### Exercise Specs
+
+1. exercises/Exercise01.java
+- Input: triangle base and height.
+- Output: computed area through overridden method.
+- Edge cases: zero dimensions; decimal dimensions.
+
+2. exercises/Exercise02.java
+- Input: predefined shape objects.
+- Output: total area through polymorphic iteration.
+- Edge cases: empty shape list; mixed shape types.
+
+## Checkpoint
+
+- [ ] I can define interfaces with polymorphic methods.
+- [ ] I can override behavior in implementing classes.
+- [ ] I can use polymorphic collections safely.
+- [ ] I completed exercises/Exercise01.java.
+- [ ] I completed exercises/Exercise02.java.

languages/java/03-advanced/inheritance-and-polymorphism/example/Main.java

@@ -0,0 +1,66 @@
+// Module focus: Treating different concrete types through one common interface.
+// Why it matters: practicing inheritance and polymorphism patterns makes exercises and checkpoints easier to reason about.
+
+import java.util.List;
+import java.util.Locale;
+
+public class Main {
+    interface Shape {
+        // The interface names the behavior generic code can rely on.
+        double area();
+
+        String name();
+    }
+
+    static class Rectangle implements Shape {
+        private final double width;
+        private final double height;
+
+        Rectangle(double width, double height) {
+            this.width = width;
+            this.height = height;
+        }
+
+        @Override
+        public double area() {
+            // Each concrete class owns its formula behind the shared interface.
+            return width * height;
+        }
+
+        @Override
+        public String name() {
+            return "Rectangle";
+        }
+    }
+
+    static class Circle implements Shape {
+        private final double radius;
+
+        Circle(double radius) {
+            this.radius = radius;
+        }
+
+        @Override
+        public double area() {
+            // Callers do not need to know this formula when they use Shape references.
+            return Math.PI * radius * radius;
+        }
+
+        @Override
+        public String name() {
+            return "Circle";
+        }
+    }
+
+    public static void main(String[] args) {
+        Locale.setDefault(Locale.US);
+
+        // Prepare sample inputs that exercise the key inheritance and polymorphism path.
+        List<Shape> shapes = List.of(new Rectangle(3.0, 4.0), new Circle(2.0));
+
+        for (Shape shape : shapes) {
+            // Report values so learners can verify the inheritance and polymorphism outcome.
+            System.out.printf("%s area: %.2f%n", shape.name(), shape.area());
+        }
+    }
+}

languages/java/03-advanced/inheritance-and-polymorphism/exercises/Exercise01.java

@@ -0,0 +1,47 @@
+import java.util.Locale;
+import java.util.Scanner;
+
+public class Exercise01 {
+    interface Shape {
+        double area();
+    }
+
+    static class Triangle implements Shape {
+        private final double base;
+        private final double height;
+
+        Triangle(double base, double height) {
+            this.base = base;
+            this.height = height;
+        }
+
+        @Override
+        public double area() {
+            return 0.5 * base * height;
+        }
+    }
+
+    public static void main(String[] args) {
+        Locale.setDefault(Locale.US);
+        Scanner scanner = new Scanner(System.in);
+        if (!scanner.hasNextDouble()) {
+            System.out.println("Invalid input.");
+            return;
+        }
+        double base = scanner.nextDouble();
+
+        if (!scanner.hasNextDouble()) {
+            System.out.println("Invalid input.");
+            return;
+        }
+        double height = scanner.nextDouble();
+
+        if (base <= 0.0 || height <= 0.0) {
+            System.out.println("Base and height must be positive.");
+            return;
+        }
+
+        Shape triangle = new Triangle(base, height);
+        System.out.printf("Triangle area: %.2f%n", triangle.area());
+    }
+}

languages/java/03-advanced/inheritance-and-polymorphism/exercises/Exercise02.java

@@ -0,0 +1,48 @@
+import java.util.List;
+import java.util.Locale;
+
+public class Exercise02 {
+    interface Shape {
+        double area();
+    }
+
+    static class Rectangle implements Shape {
+        private final double width;
+        private final double height;
+
+        Rectangle(double width, double height) {
+            this.width = width;
+            this.height = height;
+        }
+
+        @Override
+        public double area() {
+            return width * height;
+        }
+    }
+
+    static class Circle implements Shape {
+        private final double radius;
+
+        Circle(double radius) {
+            this.radius = radius;
+        }
+
+        @Override
+        public double area() {
+            return Math.PI * radius * radius;
+        }
+    }
+
+    public static void main(String[] args) {
+        Locale.setDefault(Locale.US);
+        List<Shape> shapes = List.of(new Rectangle(2.0, 5.0), new Circle(1.5));
+
+        double totalArea = 0.0;
+        for (Shape shape : shapes) {
+            totalArea += shape.area();
+        }
+
+        System.out.printf("Total area: %.2f%n", totalArea);
+    }
+}

languages/java/CHECKLIST.md

@@ -47,11 +47,12 @@
 - [ ] structs-and-classes exercises
 - [ ] constructors-and-invariants example
 - [ ] constructors-and-invariants exercises
+- [ ] inheritance-and-polymorphism example
+- [ ] inheritance-and-polymorphism exercises
 
 ## Planned
 
 - [ ] copy-and-move-semantics module
-- [ ] inheritance-and-polymorphism module
 - [ ] templates-basics module
 - [ ] 04-expert modules
 - [ ] projects/03-advanced

languages/java/README.md

@@ -6,7 +6,7 @@ This track currently covers `01-foundations` and `02-core` as a Java 21 MVP, inc
 
 - 8/8 foundations modules implemented.
 - 6/6 core modules implemented.
-- 2/5 advanced modules implemented (`structs-and-classes`, `constructors-and-invariants`).
+- 3/5 advanced modules implemented (`structs-and-classes`, `constructors-and-invariants`, `inheritance-and-polymorphism`).
 - 0/5 expert modules implemented.
 - 2/4 projects implemented (`01-foundations`, `02-core`).
 - 2/4 assessments implemented (`01-foundations`, `02-core`).
@@ -52,6 +52,7 @@ java -cp build/java Main
 - [03-advanced](./03-advanced/README.md)
   - [structs-and-classes](./03-advanced/structs-and-classes/README.md)
   - [constructors-and-invariants](./03-advanced/constructors-and-invariants/README.md)
+  - [inheritance-and-polymorphism](./03-advanced/inheritance-and-polymorphism/README.md)
 - Projects
   - [01-foundations](./projects/01-foundations/README.md)
   - [02-core](./projects/02-core/README.md)