Inheritance
> Click or hit Control-Enter to run Example.main above
Midterm Review
Overall the class did well on the midterm–congratulations!
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I’ve released two example programming questions to the HW125 problem set.
Interpreting Your Midterm Result
I’d mainly consider how you did on the programming questions:
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3/3: Great job.
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2/3: You did well, but plan to continue working hard for the rest of the semester.
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1/3: Something about how you are preparing for exams needs to change…
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0/3: Please talk to a course staff member.
Questions?
About the midterm, grading, or anything in particular?
Review: The static Keyword
public class Course {
public static int count = 0;
public static void printName() {
System.out.println("Name");
}
}
public class Example {
public static void main(String[] unused) {
// We can call printName without creating an instance
Course.printName();
// We can increment count without creating an instance
Course.count++;
}
}static methods and variables belong to the class, not to a specific
instance.
Review: static Methods
public class Course {
public static void printName() {
System.out.println("Name");
}
}
public class Example {
public static void main(String[] unused) {
// This works
Course.printName();
// This also works
Course CS125 = new Course();
CS125.printName();
}
}-
staticmethods are called directly on the class, rather than on an instance -
…but they can be called on the instance as well.
Review: static Methods and this
public class Course {
public String name;
public static void printName() {
// This doesn't work
System.out.println(name);
}
}static methods can be called without an instance, and so can’t use this
Review: static Variables
public class Course {
public static int count = 0;
public void printCount() {
System.out.println(count);
}
}
public class Example {
public static void main(String[] unused) {
Course CS125 = new Course();
Course CS225 = new Course();
Course.count++;
CS125.printCount();
CS225.printCount();
}
}static variables are shared by all instances of a given class
Review: static and public
public and private also work on static variables and methods
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public: the (static) variable can be read or written by anyone -
private: the (static) variable can only read or written by methods defined on that class -
public: the (static) method can be called by anyone -
private: the (static) method can only be called by other methods on that class
static Object Creation Method
We can also use a class method to be able to return null when creating a new
object if invalid parameters are supplied.
> Click or hit Control-Enter to run Example.main above
final and Constants
class Example {
/** The number of hours of sleep you should get per night. */
public static final int HOURS_PER_NIGHT = 8;
...
}In Java a final variable cannot be modified.
You usually see this done to establish useful constant values—which can be
either public or private
> Click or hit Control-Enter to run Example.main above
Questions About static or final?
> Click or hit Control-Enter to run Example.main above
Inheritance
public class Pet {
protected String name;
protected String type;
public void printMe() {
System.out.println("I'm a " + type + " named " + name);
}
}
public class Dog extends Pet {
Dog(String setName) {
name = setName;
type = "Dog";
}
}
Dog chuchu = new Dog("Chuchu");
chuchu.printMe();Java allows objects to inherit state and behavior from another class.
Inheritance Terminology
public class Pet { }
public class Dog extends Pet { }
public class Cat extends Pet { }In Java we establish inheritance using the extends keyword.
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DogextendsPetand so inherits state and behavior fromPet -
Catalso extendsPetand so also inherits state and behavior fromPet -
We sometimes call
PetDog's andCat's parent class -
We sometimes call
DogandCatPet's children
More Inheritance Terminology
public class Pet { }
public class Dog extends Pet { }
public class Mutt extends Dog { }In Java we can have have multiple levels of inheritance.
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DogextendsPetand so inherits state and behavior fromPet -
MuttextendsDogand so inherits state and behavior fromDogandPet -
We sometimes call
PetandDogMutt's ancestors -
We sometimes call
DogandMuttPet's descendants
protected
public class Pet {
public String name; // Anyone can set me
private String secret; // Only I can set this value
protected String type; // My descendants can use this value
}
public class Dog extends Pet {
Dog(String setName) {
name = setName;
type = "Dog";
}
}-
public: the variable can be read or written by anyone -
private: the variable can only read or written by methods defined on that class -
protected: the variable can only read or written by methods defined on that class or its descendants
> Click or hit Control-Enter to run Example.main above
The Dirty Truth About protected
public class Pet {
protected String name;
}
public class Dog extends Pet {
Dog(String setName) {
name = setName;
}
}
public class Example {
public static void main(String[] unused) {
Dog chuchu = new Dog("Chuchu");
chuchu.name = "Xyz"; // This works...
}
}-
protected: the variable can read or written by methods defined on that class or its descendants… in any package -
protected: the variable can also be read and written by any method in the same package
public, private, and protected
Variables:
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public: the variable can be read or written by anyone -
private: the variable can only read or written by methods defined on that class -
protected: the variable can be read or written by methods defined on any descendant of that class in any package or any class in the same package
Methods:
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public: the method can be called by anyone -
private: the method can only be called by other methods on that class -
protected: the method can be called by other methods defined on any descendant of that class in any package or any class in the same package
Announcements
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No Coders on this week’s quiz. Chapter 4 for next week.
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The autograder is working again. Please use it to check your MP1 scores to ensure you meet the early deadline.