9/17/2018 Programming Data Structures. Encapsulation and Inheritance

Transcription

1 9/17/2018 Programming Data Structures Encapsulation and Inheritance 1

2 Encapsulation private instance variables Exercise: 1. Create a new project in your IDE 2. Download: Employee.java, HourlyEmployee.java in the example01 folder in the GitHub repository 3. You will find an compilation error 2

3 IntelliJ BlueJ 3

4 7.2 PITFALL: Use of Private Instance Variables from the Base Class Private instance variables, private static variables, and private methods in a base class are NOT directly accessible in the derived classes ( accessed by name within the definition of a method in the derived classes). Private variables and private methods can be accessed through public methods in the base class. To fix the error on the previous slide, you can use instead of Try the above change, and see if you can solve the error 4

5 Why private? An answer from StackOverflow: In order to add something, a private method can ALWAYS be changed safely, because you know for sure that is called only from the own class, no external classes are able to call a private method (they can't even see it). Now think of a public method, anyone could call that method, so if you add/remove a parameter, you will need to change also ALL the calls to that method. Ref: 5

6 7.2 PITFALL: Private Methods Employee.java and HourlyEmployee.java in example02/ Employee.java HourlyEmployee.java 6

7 7.2 PITFALL: Private Methods Create a new project in your IDE Download java files: Employee.java and HourlyEmployee.java in example02/ Fix the error on line 110 in HourlyEmployee.java 7

8 Protected protected variables or methods in a base class can be accessed by name in the derived classes, and in any class defined in the same package Packages can be seen as folders. If the class files (java files) are in the same folder, they should be in the same package. (packages and import statements in Chapter 5.4) 8

9 Package Access == Default Access == Friendly Access If you don t put any modifier ( ) before a variable or a method definition, the variable/method has package access, also known as, default access, friendly access The variable/method can be accessed by name inside any class in the same package 9

10 Example from P Create a project, name it newproject 2. The project should have a package named somepackage 3. somepackage should have: class A, class B, class C as shown in the figure 10

11 Example from P. 489 B and C: should have a method sum (with no argument) to return the sum of v1, v2, and v3 4. The default package should have classes D and E D: should have a method sum (with no argument) to return the sum of v1 and v2 E: should have a method sum (with no argument) to return the value of v1 11

12 Example from P Upload the project to GitHub Assignment, 0.5, deadline: tomorrow noon 12

13 A few things How to create a bluej project and remember the path of the project Upload the whole project to the GitHub: Drag the whole folder of the project to GitHub upload page Do NOT click choose your files 13

14 Exercise (on paper) P.490 If a class D is derived from class B, and class B has a protected instance variable n, but the classes B and D are in different packages Question 1: can a method in class D access by name? Question 2: A method in class D can create a local object of class D. Can n of that object be accessed by name? 14

15 New situation If a class D is derived from class B, and class B has a protected instance variable n, but the classes B and D are in different packages Question 3: A method in class D can create a local object of class B. Can n of that object be accessed by name? 15

16 New situation If a class D is derived from class B, and class B has a protected instance variable n, but the classes B and D are in different packages Question 3: A method in class D can create a local object of class B. Can n of that object be accessed by name? -- No 16

17 Is-a relationship (P. 493) 17

18 has-a relationship (P. 493) Employee private String name; private Date hiredate; 18

19 Calling the parent s method explicitly P. 494 super 19

20 The class object equals tostring Clone getclass (marked final) getclass vs instanceof why not use instanceof? 20

21 In-class assignment (1.5) (Programming Project 1 in P. 508) Define a class named Person that contains two instance variables of type String that stores the first name and last name of a person and appropriate accessor and mutator methods. Also create a method named displaydetails that outputs the details of a person. 21

22 In-class assignment (cont.) (Programming Project 1 in P. 508) Next, define a class named Student that is derived from Person. Student should have instance variables of student id, course, and teacher name. The class Student should have appropriate constructor(s), which should call the constructor of Person, which should receive first name and last name from the class Student. The constructor of Student should also take values of student id, course, and teacher name and assign the values to the corresponding instance variables. This class should redefine the displaydetails method to print person details as well as details of a student. 22

23 In-class assignment (cont.) (Programming Project 1 in P. 508) Define a class named Teacher that is derived from Person. This class should include appropriate constructor(s), and contain instance variables for the subject name and salary. Finally, redefine the displaydetails method to include all teacher information in the printout. Create a main method that creates at least two student objects and two teacher objects with different values and calls displaydetails for each. 23

24 Grading rules (1.5 in total) 0.25: if class Person is correct 0.5: if class Student is correct 0.5: if class Teacher is correct 0.25: if class Main is correct Deadline tomorrow noon. 24