CS102 · Annotated Slides · Inheritance

Inheritance

CS102

1

Learning Outcomes

How inheritance promotes software reusability.
The notions of superclasses and subclasses and the relationship between them.
To use keyword extends to create a class that inherits attributes and behaviors from another class.
To access superclass members with super.
How constructors are used in inheritance hierarchies.
The methods of class Object, the direct or indirect superclass of all classes.

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Exam weight Low

Introduction

Inheritance
- A form of software reuse in which a new class is created by absorbing an existing class’s members and embellishing them with new or modified capabilities.
- Can save time during program development by basing new classes on existing proven and debugged high-quality software.
- Increases the likelihood that a system will be implemented and maintained effectively.

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Exam weight Low

Inheritance = reuse: a new class absorbs an existing class’s members and adds or modifies some.

Introduction (Cont.)

When creating a class, rather than declaring completely new members, you can designate that the new class should inherit the members of an existing class.
- Existing class is the superclass
- New class is the subclass
Each subclass can be a superclass of future subclasses.
A subclass can add its own fields and methods.
A subclass is more specific than its superclass and represents a more specialized group of objects.
The subclass exhibits the behaviors of its superclass and can add behaviors that are specific to the subclass.
- This is why inheritance is sometimes referred to as specialization.

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Exam weight High

Existing class = superclass; new class = subclass (extends).

🎯 How this can come up · 1 question

MCQ

Which keyword makes a class inherit from another?

Introduction (Cont.)

The direct superclass is the superclass from which the subclass explicitly inherits.
An indirect superclass is any class above the direct superclass in the class hierarchy.
The Java class hierarchy begins with class Object (in package java.lang)
- Every class in Java directly or indirectly extends (or “inherits from”) Object .
Java supports only single inheritance, in which each class is derived from exactly one direct superclass.

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Exam weight High

Direct superclass: the one you extend. Indirect: higher up. Every class ultimately extends Object.

🎯 How this can come up · 1 question

True / False

Java supports multiple inheritance of classes (a class can extend two classes).

Inheritance

Generic are usually the superclasses
Specific are subclasses that implement specific behavior
Why inheritance is useful?
- to reduce the size of the code and development time
- make the code modification easier and more efficient
- improve modularity and promote software reuse

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Exam weight Low

Superclasses and Subclasses

Figure 9.1 lists several simple examples of superclasses and subclasses
- Superclasses tend to be “more general” and subclasses “more specific.”
Because every subclass object is an object of its superclass, and one superclass can have many subclasses, the set of objects represented by a superclass is typically larger than the set of objects represented by any of its subclasses.

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Exam weight Low

Superclasses are more general, subclasses more specific; a subclass object IS-A superclass object.

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Exam weight Low

Slide 9: Superclasses and Subclasses (Cont.)

Exam weight Low

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Exam weight Low

Relationship

We distinguish between the is-a relationship and the has-a relationship
Is-a represents inheritance
- In an is-a relationship, an object of a subclass can also be treated as an object of its superclass
Has-a represents composition
- In a has-a relationship, an object contains as members references to other objects

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Exam weight High

is-a = inheritance (a Car is a Vehicle). has-a = composition (a Car has an Engine).

🎯 How this can come up · 1 question

MCQ

“A Manager is an Employee” suggests:

Superclasses and Subclasses (Cont.)

Not every class relationship is an inheritance relationship.
Has-a relationship
- Create classes by composition of existing classes.
- Example: Given the classes Employee BirthDate TelephoneNumber , and , it’s improper to say that an Employee BirthDate is a or that an Employee is a TelephoneNumber .
- However, an Employee has a BirthDate , and an Employee has a TelephoneNumber .

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Exam weight Low

Superclasses and Subclasses (Cont.)

Objects of all classes that extend a common superclass can be treated as objects of that superclass.
Inheritance issue
- A subclass can inherit methods that it does not need or should not have.
- Even when a superclass method is appropriate for a subclass, that subclass often needs a customized version of the method.
- The subclass can override (redefine) the superclass method with an appropriate implementation.

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Exam weight Low

A subclass may inherit methods it doesn’t want; it can override them.

