Understanding Java Inheritance & Polymorphism: Super, Overriding, & toString() in Students, Slides of Network security

Download Understanding Java Inheritance & Polymorphism: Super, Overriding, & toString() in Students and more Slides Network security in PDF only on Docsity!

Objectives

• the use of super, overriding, method

polymorphism (dynamic binding),

protected access, toString()

9. More on Inheritance

Topics

• 1. Students Example

• 2. Method Polymorphism

• 3. Extends and Private

• 4. DoME v.2 Output Problem

• 5. The Object Class's Methods

Student.java

public class Student { private int student_id; private int year; private String name;

public Student(String nm, int id, int y)

{ name = new String(nm); student_id = id; year = y; }

continued Docsity.com

public String toString() { return "Student: " + name + ", " + student_id + ", " + year; }

public int year_group() { return year; }

} // end of Student class

public String toString() { return "Grad " + super. toString() + ", " + dept + ", " + thesis; }

} // end of GradStudent class

TestStuds.java

public class TestStuds

{

public static void main(String args[]) { Student s1 = new Student("Jane Doe", 100 , 1 );

GradStudent gs1; gs1 = new GradStudent("John Smith", 200 , 4, "Pharmacy", "Retail Thesis"); :

continued Docsity.com

Compilation and Execution

$ javac Student.java

$ javac GradStudent.java

$ javac TestStuds.java

$ java TestStuds

TestStuds Output

$ java TestStuds

Student s

Student: Jane Doe, 100 , 1

Year 1

Grad student gs

GradStudent: John Smith, 200 , 4, Pharmacy,

Retail Thesis

Year 4

:

// see later

Method Lookup for s1.toString()

Student s

Student object

instance of

Method Lookup for gs1.toString()

Overriding : use the first version of toString() found in the inheritance hierarchy.

GradStudent gs

GradStudent object

instance of

Method Lookup for gs1.year_group()

Move up the inheritance hierarchy until a suitable method is found.

GradStudent gs

GradStudent object

instance of

TestStuds.java Continued

Student stud; stud = gs1; // refer to subclass

System.out.println("Student stud"); System.out.println( stud.toString () ); System.out.println("Year " + stud.year_group()); }

} // end of TestStuds class

Output

Student stud Grad Student: John Smith, 200, 4, Pharmacy, Retail Thesis Year 4

o Even though stud is of type Student, it

can refer to a GradStudent object

– because GradStudent is a subclass of

Student

Method Lookup of stud.toString()

Overriding again: use the first version found.

Student stud

GradStudent object

instance of