CS 106 Introduction to Computer Science I

Transcription

1 CS 106 Introduction to Computer Science I 03 / 23 / 2015 Instructor: Michael Eckmann

2 Today s Topics Comments and/or Questions? Objects and Object Oriented Programming Overloaded methods / constructors Set/get methods static this DecimalFormat class ternary conditional operator Michael Eckmann - Skidmore College - CS Spring 2015

3 Object orientation Class Object / object reference Instance variables (aka data members) Instance Methods Constructor Instantiation

4 class / object / constructor terminology Programmer defined classes can be used as types, just like classes in the Java API can be used as types (e.g. String.) Objects have attributes (data) and behaviors (methods.) The data are the instance variables in a class. An object of a class is instantiated by declaring a variable of that type and initializing the object by calling a constructor of that class with the new operator. e.g. TimeOfDay timeobject = new TimeOfDay( ); here, we are creating an object (timeobject), of a programmer defined class (TimeOfDay).

5 Let's create a class TimeOfDay 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

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 may 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 TimeOfDay, and therefore is accessible through timeobject by use of the dot operator: timeobject.settime ( 12, 15, 0 ); However, all the instance variables of TimeOfDay are declared as private. So, these are not accessible through timeobject. These private instance variables named, hours, minutes and seconds, are only accessible within methods of TimeOfDay.

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: TimeOfDay starttime = new TimeOfDay(); starttime.hours = 12; starttime.minutes = 100; starttime.seconds = 10; That's good because we shouldn't be able to set the minutes 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(9, 5, 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 We can update the class definition of TimeOfDay to contain set and get methods. public void sethours( int h ) { } if (( h >= 0 && h < 24 )) hours = h; else hours = 0; public int gethours( ) { return hours; }

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

14 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.

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

16 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

17 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 Let's look at example objects of type CompactDisc

18 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 members (variables or methods.)

19 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 with an object name before the dot.)

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

21 this This this contains the value of the current object is a way to explicitly say you are referring to an instance variable inside the same class example usage: this.minutes (would refer to minutes instance variable) 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

22 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)

23 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));

24 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.

25 Ternary operator This acts like if/else but is an operator that works on operands and evaluates to some value/result The ternary conditional operator? : works on three operands separated by a? and a : Example: int hour = 14, nonmilhr; nonmilhr = ((hour == 12 hour == 0)? 12 : hour % 12 );