20.2 Java inheritance
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Inheritance allows classes to extend and inherit properties from base classes. This creates class hierarchies where subclasses inherit and can override methods from superclasses. Inheritance promotes code reuse through extension when subclasses share the same role as the base class. Composition and delegation are alternative approaches to code reuse that may be preferable in some cases over inheritance.
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20.2 Java inheritance
- 1. Inheritance Software University http://softuni.bg SoftUni Team Technical Trainers Extending Classes
- 2. Table of Contents 1. Inheritance 2. Class Hierarchies 3. Inheritance in Java 4. Accessing Members of the Base Class 5. Types of Class Reuse Extension, Composition, Delegation 6. When to Use Inheritance 2
- 5. Superclass - Parent class, Base Class The class giving its members to its child class Subclass - Child class, Derived Class The class taking members from its base class Inheritance 5 Superclass Subclass Derived Base
- 6. Inheritance – Example 6 Person +Name: String +Address: String Employee +Company: String Student +School: String Derived class Derived class Base class
- 7. Inheritance leads to hierarchies of classes and/or interfaces in an application: Class Hierarchies 7 Game MultiplePlayersGame BoardGame Chess Backgammon SinglePlayerGame Minesweeper Solitaire Base class holds common characteristics … …
- 8. Class Hierarchies – Java Collection 8 Collection Queue Deque ArrayDeque HashSet List ArrayList PriorityQueue Iterable Set LinkedList Vector Stack LinkedHashSet SortedSet TreeSet
- 9. Object is at the root of Java Class Hierarchy Java Platform Class Hierarchy 9
- 10. Java supports inheritance through extends keyword Inheritance in Java 10 class Person { … } class Student extends Person { … } class Employee extends Person { … } Person Employee Student extends Person Student
- 11. Class taking all members from another class Inheritance - Derived Class 11 Person Student Employee Mother : Person Father : Person School: School Org: Organization Reusing Person
- 12. You can access inherited members Using Inherited Members 12 class Person { public void sleep() { … } } class Student extends Person { … } class Employee extends Person { … } Student student = new Student(); student.sleep(); Employee employee = new Employee(); employee.sleep();
- 13. Constructors are not inherited Constructors can be reused by the child classes Reusing Constructors 13 class Student extends Person { private School school; public Student(String name, School school) { super(name); this.school = school; } } Constructor call should be first
- 14. A derived class instance contains an instance of its base class Thinking About Inheritance - Extends 14 Employee (Derived Class) +work():void Student (Derived Class) +study():void Person (Base Class) +sleep():void
- 15. Inheritance has a transitive relation Inheritance 15 class Person { … } class Student extends Person { … } class CollegeStudent extends Student { … } Person CollegeStudent Student
- 16. In Java there is no multiple inheritance Only multiple interfaces can be implemented Multiple Inheritance 16 Person CollegeStudent Student
- 17. Use the super keyword Access to Base Class Members 17 class Person { … } class Employee extends Person { public void fire(String reasons) { System.out.println( super.name + " got fired because " + reasons); } }
- 18. Problem: Single Inheritance 18 Animal +eat():void Dog +bark():void Check your solution here :https://judge.softuni.bg/Contests/1574/Inheritance-Lab
- 21. Reusing Code at Class Level Reusing Classes
- 22. Derived classes can access all public and protected members Derived classes can access default members if in same package Private fields aren't inherited in subclasses (can't be accesssed) Inheritance and Access Modifiers 22 class Person { protected String address; public void sleep(); String name; private String id; } Can be accessed through other methods
- 23. Derived classes can hide superclass variables Shadowing Variables 23 class Patient extends Person { protected float weight; public void method() { double weight = 0.5d; } } class Person { protected int weight; } hides int weight hides both
- 24. Use super and this to specify member access Shadowing Variables - Access 24 class Patient extends Person { protected float weight; public void method() { double weight = 0.5d; this.weight = 0.6f; super.weight = 1; } } class Person { protected int weight; } Instance member Base class member Local variable
- 25. A child class can redefine existing methods Overriding Derived Methods 25 public class Person { public void sleep() { System.out.println("Person sleeping"); } } public class Student extends Person { @Override public void sleep(){ System.out.println("Student sleeping"); } } Signature and return type should match Method in base class must not be final
- 26. final – defines a method that can't be overridden Final Methods 26 public class Animal { public final void eat() { … } } public class Dog extends Animal { @Override public void eat() {} // Error… }
- 27. Inheriting from a final classes is forbidden Final Classes 27 public final class Animal { … } public class Dog extends Animal { } // Error… public class MyString extends String { } // Error… public class MyMath extends Math { } // Error…
- 28. One approach for providing abstraction Inheritance Benefits - Abstraction 28 Person person = new Person(); Student student = new Student(); List<Person> people = new ArrayList(); people.add(person); people.add(student); Student (Derived Class) Person (Base Class) Focus on common properties
- 29. We can extend a class that we can't otherwise change Inheritance Benefits – Extension 29 Collections ArrayList CustomArrayList Extends
- 30. Create an array list that has All functionality of an ArrayList Function that returns and removes a random element Problem: Random Array List 30 Collections ArrayList RandomArrayList +getRandomElement():Object
- 31. Solution: Random Array List 31 public class RandomArrayList extends ArrayList { private Random rnd; // Initialize this… public Object getRandomElement() { int index = this.rnd.nextInt(super.size()); Object element = super.get(index); super.remove(index); return element; } }
- 32. Types of Class Reuse Extension, Composition, Delegation
- 33. Duplicate code is error prone Reuse classes through extension Sometimes the only way Extension 33 Collections ArrayList CustomArrayList
- 34. Using classes to define classes Composition 34 class Laptop { Monitor monitor; Touchpad touchpad; Keyboard keyboard; … } Reusing classes Laptop Monitor Touchpad Keyboard
- 35. Delegation 35 class Laptop { Monitor monitor; void incrBrightness() { monitor.brighten(); } void decrBrightness() { monitor.dim(); } } Laptop increaseBrightness() decreaseBrightness() Monitor
- 36. Create a simple Stack class which can store only strings Problem: Stack of Strings 36 StackOfStrings -data: List<String> +push(String) :void +pop(): String +peek(): String +isEmpty(): boolean StackOfStrings ArrayList
- 37. Solution: Stack of Strings 37 public class StackOfStrings { private List<String> container; // TODO: Create a constructor public void push(String item) { this.container.add(item); } public String pop() { // TODO: Validate if list is not empty return this.container.remove(this.container.size() - 1); } }
- 38. Classes share IS-A relationship Derived class IS-A-SUBSTITUTE for the base class Share the same role Derived class is the same as the base class but adds a little bit more functionality When to Use Inheritance 38 Too simplistic
- 39. … … … Summary 39 Inheritance is a powerful tool for code reuse Subclass inherits members from Superclass Subclass can override methods Look for classes with the same role Look for IS-A and IS-A-SUBSTITUTE for relationship Consider Composition and Delegation instead
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