Inheritance (Part 1)
Fattane Zarrinkalam
Problem
• Three main concepts in an information system of an educational institute:
• Teacher
• Student
• Employee
public class Student {
private String name;
private String lastname;
private Date birthDate;
private int id;
…
}
public class Teacher {
private String name;
private String lastname;
private Date birthDate;
private int rank;
…
}
public class Employee {
private String name;
private String lastname;
private Date birthDate;
private int empCode;
…
}
2
Problem
• Issues with this design
•
•
•
•
Duplicate codes
Define duplicate members in classes
Increase class size
Repetition is the source of many problems
• Incomplete update
• Increased likelihood of bugs
• Lack of attention to the semantic connection of concepts in the real world
• All three concepts of teacher, student and employee have a lot in common
• Because all three are a special form of the concept of person / person / human
3
Problem
• Solution: inheritance
• Identify common features between classes
• Define common features (attributes and methods) in a class
• Superclass
• Define the specific features of each concept in a separate class
• Subclass
• Establish a connection between subclasses and the superclass
4
Inheritance
• It is the mechanism in java by which one class is allowed to inherit the features
(attributes and methods) of another class.
• The class which inherits the properties of other is known as subclass (derived
class, child class) and the class whose properties are inherited is known as
superclass (base class, parent class).
5
Superclass : Person
public class Person {
private String name;
private String lastname;
private Date birthDate;
public Person(String name, String lastname) {
setName(name);
setLastname(lastname);
}
Why is this constructor defined?
public Person() {
}
public void print() {
System.out.println(“name: “ + name);
System.out.println(“lastname: “ + lastname);
}
public void setName(String name) {
this.name = name;
}
(continued)6
Superclass : Person
public String getName() {
return name;
}
public void setLastname(String lastname) {
this.lastname = lastname;
}
public String getLastname() {
return lastname;
}
public void setBirthDate(Date birthDate) {
this.birthDate = birthDate;
}
public Date getBirthDate() {
return birthDate;
}
…
}
7
Subclass1 : Student
The keyword used for inheritance is extends.
public class Student extends Person {
private int id;
public Student(String name, String lastname, int id) {
setName(name);
setLastname(lastname);
setId(id);
}
public void setId(int id) {
this.id = id;
}
The class Student inherits from the Person class
public int getId() {
return id;
}
…
}
8
Subclass2 : Teacher
public class Teacher extends Person {
private int rank;
public Teacher(String name, String lastname, int rank) {
setName(name);
setLastname(lastname);
setRank(rank);
}
public void setRank(int rank) {
this.rank = rank;
}
public int getRank() {
return rank;
}
…
}
9
Subclass3 : Employee
public class Employee extends Person {
private int empCode;
public Employee(String name, String lastname, int empCode)
{
setName(name);
setLastname(family);
setEmpCode(empCode);
}
public void setEmpCode(int empCode) {
this.empCode = empCode;
}
public int getEmpCode() {
return empCode;
}
…
}
10
Inheritance Hierarchy
Superclass
Person
is-a relationship
Teacher
Student
Employee
Subclasses
11
Inheritance Hierarchy
• A class can be a superclass of other classes and itself a subclass of another class
• A class can have several subclasses (direct or indirect)
Shape
Shape2D
Circle
Shape3D
Rectangle
Cube
Class Hierarchy
Sphere
12
Types of Inheritance
• Single inheritance
• A class can only have one direct superclass
• Multiple inheritance
• A class can have several direct superclasses
Java doesn’t support multiple inheritance
13
Example
public class Person {
private String name;
private String lastname;
private Date birthDate;
public class Student extends Person {
private int id;
public Student(String name, String lastname, int id) {
setName(name);
setLastname(lastname);
setId(id);
}
public
void setId(int
id) {
public Person(String name, String
lastname)
{
this.id = id;
setName(name);
}
setLastname(lastname);
}
public int getId() {
public Person() {
return id;
}
}
public void setName(String name)
{
…
this.name = name;
}
}
public void setLastname(String lastname) {
this.lastname = lastname;
}
…
}
14
Example
public class Student extends Person {
private int id;
public Student(String name, String lastname, int id) {
setName(name);
setLastname(lastname);
setId(id);
}
public void setId(int id) {
this.id = id;
}
Student stu = new Student(“John”, “River”, 87231);
stu.setName(“Johnecca”);
public int getId() {
return id;
}
…
}
15
Example
public class Person {
private String name;
private String lastname;
private Date birthDate;
public class Student extends Person {
private int id;
public Student(String name, String lastname, int id) {
this.name = name;
this.lastname = lastname;
this.Id = id;
}
Compile time error
public
void setId(int
id) {
public Person(String name, String
lastname)
{
this.id = id;
setName(name);
}
setLastname(lastname);
}
public int getId() {
public Person() {
return id;
}
}
public void setName(String name)
{
…
this.name = name;
}
}
public void setLastname(String lastname) {
this.lastname = lastname;
}
…
}
16