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Module 07 - OOP: Classes & Objects

What You Will Learn

Topic	Key Insight
Class anatomy	fields, constructors, methods, static members - where each lives in memory
Constructor chaining	this(...) eliminates duplication; telescoping constructors vs builder
this reference	disambiguation, chaining, passing self as argument
Static members	class-level state and behaviour; initialisation order
Object lifecycle	allocation on heap, GC eligibility, finalize is dead - use Cleaner
equals / hashCode contract	symmetry, transitivity, consistency; why violating it breaks HashMap
toString	always override it - the debugger and logs will thank you
Records (Java 16+)	immutable data carriers; compact constructor for validation
var with objects	type inference at declaration site

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Memory Layout

Stack                       Heap
─────                       ────────────────────────────────────────
main() frame                ┌─────────────────────┐
  p ──────────────────────► │  Person object       │
                            │  name: ──────────────┼──► "Alice" (String pool)
                            │  age:  30            │
                            │  email: ─────────────┼──► "alice@x.com"
                            └─────────────────────┘

Two variables, one object:
  a ──────────────────────► ┌───────────────┐
  b ───────────────────────►│  same object  │
                            └───────────────┘
  a == b   → true  (same reference)
  a.equals(b) → true (same content, if properly implemented)

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equals / hashCode Contract

                 ┌──────────────────────────────────────────┐
                 │            THE CONTRACT                   │
                 │                                          │
  Reflexive  ──► │  x.equals(x) == true                    │
  Symmetric  ──► │  x.equals(y) == y.equals(x)             │
  Transitive ──► │  x.equals(y) && y.equals(z) → x.equals(z)│
  Consistent ──► │  same result across multiple calls        │
  Null-safe  ──► │  x.equals(null) == false                 │
                 │                                          │
                 │  CRITICAL LINK:                          │
                 │  x.equals(y) → x.hashCode()==y.hashCode()│
                 │  (converse NOT required)                 │
                 └──────────────────────────────────────────┘

Violation consequence:
  HashSet<Point> set = new HashSet<>();
  set.add(new Point(1, 2));
  set.contains(new Point(1, 2));  // FALSE if hashCode broken!

---

Static Initialisation Order

Class loaded
    │
    ▼
1. static fields (in declaration order)
2. static initialiser blocks (in declaration order)
    │
    ▼
new MyClass()
    │
    ▼
3. instance fields (in declaration order)
4. instance initialiser blocks
5. constructor body

---

Record vs Class

// class - 30+ lines boilerplate
class Point {
    private final int x, y;
    public Point(int x, int y) { this.x = x; this.y = y; }
    public int x() { return x; }
    public int y() { return y; }
    @Override public boolean equals(Object o) { ... }
    @Override public int hashCode() { ... }
    @Override public String toString() { ... }
}

// record - 1 line, same semantics
record Point(int x, int y) {}

// compact constructor - add validation
record Point(int x, int y) {
    Point {                              // no parameter list here
        if (x < 0 || y < 0)
            throw new IllegalArgumentException("negative coordinate");
        // x and y are implicitly assigned after the block
    }
}

---

Source Files

File	What it Demonstrates
ClassAnatomy.java	Fields, constructors, this, static members, initialisation order
EqualsHashCode.java	Correct equals/hashCode with all five contract rules, HashMap/HashSet behaviour
RecordsDemo.java	Records, compact constructors, record patterns (Java 21), with-style copying
BankAccount.java	Full class design: encapsulation, invariants, equals/hashCode, toString, static factory, object lifecycle

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Running

cd module-07-oop-classes
mvn test          # compile + run all JUnit 5 tests
mvn compile exec:java -Dexec.mainClass=com.javatraining.oop.ClassAnatomy