Lecture 5 inheritance

Introduction toIntroduction to Computer Science IIComputer Science II COSC 1320/6305 Lecture 5:Lecture 5: InheritanceInheritance (Chapter 7)(Chapter 7)

Introduction to Inheritance • Inheritance is one of the main techniques of object-oriented programming (OOP) • Using this technique, a very general form of a class is first defined and compiled, and then more specialized versions of the class are defined by adding instance variables and methods – The specialized classes are said to inherit the methods and instance variables of the general class 7-3Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

Introduction to Inheritance • Inheritance is the process by which a new class is created from another class – The new class is called a derived class – The original class is called the base class • A derived class automatically has all the instance variables and methods that the base class has, and it can have additional methods and/or instance variables as well • Inheritance is especially advantageous because it allows code to be reused, without having to copy it into the definitions of the derived classes 7-4Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

Derived Classes • When designing certain classes, there is often a natural hierarchy for grouping them – In a record-keeping program for the employees of a company, there are hourly employees and salaried employees – Hourly employees can be divided into full time and part time workers – Salaried employees can be divided into those on technical staff, and those on the executive staff 7-6Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

Derived Classes • All employees share certain characteristics in common – All employees have a name and a hire date – The methods for setting and changing names and hire dates would be the same for all employees • Some employees have specialized characteristics – Hourly employees are paid an hourly wage, while salaried employees are paid a fixed wage – The methods for calculating wages for these two different groups would be different 7-7Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

Derived Classes • Within Java, a class called Employee can be defined that includes all employees • This class can then be used to define classes for hourly employees and salaried employees – In turn, the HourlyEmployee class can be used to define a PartTimeHourlyEmployee class, and so forth 7-8Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

Derived Classes • Since an hourly employee is an employee, it is defined as a derived class of the class Employee – A derived class is defined by adding instance variables and methods to an existing class – The existing class that the derived class is built upon is called the base class – The phrase extends BaseClass must be added to the derived class definition: public class HourlyEmployee extends Employee 7-13Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

Derived Classes • When a derived class is defined, it is said to inherit the instance variables and methods of the base class that it extends – Class Employee defines the instance variables name and hireDate in its class definition – Class HourlyEmployee also has these instance variables, but they are not specified in its class definition – Class HourlyEmployee has additional instance variables wageRate and hours that are specified in its class definition 7-15Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

Derived Classes • Just as it inherits the instance variables of the class Employee, the class HourlyEmployee inherits all of its methods as well – The class HourlyEmployee inherits the methods getName, getHireDate, setName, and setHireDate from the class Employee – Any object of the class HourlyEmployee can invoke one of these methods, just like any other method 7-17Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

Derived Class (Subclass) • A derived class, also called a subclass, is defined by starting with another already defined class, called a base class or superclass, and adding (and/or changing) methods, instance variables, and static variables – The derived class inherits all the public methods, all the public and private instance variables, and all the public and private static variables from the base class – The derived class can add more instance variables, static variables, and/or methods 7-19Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

Inherited Members • A derived class automatically has all the instance variables, all the static variables, and all the public methods of the base class – Members from the base class are said to be inherited • Definitions for the inherited variables and methods do not appear in the derived class – The code is reused without having to explicitly copy it, unless the creator of the derived class redefines one or more of the base class methods 7-20Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

Parent and Child Classes • A base class is often called the parent class – A derived class is then called a child class • These relationships are often extended such that a class that is a parent of a parent . . . of another class is called an ancestor class – If class A is an ancestor of class B, then class B can be called a descendent of class A 7-22Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

Overriding a Method Definition • Although a derived class inherits methods from the base class, it can change or override an inherited method if necessary – In order to override a method definition, a new definition of the method is simply placed in the class definition, just like any other method that is added to the derived class 7-24Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

Changing the Return Type of an Overridden Method • Ordinarily, the type returned may not be changed when overriding a method • However, if it is a class type, then the returned type may be changed to that of any descendent class of the returned type • This is known as a covariant return type – Covariant return types are new in Java 5.0; they are not allowed in earlier versions of Java 7-25Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

Covariant Return Type • Given the following base class: public class BaseClass { . . . public Employee getSomeone(int someKey) . . . • The following is allowed in Java 5.0: public class DerivedClass extends BaseClass { . . . public HourlyEmployee getSomeone(int someKey) . . . 7-26Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

