CS 106 Introduction to Computer Science I

Transcription

1 CS 106 Introduction to Computer Science I 04 / 06 / 2015 Instructor: Michael Eckmann

2 Today s Topics Comments and/or Questions? Let's finish Pet/Dog/Cat/TalkingDog classes Overriding methods Abstract New Topic Decimal numbers vs. Binary numbers Bit shifting Michael Eckmann - Skidmore College - CS Spring 2015

3 Inheritance Super class vs. Subclass (aka Parent class vs. Child class) Use extends keyword to describe the inheritance e.g. public class Cat extends Mammal Cat is subclass, Mammal is the superclass Classes that do not have the extends keyword implicitly extend Object (a class in the Java API) A subclass inherits all the instance variables (even private ones) from the super class. A subclass's constructor should first call the superclass's constructor (by calling super(... )) Place any parameters that the super class's constructor has inside the parens

4 Pet, Dog & Cat classes Let's create a small class hierarchy with Pet as the superclass and Dog and Cat as subclasses. All Pets have name breed Dogs additionally have a skill Let's say Cats have no additional data Let's say all Pets sleep the same... Zzzzz. Let's also have Dogs and Cats be able to speak (Dogs go woof, Cats go meow. Let's create some objects of type Cat and Dog and have them speak.

5 Pet, Dog & Cat classes Let's create an array of Pet objects. Can we call sleep() on each Pet? yes Can we call getname() on each Pet? yes Can we call speak() on each Pet? no, not as written because speak is not in the Pet class (it is only in the Cat and Dog classes) Could cast the Pet to the correct subclass type and then call speak, but there's a better way

6 Overriding methods A subclass can override a superclass's method by providing a definition for a method that exists in the superclass with the same name and number and type of parameters.

7 More on overriding methods Using the classes Pet, Dog, and Cat - We provide a speak() method in Pet but with no body. Then, any class that inherits from Pet, must implement this method (if the class intends to be instantiable (not abstract)). Further, let's assume we have a class TalkingDog which doesn't bark when he speaks, instead he speaks English. To implement this stuff...

8 Abstract classes Abstract classes Can never be instantiated Can contain both abstract methods and actual (non-abstract) methods Can contain instance variables as well as constants If a class contains any abstract methods then it MUST BE an abstract class But an abstract class is not required to have abstract methods

9 More on overriding methods So we have classes Pet (abstract), Dog, TalkingDog and Cat. We provided a speak() method in Pet with no body (and we make it abstract) which forced the Pet class to be abstract. Then, any class that inherits from Pet, must implement this method. We provided an actual (non-abstract) method sleeps() in Pet. And the class TalkingDog which doesn't bark when he speaks, instead he speaks English.

10 More on overriding methods Dog and Cat inherit from Pet. TalkingDog inherits from Dog. Note: because Pet contains an abstract method, the class is not instantiable --- the class itself must be declared as abstract.

11 abstract methods The reason we create an abstract method (with an empty method body) in the superclass is so that this guarantees that all subclasses must provide this method if they are to be instantiable. Make the method abstract if the superclass doesn't need to be instantiable and if there is no obvious default behavior for the method in the superclass --- this passes the requirement of the behavior of the abstract method to any subclass that wants to be instantiable. Notice: a subclass of Pet had to include code for the speak() method we couldn't instantiate an object of type Pet because it is abstract

12 Decimal numbers Decimal numbers are numbers in base 10. Each position in the number can be one of 10 digits (0-9). Each place represents a 10 i 's place in the number. eg. the base 10 number 965 has a 5 in the 10 0 place, a 6 in the 10 1 place and a 9 in the 10 2 place. The value of this number is 5* * *10 2 = 965.

13 Binary numbers Recall that the primitive data types (int in particular) are stored in binary. Let's review binary numbers here. Binary numbers are numbers in base 2. Each position in the number can be one of 2 digits (0-1). Each place represents a 2 i 's place in the number. eg. the binary number 100 has a 0 in the 2 0 place, a 0 in the 2 1 place and a 1 in the 2 2 place. The value of this number is 0* * *2 2 = 4 in base 10. Let's look at more examples on the board and compare to decimal numbers.

14 Binary numbers What is the value in base 10 of the binary number: 10101? What is the value in base 10 of the binary number: 11101? What is the value in base 10 of the binary number: 10111? What is the value in base 10 of the binary number: ?

15 Binary numbers What is the value in base 10 of the binary number: 10101? = 21 What is the value in base 10 of the binary number: 11101? = 29 What is the value in base 10 of the binary number: 10111? = 23 What is the value in base 10 of the binary number: ? =

16 shifting When you multiply a decimal number by 10 the result is the number shifted left by 1 position (with a 0 after it). When you multiply a decimal number by 100 the result is the number shifted left by 2 positions (with 00 after it). When you divide a decimal number by 10 the result is the number shifted to the right by 1 position. When you divide a decimal number by 100 the result is the number shifted to the right by 2 positions. And so on...

17 Bit shifting When you multiply a binary number by 2 the result is the number shifted left by 1 position (with a 0 after it). When you multiply a binary number by 4 the result is the number shifted left by 2 positions (with 00 after it). When you divide a binary number by 2 the result is the number shifted to the right by 1 position. When you divide a binary number by 4 the result is the number shifted to the right by 2 positions. And so on...

18 Bit shifting >> and << are bit shift operators. They each work on two operands. The operand to the left is the number that will be shifted. The operand to the right is the number of positions to shift.

19 Bit shifting Another programming assignment will be assigned which you will be able to start tomorrow during lab. It will require you to use the ppm images and bit shifting and more to be able to compare images based on their color content. Example idea of histogram bins on the board.