Inheritance #1

1. Inheritance #1

2. First questions • Similar to Python? • What about visibility and encapsulation? • can an object of the child class access private members of its parent? • Is the inheritance hierarchy reflected in type declaration? • How does method overriding work?

3. Stepper in Python from Counter import Counter class Stepper (Counter): # from Python Review #1 def __init__(self): Counter.__init__(self) def reset(self): self.value = 0 # where is my parent? # parent’s constructor called explicitly # child method; others inherited

4. Testing Stepper # Testing the child class: s = Stepper() print "initial value", s.current() s.step() print "after one step", s.current() s.reset() print "after reset", s.current() # inherited # new method

5. Stepper in Java public class Stepper extends Counter { public Stepper () { super(); } public void reset() { this.value = 0; } } // parent's constructor called implicitly // child method; others inherited

6. Recall Java’s Counter public class Counter { private int value; public Counter () { this.value = 0; } public void step() { this.value += 1; } public int current() { return this.value; } } // static typing // encapsulation enforced

7. What is inherited? • Methods are public – they are inherited • Instance variable “value” is private – it is not inherited • So how do we access “value” in Stepper?

8. Use protected visibility public class Counter { protected int value; public Counter () { this.value = 0; } public void step() { this.value += 1; } public int current() { return this.value; } } // only “package” encapsulation enforced // child class has access

9. TestStepper in Java public class TestStepper { public static void main (String[] args) { // Testing the child class: Stepper s = new Stepper(); System.out.println("initial value " + s.current()); s.step(); System.out.println("after one step " + s.current()); s.reset(); System.out.println("after reset " + s.current()); } } // inherited // new method

10. Will this work? public class TestStepper { public static void main (String[] args) { // Testing the child class: Counter s = new Stepper(); System.out.println("initial value " + s.current()); s.step(); System.out.println("after one step " + s.current()); s.reset(); System.out.println("after reset " + s.current()); } } // Stepper is a “kind of” Counter NO! // no method reset() in class Counter!

11. Must use a cast public class TestStepper { public static void main (String[] args) { // Testing the child class: Counter s = new Stepper(); // Stepper is a “kind of” Counter System.out.println("initial value " + s.current()); s.step(); System.out.println("after one step " + s.current()); ((Stepper)s).reset(); System.out.println("after reset " + s.current()); } } // tell javac “s” is actually a Stepper object

12. Will this work? public class TestStepper { public static void main (String[] args) { // Testing the child class: Stepper s = new Counter(); System.out.println("initial value " + s.current()); s.step(); System.out.println("after one step " + s.current()); s.reset(); System.out.println("after reset " + s.current()); } } NO! // Counter a “kind of” Stepper?

13. Incrementer in Python Recall this method overriding example: from Counter import Counter class Incrementer (Counter): # from Python Review #1 def __init__(self, increment=1): Counter.__init__(self) self.increment = increment def step(self): self.value += self.increment # parent’s constructor called explicitly # child instance variable # overrides parent step()

14. Testing Incrementer # Testing the parent class: obj = Counter() for i in range(2): obj.step() print "Counter (parent)", i, ":", obj.current() # Testing the child class: obj = Incrementer(3) for i in range(2): obj.step() print "Incrementer (child)", i, ":", obj.current() Counter (parent) 0 : 1 Counter (parent) 1 : 2 Incrementer (child) 0 : 3 Incrementer (child) 1 : 6

15. Incrementer in Java public class Incrementer extends Counter { private int increment; public Incrementer (int increment) { super(); this.increment = increment; } public Incrementer () { super(); this.increment = 1; } public void step() { this.value += this.increment; } } // child instance variable // constructor for increment // constructor for default increment Overloaded! // overrides parent step()

16. TestIncrementer in Java public class TestIncrementer { public static void main (String[] args) { Counter obj; // Testing the parent class: obj = new Counter(); for (int i = 0; i < 2; i++) { obj.step(); System.out.println("Counter (parent) " + i + ":“ + obj.current()); } // Testing the child class: obj = new Incrementer(3); for (int i = 0; i < 2; i++) { obj.step(); System.out.println("Incrementer (child) " + i + ":“ + obj.current()); } } } Counter (parent) 0 : 1 Counter (parent) 1 : 2 Polymorphism! Incrementer (child) 0 : 3 Incrementer (child) 1 : 6