Changing the Access Permission of an Overridden Method • The access permission of an overridden method can be changed from private in the base class to public (or some other more permissive access) in the derived class • However, the access permission of an overridden method can not be changed from public in the base class to a more restricted access permission in the derived class 7-27Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

Changing the Access Permission of an Overridden Method • Given the following method header in a base case: private void doSomething() • The following method header is valid in a derived class: public void doSomething() • However, the opposite is not valid • Given the following method header in a base case: public void doSomething() • The following method header is not valid in a derived class: private void doSomething() 7-28Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

Pitfall: Overriding Versus Overloading • Do not confuse overriding a method in a derived class with overloading a method name – When a method is overridden, the new method definition given in the derived class has the exact same number and types of parameters as in the base class – When a method in a derived class has a different signature from the method in the base class, that is overloading – Note that when the derived class overloads the original method, it still inherits the original method from the base class as well 7-29Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

The final Modifier • If the modifier final is placed before the definition of a method, then that method may not be redefined in a derived class • It the modifier final is placed before the definition of a class, then that class may not be used as a base class to derive other classes 7-30Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

The super Constructor • A derived class uses a constructor from the base class to initialize all the data inherited from the base class – In order to invoke a constructor from the base class, it uses a special syntax: public derivedClass(int p1, int p2, double p3) { super(p1, p2); instanceVariable = p3; } – In the above example, super(p1, p2); is a call to the base class constructor 7-32Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

The super Constructor • A call to the base class constructor can never use the name of the base class, but uses the keyword super instead • A call to super must always be the first action taken in a constructor definition • An instance variable cannot be used as an argument to super 7-33Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

The super Constructor • If a derived class constructor does not include an invocation of super, then the no-argument constructor of the base class will automatically be invoked – This can result in an error if the base class has not defined a no- argument constructor • Since the inherited instance variables should be initialized, and the base class constructor is designed to do that, then an explicit call to super should always be used 7-34Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

The this Constructor • Within the definition of a constructor for a class, this can be used as a name for invoking another constructor in the same class – The same restrictions on how to use a call to super apply to the this constructor • If it is necessary to include a call to both super and this, the call using this must be made first, and then the constructor that is called must call super as its first action 7-36Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

The this Constructor • Often, a no-argument constructor uses this to invoke an explicit- value constructor – No-argument constructor (invokes explicit-value constructor using this and default arguments): public ClassName() { this(argument1, argument2); } – Explicit-value constructor (receives default values): public ClassName(type1 param1, type2 param2) { . . . } 7-37Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

The this Constructor public HourlyEmployee() { this("No name", new Date(), 0, 0); } • The above constructor will cause the constructor with the following heading to be invoked: public HourlyEmployee(String theName, Date theDate, double theWageRate, double theHours) 7-38Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

Tip: An Object of a Derived Class Has More than One Type An object of a derived class has the type of the derived class, and it also has the type of the base class More generally, an object of a derived class has the type of every one of its ancestor classes – Therefore, an object of a derived class can be assigned to a variable of any ancestor type 7-40Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

Tip: An Object of a Derived Class Has More than One Type An object of a derived class can be plugged in as a parameter in place of any of its ancestor classes In fact, a derived class object can be used anyplace that an object of any of its ancestor types can be used Note, however, that this relationship does not go the other way – An ancestor type can never be used in place of one of its derived types 7-41Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

Pitfall: The Terms "Subclass" and "Superclass" • The terms subclass and superclass are sometimes mistakenly reversed – A superclass or base class is more general and inclusive, but less complex – A subclass or derived class is more specialized, less inclusive, and more complex • As more instance variables and methods are added, the number of objects that can satisfy the class definition becomes more restricted 7-42Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

An Enhanced StringTokenizer Class • Thanks to inheritance, most of the standard Java library classes can be enhanced by defining a derived class with additional methods • For example, the StringTokenizer class enables all the tokens in a string to be generated one time – However, sometimes it would be nice to be able to cycle through the tokens a second or third time 7-43Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

An Enhanced StringTokenizer Class • This can be made possible by creating a derived class: – For example, EnhancedStringTokenizer can inherit the useful behavior of StringTokenizer – It inherits the countTokens method unchanged • The new behavior can be modeled by adding new methods, and/or overriding existing methods – A new method, tokensSoFar, is added – While an existing method, nextToken, is overriden 7-44Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

