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Add Java constructors and invariants module
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‎CONCEPT_INDEX.md‎

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| Concept | C++ | C# | Go | Java | Python | TypeScript |
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| --- | --- | --- | --- | --- | --- | --- |
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| 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) |
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| 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) | Planned | [Python](languages/python/03-advanced/constructors-and-invariants/README.md) | [TypeScript](languages/typescript/03-advanced/constructors-and-invariants/README.md) |
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| 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) |
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| 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) |
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| 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) |
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| 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) |

‎LANGUAGE_PARITY_MATRIX.md‎

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- C#: `5/5` modules complete in `03-advanced`, `5/5` in `04-expert`
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- Go: `5/5` modules complete in `03-advanced`, `5/5` in `04-expert`
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- Java: `1/5` modules complete in `03-advanced`, `0/5` in `04-expert`
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- Java: `2/5` modules complete in `03-advanced`, `0/5` in `04-expert`
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- Python: `5/5` modules complete in `03-advanced`, `5/5` in `04-expert`
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- TypeScript: `5/5` modules complete in `03-advanced`, `5/5` in `04-expert`
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| Order | Module | C++ | C# | Go | Java | Python | TypeScript |
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| --- | --- | --- | --- | --- | --- | --- | --- |
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| 1 | structs-and-classes | Done | Done | Done | Done | Done | Done |
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| 2 | constructors-and-invariants | Done | Done | Done | Planned | Done | Done |
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| 2 | constructors-and-invariants | Done | Done | Done | Done | Done | Done |
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| 3 | copy-and-move-semantics | Done | Done | Done | Planned | Done | Done |
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| 4 | inheritance-and-polymorphism | Done | Done | Done | Planned | Done | Done |
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| 5 | templates-basics | Done | Done | Done | Planned | Done | Done |

‎README.md‎

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| C++ | 00-setup, 01-foundations, 02-core, 03-advanced, 04-expert | Foundations, Core, Advanced, Expert, projects, assessments | Most complete and primary track |
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| 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 |
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| 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 |
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| Java | 01-foundations, 02-core, 03-advanced partial | 8/8 foundations modules, 6/6 core modules, 1/5 advanced modules, 2/4 projects, 2/4 assessments | Java 21 MVP through core plus first advanced module; remaining advanced, expert, and later checkpoints planned |
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| 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 |
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| 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 |
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| 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 |
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‎languages/java/03-advanced/README.md‎

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# Java 03 Advanced
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This level starts the advanced Java track with object modeling. It is currently a partial expansion.
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This level starts the advanced Java track with object modeling and constructor-enforced invariants. It is currently a partial expansion.
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## Learning Metadata
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- Difficulty: Intermediate.
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- Estimated Time: 45-60 minutes for the current module.
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- Estimated Time: 1.5-2 hours for the current modules.
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- Prerequisites: Completion of `01-foundations` and `02-core`.
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- Study Strategy: Focus on data plus behavior first, then compare Java classes with records, structs, and classes in other tracks.
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## Module Order
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1. [structs-and-classes](./structs-and-classes/README.md)
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2. [constructors-and-invariants](./constructors-and-invariants/README.md)
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## Level Outcomes
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- Model related data and behavior with Java records and classes.
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- Protect object state through constructors and methods.
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- Define invariants that remain true after construction and updates.
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- Compare Java object modeling with C++, C#, Go, Python, and TypeScript.
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## Done When
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- [ ] I ran `structs-and-classes/example/Main.java`.
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- [ ] I completed both `structs-and-classes` exercises.
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- [ ] I ran `constructors-and-invariants/example/Main.java`.
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- [ ] I completed both `constructors-and-invariants` exercises.
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- [ ] I can explain when a Java record is enough and when a class is better.
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- [ ] I can keep state updates behind methods instead of editing fields directly.
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- [ ] I can explain how constructors protect invariants.
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## Study Tip
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This level is intentionally partial. Finish `structs-and-classes` before expanding into constructors, copy/reference behavior, inheritance, and generics.
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This level is intentionally partial. Finish these object-state modules before expanding into copy/reference behavior, inheritance, and generics.

‎languages/java/03-advanced/constructors-and-invariants/README.md‎

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# Constructors and Invariants (Java)
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This module shows how constructors enforce valid object state.
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## Learning Metadata
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- Difficulty: Intermediate.
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- Estimated Time: 35-50 minutes.
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- Prerequisites: `03-advanced/structs-and-classes`.
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- Cross-Language Lens: Compare constructor rules, validation points, and object setup guarantees in each track.
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## Quick Run
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~~~bash
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javac -d build/java languages/java/03-advanced/constructors-and-invariants/example/Main.java
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java -cp build/java Main
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~~~
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## Topics Covered
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- Constructor guards that normalize invalid values.
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- Defining invariants as always-true object rules.
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- Safe update methods that reject invalid transitions.
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- Returning explicit operation success values.
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## Common Pitfalls
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- Accepting invalid constructor values and fixing later.
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- Writing directly into internal fields from outside methods.
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- Ignoring failed updates and assuming state changed.
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## Exercise Focus
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- exercises/Exercise01.java: bank account with non-negative balance invariant.
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- exercises/Exercise02.java: date model with month/day validation.
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### Exercise Specs
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1. exercises/Exercise01.java
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- Input: initial balance and transaction values.
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- Output: updated balance with validity checks.
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- Edge cases: negative initial balance; withdrawal beyond balance.
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2. exercises/Exercise02.java
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- Input: month and day values.
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- Output: valid/invalid date result.
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- Edge cases: month out of range; day out of range for month.
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## Checkpoint
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- [ ] I can define clear invariants for a class.
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- [ ] I can enforce invariants in constructors and methods.
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- [ ] I can reject invalid updates safely.
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- [ ] I completed exercises/Exercise01.java.
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- [ ] I completed exercises/Exercise02.java.

