Abstract Classes

Abstract Classes

1. What Is Abstract? (beginner)

Abstract class = half-built blueprint (has SOME ready methods + SOME empty promises!). Can't new it (incomplete!) — child MUST fill promises (or stay abstract!). Like flat floor-plan: walls promised, paint your choice!

2. Code — Shape Area (exam classic!)

abstract class Shape { // abstract (no new Shape()!)
    String color;
    Shape(String c) { color = c; } // ctor OK! (child via super!)
    void show() { System.out.println("color=" + color); } // concrete (ready!)
    abstract double area(); // PROMISE (no body! child MUST write!)
}
class Circle extends Shape {
    double r;
    Circle(double r) { super("red"); this.r = r; }
    @Override double area() { return Math.PI * r * r; } // fulfilled!
}
// Shape s = new Shape("x"); // ERROR! (abstract — incomplete!)
// Shape s = new Circle(7); s.area(); // 153.94 (polymorphism!)

3. Rules (exam traps!)

• abstract method: NO body (; only!), in abstract class only!
• Child must implement ALL abstracts (else child abstract too!).
• Abstract CAN have ctor, fields, concrete methods, static, main!
• abstract + final together = ERROR (final bans child, abstract needs child!).

4. Abstract vs Interface (preview!)

	Abstract class	Interface
new?	No	No
Methods	mix (ready + abstract)	all abstract (Java 7) / default ok (8+)
Extends	one parent	many interfaces!

5. Exam Points

• Shape program + rules — 7 marks, very frequent.
• Abstract vs interface + final clash — 5 marks, frequent.