12 — Inheritance & Polymorphism

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▶️ java pkg1core/core11Inheritance.java · java pkg1core/core12Polymorphism.java

Inheritance · is-a relationship

1class Animal {2    void speak() { System.out.println("..."); }3}4 5class Dog extends Animal {6    @Override7    void speak() { System.out.println("Woof!"); }8}

• extends for classes (single inheritance only)
• Child inherits fields and methods
• @Override recommended · compiler catches typos

`super` keyword

1class Dog extends Animal {2    Dog(String name) {3        super();              // call parent constructor (must be first)4    }5    @Override6    void speak() {7        super.speak();        // call parent version8        System.out.println("Dog done");9    }10}

Polymorphism · one interface, many forms

1Animal a = new Dog();    // upcasting · always safe2a.speak();               // prints "Woof!" · runtime dispatch

▶️ Dynamic dispatch: JVM calls the actual object's method, not the reference type.

Overloading vs overriding

When NOT to inherit

▶️ Don't inherit just to reuse code · use composition (has-a).

Bad: class Stack extends ArrayList
Good: class Stack { private Deque<T> data; }

Related → 03-interview/02-OopAndSolid.md

Related → 01 Core Java

Next → 13 Abstraction & Interfaces