Example Without Using Inheritance

In what follows, we have two classes of CommissionEmployee, and BasePlusComissionEmployee.
The only difference is that the BasePlusComissionEmployee has a base salary in addition to attributes of ComissionEmployee, and thus its earning method is slightly different.
The code shows the implementation of both classes without using inheritance.
Observe the following: There is a lot of redundancy in both codes
BasePlusComissionEmployee has more than 110 lines of code.
Certainly, this programming style is not efficient. It is also not easy to modify the code of the related classes.
Solution: using inheritance as shown in the next section

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Exam weight Low

Without inheritance, BasePlusCommissionEmployee copies almost all of CommissionEmployee.

See code Without Using Inheritance

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Exam weight Low

Relationship between Superclasses and Subclasses

Inheritance hierarchy containing types of employees in a company’s payroll application
Commission employees are paid a percentage of their sales
Base-salaried commission employees receive a base salary plus a percentage of their sales.

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Exam weight Low

Example Using Inheritance

Considering the drawbacks of the first approach, we now use an important concept in Object Oriented Programming that is inheritance. Observe the following:
The class CommissionEmployee, that is the generic class (or superclass) remains unchanged (same code as above)
The class BasePlusComissionEmployee now extends the class CommissionEmployee
How much code is reduced with using inheritance: 50 lines with inheritance as compared to more than 110 lines without using inheritance in the BasePlusComissionEmployee

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Exam weight Low

See Code Using Inheritance

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Exam weight Low

Creating and Using a CommissionEmployee Class

Constructors are not inherited.
The first task of a subclass constructor is to call its direct superclass’s constructor explicitly or implicitly
- Ensures that the instance variables inherited from the superclass are initialized properly.
If the code does not include an explicit call to the superclass constructor, Java implicitly calls the superclass’s default or no-argument constructor.
A class’s default constructor calls the superclass’s default or no-argument constructor.

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Exam weight High

Constructors are NOT inherited. A subclass constructor must call a superclass constructor first (explicitly with super(...) or implicitly).

🎯 How this can come up · 1 question

True / False

Constructors are inherited by subclasses.

Creating and Using a CommissionEmployee Class (Cont.)

To override a superclass method, a subclass must declare a method with the same signature as the superclass method
@Override annotation
- Indicates that a method should override a superclass method with the same signature.
- If it does not, a compilation error occurs.

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Exam weight High

Overriding: same signature in the subclass; @Override asks the compiler to check it.

Exam trapOverloading = same name, different parameters (same class). Overriding = same signature in a subclass.

🎯 How this can come up · 1 question

MCQ

A subclass method with the same signature as a superclass method:

Slide 21: Creating and Using a BasePlus- CommissionEmployee Class

Exam weight Low

Creating and Using a BasePlus- CommissionEmployee Class (Cont.)

We literally copied CommissionEmployee ’s code, pasted it into BasePlusCommissionEmployee, then modified the new class to include a base salary and methods that manipulate the base salary.
- This “copy-and-paste” approach is often error prone and time consuming.
- It spreads copies of the same code throughout a system, creating a code-maintenance nightmare.

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Exam weight Low

Copy-and-paste reuse is error-prone and a maintenance nightmare → use inheritance.

Slide 24: Creating a CommissionEmployee – BasePlusCommissionEmployee Inheritance Hierarchy

Exam weight Low

class BasePlusCommissionEmployee extends CommissionEmployee: only the base salary is new.

Creating a CommissionEmployee – BasePlusCommissionEmployee Inheritance Hierarchy (Cont.)

Each subclass constructor must implicitly or explicitly call its superclass constructor to initialize the instance variables inherited from the superclass.
- Superclass constructor call syntax—keyword super , followed by a set of parentheses containing the superclass constructor arguments.
- Must be the first statement in the subclass constructor’s body.
If the subclass constructor did not invoke the superclass’s constructor explicitly, Java would attempt to invoke the superclass’s no-argument or default constructor.
- Class CommissionEmployee does not have such a constructor, so the compiler would issue an error.
You can explicitly use super to call the superclass’s no-argument or default constructor, but this is rarely done.

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Exam weight High

super(args) must be the FIRST statement in the subclass constructor.

🎯 How this can come up · 1 question

MCQ

Where must the call super(first, last, ssn) appear in a subclass constructor?

Creating a CommissionEmployee – BasePlusCommissionEmployee Inheritance Hierarchy (Cont.)

Compilation errors occur when the subclass attempts to access the superclass’s private instance variables.
These lines could have used appropriate get methods to retrieve the values of the superclass’s instance variables.

26

Exam weight High

A subclass can’t access the superclass’s PRIVATE fields directly → use get methods (or protected).

🎯 How this can come up · 1 question

True / False

A subclass can directly read its superclass’s private instance variables.