CS 106 Introduction to Computer Science I

Transcription

1 CS 106 Introduction to Computer Science I 03 / 28 / 2012 Instructor: Michael Eckmann

2 Today s Topics Questions? Comments? Object oriented programming More of creating our own classes Valid ways to call methods DecimalFormat class Ternary conditional operator Michael Eckmann - Skidmore College - CS Spring 2012

3 Let's create a class Time What instance variables might we have to represent a time? What types will they be? What are the valid values of each of these variables? We can make sure that these variables ALWAYS have valid data in them make the instance variables private and in any method that can change the value of an instance variable make sure to not allow it to have an invalid value

4 class Time Time timeobject = new Time( ); The above line would appear in a method of some class (not Time) that wishes to use a Time object. When this happens, memory for the object is allocated (by the new operator) and the instance variables of the class are initialized (by Time( ) ). The object has access to any public methods and data defined in the class. i.e. timeobject is the object variable that has access to the public data and methods of class Time1.

5 Referring to members of a class via an object The. operator is used along with object to access particular members (data or methods) of a class. E.g. Time starttime = new Time(12,55,0); starttime.settime(13,15,0); // dot operator used We may only access public members.

6 public / private members Instance variables and methods of a class are called members of the class. members can be specified as public or private (or protected --- which we ll discuss later.) private members of a class are accessible only to methods within the class. public members of a class are accessible within that class as well as in other classes that contain a reference to an object of that class (via the dot operator).

7 public / private members So, in our example, settime is a public method of class Time, and therefore is accessible through starttime by use of the dot operator: starttime.settime ( 12, 15, 0 ); However, all the instance variables of Time are declared as private. So, these are not accessible through starttime. These private instance variables named, hour, minute and second, are only accessible within methods of Time.

8 public / private members These private instance variables named, hour, minute and second, are only accessible within methods of Time. Because they are private, we cannot write code like: Time starttime = new Time(); starttime.hour = 12; starttime.minute = 100; starttime.second = 10; That's good because we shouldn't be able to set the minute to be 100 (it's valid range is 0-59). But then how would we change the hour or minute or second?

9 public / private members That's good because we shouldn't be able to set the minute to be 100 (it's valid range is 0-59). But then how would we change the hour, minute and second? This way: starttime.settime(12, 100, 10); The above code will be allowed to be called but when settime sees that minute is trying to be set to 100 which is not within the valid range it will set starttime's minute to be 0. An example call with a valid time (the time CS106 starts): starttime.settime(13, 25, 0);

10 overloaded constructors When we create a class, we usually provide constructors that we want to have called when objects of our class are instantiated. We can provide many different constructors, each taking different parameters. When we do this, we are said to be overloading the constructor because we are providing different methods of the same name (but different parameters.) Java calls the correct constructor, based on the number of and type of the arguments passed in to the method call.

11 set and get methods When instance variables of a class are declared as private, we sometimes want other classes to have access to their values and be able to change them in a controlled way. To this end, we usually provide what are called set and get methods for this purpose. A set method usually takes in a value as a parameter and checks it for validity. If it is a valid value, then the appropriate instance variable is set (has its value changed to the one passed in.) We usually create a get method for each instance variable of which some other class might want to know the value. get methods are sometimes called accessor methods and set methods are sometimes called mutator methods

12 set and get methods so, we would then be able to call these methods in a program like: Time t1 = new Time(); Time t2 = new Time(); t1.sethour( 11 ); t2.sethour( 45 ); // what will happen here? int hourvalue; hourvalue = t1.gethour( ); hourvalue = t2.gethour( ); If hour were a public (instead of private) instance variable of class Time2 then we would be able to: t2.hour = 45; // bad idea...

13 static variables A static class variable means there is only one copy of the variable, regardless of the number of objects instantiated. one use of static class variables is to keep a count of how many objects of the class have been instantiated. This can be done by incrementing a static variable in the constructor.

14 Static variables Static variables represent classwide information. Instance variables represent object specific information. What does that mean?

15 Static variables Static variables represent classwide information. Instance variables represent object specific information. What does that mean? Example: class BankCheck to represent a check in a checkbook. amount date lastchecknumberused Let's look at example objects of type BankCheck

16 Static variables Static variables represent classwide information. Instance variables represent object specific information. What does that mean? Example: a class to represent a CompactDisc Artist Title TotalTime (max 80 minutes) Let's look at example objects of type CompactDisc

17 Static variables Because static variables represent classwide information, we don't need any objects to be instantiated if we want to use them. Instead, we use the class name and dot operator to access any public static variables or public static methods.