Encapsulation and Inheritance Pitfall: Use of Private Instance Variables from the Base Class • An instance variable that is private in a base class is not accessible by name in the definition of a method in any other class, not even in a method definition of a derived class – For example, an object of the HourlyEmployee class cannot access the private instance variable hireDate by name, even though it is inherited from the Employee base class • Instead, a private instance variable of the base class can only be accessed by the public accessor and mutator methods defined in that class – An object of the HourlyEmployee class can use the getHireDate or setHireDate methods to access hireDate 7-49Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

Encapsulation and Inheritance Pitfall: Use of Private Instance Variables from the Base Class • If private instance variables of a class were accessible in method definitions of a derived class, then anytime someone wanted to access a private instance variable, they would only need to create a derived class, and access it in a method of that class – This would allow private instance variables to be changed by mistake or in inappropriate ways (for example, by not using the base type's accessor and mutator methods only) 7-50Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

Pitfall: Private Methods Are Effectively Not Inherited • The private methods of the base class are like private variables in terms of not being directly available • However, a private method is completely unavailable, unless invoked indirectly – This is possible only if an object of a derived class invokes a public method of the base class that happens to invoke the private method • This should not be a problem because private methods should just be used as helping methods – If a method is not just a helping method, then it should be public, not private 7-51Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

Protected and Package Access • If a method or instance variable is modified by protected (rather than public or private), then it can be accessed by name – Inside its own class definition – Inside any class derived from it – In the definition of any class in the same package • The protected modifier provides very weak protection compared to the private modifier – It allows direct access to any programmer who defines a suitable derived class – Therefore, instance variables should normally not be marked protected 7-53Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

Protected and Package Access • An instance variable or method definition that is not preceded with a modifier has package access – Package access is also known as default or friendly access • Instance variables or methods having package access can be accessed by name inside the definition of any class in the same package – However, neither can be accessed outside the package 7-54Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

Protected and Package Access • Note that package access is more restricted than protected – Package access gives more control to the programmer defining the classes – Whoever controls the package directory (or folder) controls the package access 7-55Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

Pitfall: Forgetting About the Default Package • When considering package access, do not forget the default package – All classes in the current directory (not belonging to some other package) belong to an unnamed package called the default package • If a class in the current directory is not in any other package, then it is in the default package – If an instance variable or method has package access, it can be accessed by name in the definition of any other class in the default package 7-57Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

Pitfall: A Restriction on Protected Access If a class B is derived from class A, and class A has a protected instance variable n, but the classes A and B are in different packages, then the following is true: – A method in class B can access n by name (n is inherited from class A) – A method in class B can create a local object of itself, which can acces n by name (again, n is inherited from class A) 7-58Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

Pitfall: A Restriction on Protected Access • However, if a method in class B creates an object of class A, it can not access n by name – A class knows about its own inherited variables and methods – However, it cannot directly access any instance variable or method of an ancestor class unless they are public – Therefore, B can access n whenever it is used as an instance variable of B, but B cannot access n when it is used as an instance variable of A • This is true if A and B are not in the same package – If they were in the same package there would be no problem, because protected access implies package access 7-59Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

Tip: "Is a" Versus "Has a" • A derived class demonstrates an "is a" relationship between it and its base class – Forming an "is a" relationship is one way to make a more complex class out of a simpler class – For example, an HourlyEmployee "is an" Employee – HourlyEmployee is a more complex class compared to the more general Employee class 7-60Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

Tip: "Is a" Versus "Has a" • Another way to make a more complex class out of a simpler class is through a "has a" relationship – This type of relationship, called composition, occurs when a class contains an instance variable of a class type – The Employee class contains an instance variable, hireDate, of the class Date, so therefore, an Employee "has a" Date 7-61Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

Tip: "Is a" Versus "Has a" • Both kinds of relationships are commonly used to create complex classes, often within the same class – Since HourlyEmployee is a derived class of Employee, and contains an instance variable of class Date, then HourlyEmployee "is an" Employee and "has a" Date 7-62Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

Tip: Static Variables Are Inherited • Static variables in a base class are inherited by any of its derived classes • The modifiers public, private, and protected, and package access have the same meaning for static variables as they do for instance variables 7-63Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

