Object Oriented Programming. What is this Object? Using the Object s Slots

Transcription

1 1 Object Oriented Programming Chapter 2 introduces Object Oriented Programming. OOP is a relatively new approach to programming which supports the creation of new data types and operations to manipulate those types. What is this Object? There is no real answer to the question, but we ll call it a thinking cap. The plan is to describe a thinking cap by telling you what actions can be done to it. 2 Using the Object s Slots You may put a piece of paper in each of the two slots (green and red), with a sentence written on each. You may push the green button and the thinking cap will speak the sentence from the green slot s paper. And same for the red button. 3

2 4 Example Example That test was a breeze! 5 Example I should study harder! 6

3 7 We can implement the thinking cap using a data type called a class. public class Thinking... The class will have two components called greenwords and redwords. These components are strings which hold the information that is placed in the two slots. Using a class permits two features... public class Thinking String greenwords; String redwords;... 8 The two components will be private instance variables. This ensures that nobody can directly access this information. The only access is through methods that we provide for the class. public class Thinking private char greenwords; private char redwords;... 9

4 10 In a class, the methods which manipulate the objects of the class are also listed. class Thinking private char greenwords; private char redwords;... s of the thinking cap methods go here. Our thinking cap has at least three methods: public class Thinking private char greenwords; private char redwords; public void slots(string newgreen, String newred)... public void pushgreen( )... public void pushred( ) The code for a new class is generally put in a Java package, as shown here: package myclassses.225; public class Thinking private char greenwords; private char redwords; This means that Thinking.java and Thinking.class files will be a subdirectory myclasses/225 public void slots(string newgreen, String newred)... public void pushgreen( )... public void pushred( )... 12

5 13 A program that wants to use the thinking cap can import the Thinking class. import myclasses.225.thinking;... Just for fun, the example program will declare two Thinking variables named student and fan. import myclasses.225.thinking; public class Example public static void main( ) Thinking student; Thinking fan; 14 The variables are examples of reference variables, which means that they have the capability of referring to Thinking objects that we create with the new operator. import myclasses.225.thinking; public class Example public static void main( ) Thinking student; Thinking fan; student = new Thinking( ); fan = new Thinking( ); 15

6 16 Once the Thinkings are created, we can activate methods such as slots for the student Thinking. import myclasses.225.thinking; public class Example public static void main( ) Thinking student; Thinking fan; student = new Thinking( ); fan = new Thinking( ); The method activation consists of four parts, starting with the variable name. Name Name of of the the variable variable 17 The variable name is followed by a period. A Period Period 18

7 19 After the period is the name of the method that you are activating. Name Name of of the the method method Finally, the arguments for the method. In this example the first argument (newgreen) is "Hello" and the second argument (newred) is "Bye". student.slots( "Hello", "Bye" ); Arguments Arguments 20 A Quiz How would you activate student's pushgreen method? What would be the output of student's pushgreen method at this point in the program? public static void main(string[ ] args) Thinking student; Thinking fan; student = new Thinking( ); fan = new Thinking( ); 21

8 22 A Quiz Notice that the pushgreen method has no arguments. At this point, activating student.pushgreen will print the string Hello. public static void main(string[ ] args) Thinking student; Thinking fan; student = new Thinking( ); fan = new Thinking( ); student.pushgreen( ); A Quiz public static void main(string[ ] args) Thinking student; Thinking fan; student = new Thinking( ); fan = new Thinking( ); fan.slots( "Go Cougars!", "Boo!"); student.pushgreen( ); fan.pushgreen( ); student.pushred( );... Trace through this program, and tell me the complete output. 23 We will look at the body of slots, which must copy its two arguments to the two private instance variables. public class Thinking private String greenwords; private String redwords; public void slots(string newgreen, String newred)... public void pushgreen( )... public void pushred( )... 24

9 25 The method s s implementation occurs after the parameter list public void slots(string newgreen, String newred) greenwords = newgreen; redwords = newred; Within the body of the method, the class s instance variables and other methods may all be accessed. public void slots(string newgreen, String newred) greenwords = newgreen; redwords = newred; 26 Within the body of the method, the class s instance variables and other methods may all be accessed. But, whose instance variables are these? Are they public void slots(string newgreen, student.greenwords String newred) student.redwords greenwords = newgreen; fan.greenwords redwords = newred; fan.redwords? 27

10 28 Within the body of the method, the class s instance variables and other methods may all be accessed. If we activate student.slots: public void slots(string newgreen, String newred) student.greenwords greenwords = newgreen; student.redwords redwords = newred; Here is the implementation of the pushgreen method, which prints the green Words: public void pushgreen( ) System.out.println(greenWords); Notice how this method implementation uses the greenwords instance variable of the object. 29 A Common Pattern Often, one or more methods will place data in the instance variables... slots public class Thinking private String greenwords; private String redwords;... pushgreen & pushred...so that other methods may use that data. 30

11 31 Constructors Look like methods without a return value Name is the same as the name of the class Must not be preceded by a return type (not even void) Constructors A class may have any number of constructors, each with a different list of parameters. Constructors initialize values of instance variables If no constructors are provided, the system provides one default no arguments constructor 32 Method Types Accessor Methods Provide information about the instance variables Modification Methods Modify the instance variables 33

12 34 Type Casting Data may be represented as a different data type by typecasting, e.g.: int x = 5; int y = 7; double z; z = (double) x / (double) y; Assignment Statements Referring to the example, what is the effect of the following statements? Thinking student; Thinking fan; student = new Thinking( ); fan = student; fan.slots( "Go Cougars!", "Boo!"); student.pushgreen( ); fan.pushgreen( ); student.pushred( ); 35 Testing for Equality The test (student == fan) returns true only if both variables refer to the same object. What if we need a test that returns true if both variables refer to two objects with the same values for all slots? Solution: use an equals method 36

13 37 equals Methods An equals method is a programmer provided method to test for equality other than by testing if the 2 variables refer to the same object Typical format: public boolean equals (Object obj) if (obj instanceof ) //test goes here else return false; A default equals method is provided by the system and works like == clone Methods clone methods make a new object that is a copy of an existing object Form: public Object clone() Class header must indicate intent to use a clone method: public class implements Cloneable 38 clone Methods Typical format: public Object clone() answer; try answer = ( ) super.clone(); catch (CloneNotSupportedException e) throw new RuntimeException ( This class does not implement Cloneable ); //do any necessary modifications to answer Return answer; 39

14 40 Java Parameters Parameters from any of the eight primitive data types are treated as local variables, any changes to them within a method does not affect the argument Parameters that are object reference act as references to the original object and thus changes are reflected in the argument s object