‎languages/java/03-advanced/constructors-and-invariants/example/Main.java‎

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// Module focus: Building objects that start valid and stay valid through guarded updates.
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// Why it matters: practicing constructors and invariants patterns makes exercises and checkpoints easier to reason about.
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import java.util.Locale;
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public class Main {
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static class Temperature {
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private double celsius;
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Temperature(double celsiusValue) {
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// The constructor clamps invalid starting data so every object starts valid.
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celsius = Math.max(-273.15, celsiusValue);
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}
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boolean setCelsius(double newValue) {
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// Updates must protect the same invariant as construction.
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if (newValue < -273.15) {
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return false;
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}
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celsius = newValue;
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return true;
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}
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double celsius() {
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// Accessors expose the valid state without letting callers edit fields directly.
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return celsius;
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}
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}
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public static void main(String[] args) {
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Locale.setDefault(Locale.US);
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// Prepare sample inputs that exercise the key constructors and invariants path.
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Temperature temperature = new Temperature(-500.0);
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// Report values so learners can verify the constructors and invariants outcome.
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System.out.printf("Initial value (clamped): %.2f C%n", temperature.celsius());
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boolean updated = temperature.setCelsius(25.0);
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System.out.println("Set to 25.0 success: " + updated);
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System.out.printf("Current value: %.2f C%n", temperature.celsius());
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boolean rejected = temperature.setCelsius(-300.0);
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System.out.println("Set to -300.0 success: " + rejected);
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System.out.printf("Current value: %.2f C%n", temperature.celsius());
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}
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}

‎languages/java/03-advanced/constructors-and-invariants/exercises/Exercise01.java‎

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import java.util.Locale;
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import java.util.Scanner;
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public class Exercise01 {
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static class BankAccount {
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private double balance;
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BankAccount(double initialBalance) {
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balance = Math.max(0.0, initialBalance);
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}
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boolean deposit(double amount) {
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if (amount <= 0.0) {
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return false;
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}
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balance += amount;
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return true;
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}
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boolean withdraw(double amount) {
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if (amount <= 0.0 || amount > balance) {
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return false;
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}
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balance -= amount;
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return true;
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}
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double balance() {
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return balance;
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}
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}
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public static void main(String[] args) {
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Locale.setDefault(Locale.US);
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Scanner scanner = new Scanner(System.in);
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double initialBalance = scanner.nextDouble();
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BankAccount account = new BankAccount(initialBalance);
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double depositAmount = scanner.nextDouble();
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account.deposit(depositAmount);
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double withdrawAmount = scanner.nextDouble();
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if (!account.withdraw(withdrawAmount)) {
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System.out.println("Withdrawal rejected.");
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}
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System.out.printf("Final balance: %.2f%n", account.balance());
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}
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}

‎languages/java/03-advanced/constructors-and-invariants/exercises/Exercise02.java‎

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import java.util.Scanner;
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public class Exercise02 {
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static class SimpleDate {
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private static final int[] DAYS_IN_MONTH = {
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31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31
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};
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private final int month;
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private final int day;
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SimpleDate(int month, int day) {
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this.month = month;
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this.day = day;
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}
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boolean isValid() {
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if (month < 1 || month > 12) {
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return false;
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}
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return day >= 1 && day <= DAYS_IN_MONTH[month - 1];
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}
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}
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public static void main(String[] args) {
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Scanner scanner = new Scanner(System.in);
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if (!scanner.hasNextInt()) {
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System.out.println("Invalid input.");
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return;
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}
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int month = scanner.nextInt();
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if (!scanner.hasNextInt()) {
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System.out.println("Invalid input.");
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return;
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}
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int day = scanner.nextInt();
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SimpleDate date = new SimpleDate(month, day);
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System.out.println(date.isValid() ? "Valid date" : "Invalid date");
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}
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}

‎languages/java/CHECKLIST.md‎

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- [ ] structs-and-classes example
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- [ ] structs-and-classes exercises
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- [ ] constructors-and-invariants example
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- [ ] constructors-and-invariants exercises
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## Planned
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- [ ] constructors-and-invariants module
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- [ ] copy-and-move-semantics module
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- [ ] inheritance-and-polymorphism module
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- [ ] templates-basics module

‎languages/java/README.md‎

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- 8/8 foundations modules implemented.
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- 6/6 core modules implemented.
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- 1/5 advanced modules implemented (`structs-and-classes`).
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- 2/5 advanced modules implemented (`structs-and-classes`, `constructors-and-invariants`).
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- 0/5 expert modules implemented.
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- 2/4 projects implemented (`01-foundations`, `02-core`).
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- 2/4 assessments implemented (`01-foundations`, `02-core`).
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- [error-handling-and-defensive-programming](./02-core/error-handling-and-defensive-programming/README.md)
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- [03-advanced](./03-advanced/README.md)
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- [structs-and-classes](./03-advanced/structs-and-classes/README.md)
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- [constructors-and-invariants](./03-advanced/constructors-and-invariants/README.md)
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- Projects
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- [01-foundations](./projects/01-foundations/README.md)
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- [02-core](./projects/02-core/README.md)

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