Access to a Redefined Base Method • Within the definition of a method of a derived class, the base class version of an overridden method of the base class can still be invoked – Simply preface the method name with super and a dot public String toString() { return (super.toString() + "$" + wageRate); } • However, using an object of the derived class outside of its class definition, there is no way to invoke the base class version of an overridden method 7-65Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

You Cannot Use Multiple supers • It is only valid to use super to invoke a method from a direct parent – Repeating super will not invoke a method from some other ancestor class • For example, if the Employee class were derived from the class Person, and the HourlyEmployee class were derived form the class Employee , it would not be possible to invoke the toString method of the Person class within a method of the HourlyEmployee class super.super.toString() // ILLEGAL! 7-66Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

The Class Object • In Java, every class is a descendent of the class Object – Every class has Object as its ancestor – Every object of every class is of type Object, as well as being of the type of its own class • If a class is defined that is not explicitly a derived class of another class, it is still automatically a derived class of the class Object 7-67Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

The Class Object • The class Object is in the package java.lang which is always imported automatically • Having an Object class enables methods to be written with a parameter of type Object – A parameter of type Object can be replaced by an object of any class whatsoever – For example, some library methods accept an argument of type Object so they can be used with an argument that is an object of any class 7-69Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

The Class Object The class Object has some methods that every Java class inherits – For example, the equals and toString methods Every object inherits these methods from some ancestor class – Either the class Object itself, or a class that itself inherited these methods (ultimately) from the class Object However, these inherited methods should be overridden with definitions more appropriate to a given class – Some Java library classes assume that every class has its own version of such methods 7-70Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

The Right Way to Define equals • Since the equals method is always inherited from the class Object, methods like the following simply overload it: public boolean equals(Employee otherEmployee) { . . . } • However, this method should be overridden, not just overloaded: public boolean equals(Object otherObject) { . . . } 7-71Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

The Right Way to Define equals • The overridden version of equals must meet the following conditions – The parameter otherObject of type Object must be type cast to the given class (e.g., Employee) – However, the new method should only do this if otherObject really is an object of that class, and if otherObject is not equal to null – Finally, it should compare each of the instance variables of both objects 7-72Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

A Better equals Method for the Class Employee public boolean equals(Object otherObject) { if(otherObject == null) return false; else if(getClass( ) != otherObject.getClass( )) return false; else { Employee otherEmployee = (Employee)otherObject; return (name.equals(otherEmployee.name) && hireDate.equals(otherEmployee.hireDate)); } } 7-73Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

Tip: getClass Versus instanceof • Many authors suggest using the instanceof operator in the definition of equals – Instead of the getClass() method • The instanceof operator will return true if the object being tested is a member of the class for which it is being tested – However, it will return true if it is a descendent of that class as well • It is possible (and especially disturbing), for the equals method to behave inconsistently given this scenario 7-75Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

Tip: getClass Versus instanceof Here is an example using the class Employee . . . //excerpt from bad equals method else if(!(OtherObject instanceof Employee)) return false; . . . Now consider the following: Employee e = new Employee("Joe", new Date()); HourlyEmployee h = new HourlyEmployee("Joe", new Date(),8.5,40); boolean testH = e.equals(h); boolean testE = h.equals(e); 7-76Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

Tip: getClass Versus instanceof • testH will be true, because h is an Employee with the same name and hire date as e • However, testE will be false, because e is not an HourlyEmployee, and cannot be compared to h • Note that this problem would not occur if the getClass() method were used instead, as in the previous equals method example 7-77Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

instanceof and getClass • Both the instanceof operator and the getClass() method can be used to check the class of an object • However, the getClass() method is more exact – The instanceof operator simply tests the class of an object – The getClass() method used in a test with == or != tests if two objects were created with the same class 7-78Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

The instanceof Operator • The instanceof operator checks if an object is of the type given as its second argument Object instanceof ClassName – This will return true if Object is of type ClassName, and otherwise return false – Note that this means it will return true if Object is the type of any descendent class of ClassName 7-79Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

The getClass() Method • Every object inherits the same getClass() method from the Object class – This method is marked final, so it cannot be overridden • An invocation of getClass() on an object returns a representation only of the class that was used with new to create the object – The results of any two such invocations can be compared with == or != to determine whether or not they represent the exact same class (object1.getClass() == object2.getClass()) 7-80Copyright © 2010 Pearson Addison-Wesley. All rights reserved.

Lecture 5 inheritance

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