18 static variables static variables are accessible in other classes if the static variables are declared public. They are accessible by using the class name followed by the dot operator and then the static variable name. e.g. if there was a public static variable named timeobjectcount in the Time class, we could access it in another class by referring to: Time.timeObjectCount If it was declared as private, then we would need to have a public static method in Time to provide read access to it. e.g. a method like: public static int getcount( ) { return timeobjectcount; }

19 static methods we could call this method in the following way: int currentcount; currentcount = Time.getCount( ); Notice that we used the class name Time (not an object name) before the dot operator to reference the static members of a class. We have seen this before, such as when we called a static method of class Integer: Integer.parseInt( somestr ) Note however that if we have instantiated objects of class Time, we would be able to access the static members either by the class name or the object name.

20 static methods so, assuming t1 is an object of type Time, after it is instantiated, we would be able to call a static method getcount( ) either by: Time.getCount( ) or t1.getcount( ) It is best (for readability purposes) however to use the class name way. That is, Time.getCount( ) is preferred because someone reading your code will know that getcount ( ) is a static method of class Time, which may be valuable in understanding the code.

21 static methods The methods in class Math that we ve used are static methods. Recall that we don t create an object of type Math. Instead, we call the methods in Math with the class name (Math) followed by the dot operator, followed by the method name. This is how static methods are called (with the class name, not an object name.)

22 Concepts recap Public vs. private instance variables Public vs. private methods Static vs. instance variables Static vs. non-static methods

23 this this it contains the value of the current object I'll show an example of this in the Time class. example usage: this.minute (would refer to minute instance variable within a method of the Time class) Why use this? if you name a parameter of a method the same name as an instance variable, you can refer to the instance variable inside the method by using this. in front of it and refer to the parameter by its name alone

24 Let's create another class Let's create a brand new class using all these concepts. Let's create a class to represent a Movie Let's assume we want to store for each movie: Title Director Running time Year of release Let's also assume we want to store a count of all the movies we have instantiated This Movie class will be intended to be used in a program that would hold the inventory of a video/dvd rental store.

25 ways to call methods a method that is defined in the same class as another method can call it by just using the method name and arguments in parentheses (regardless of whether the method is public or private). we saw many examples of this: one was in the sorting program we wrote --- the swap method was defined in our class and was called like so swap(arr, i, i+1);

26 ways to call methods a public static method that is defined in some other class can be called by using the class name followed by the dot operator and then the method name and arguments in parentheses. Note: we don t have to first define a reference to an object of that class. we ve seen examples of this with the Integer.parseInt and JOptionPane.showMessageDialog methods (among many others) We do NOT need to declare a variable to hold a reference to an object of these classes if all we want to do is call a static method in it. We call one of these like so: someint = Integer.parseInt(someStr);

27 ways to call methods A non-static public method that is defined in some other class must be called by using a reference to the object of the class followed by the dot operator and then the method name and arguments in parentheses. we ve seen examples of this with the Time class First we declare a variable to hold a reference to a Time object, then we can call one of Time s methods. E.g. the call to tostring() in the second line below Time starttime = new Time(); String s = starttime.tostring(); System.out.print(" start time = " + s);

28 ways to call methods A private method that is defined in some other class cannot be called from outside that class.

29 Recap - ways to call methods a method (regardless of whether the method is public or private) that is defined in the same class as another method can call it by just using the method name and arguments in parentheses. swap(arr, i, i+1); a non-static public method that is defined in some other class (from where it is being called) must be called by using a reference to the object of the class followed by the dot operator and then the method name and arguments in parentheses. Time starttime = new Time(); String s = starttime.tostring(); System.out.print(" start time = " + s); a public static method that is defined in some other class can be called by using the class name followed by the dot operator and then the method name and arguments in parentheses. someint = Integer.parseInt(someStr);

30 DecimalFormat class Let's look online at the Java API for this class (and others) Allows us to make output nicer by rounding floats and doubles etc. for output Specify a pattern when creating an object of DecimalFormat class 0 vs. # 0 is a required digit position To the left of decimal --- means at least that many To the right of decimal --- means exactly that many places # to the right of decimal means that a 0 will not display. % at end of pattern causes number to be displayed as a percent E-notation is also possible (scientific notation)

31 DecimalFormat class DecimalFormat df = new DecimalFormat( ); To use it : (assuming we have a double var temperature) df.format(temperature) /* this will return a String based on the value of temperature and the pattern specified when the df object was constructed */ Since it returns a String we can place it anywhere a String can be used, like: System.out.println("The temperature is: " + df.format(temperature));

32 DecimalFormat class Let's try a few different formats and see what is produced. Note: no truncation of numbers to the LEFT of the decimal point will ever occur with this formatting even if we have: the format is 00.0# assume the value to format is The number will be formatted as because the amount of 0's to the LEFT of the decimal in the format means at least that many places.

33 Ternary operator (acts like if /else ) Ternary conditional operator contains three parts separated by a? and a : Example: int hour = 14, nonmilhr; nonmilhr = ((hour == 12 hour == 0)? 12 : hour % 12 );