Introduction to Programming II Constructors Chris Brooks Department of Computer Science University of San Francisco Department of Computer Science University of San Francisco p 1 4 2 Anatomy of an Object Recall that an object consists of Instance data Methods Methods consist of Parameters Local data Method body Return statement Department of Computer Science University of San Francisco p 2 4 3 Constructors A constructor is a method that is called when an object is first created Its responsibility is to initialize an object s instance variables It must have the same name as the class in constructs It has no return type It is called when new is invoked Department of Computer Science University of San Francisco p 3 4 4 Example public class Point public int xval public int yval Point int x int y xval x yval y Point p new Point 3 4 Department of Computer Science University of San Francisco p 4 4 5 Multiple Constructors It s often helpful to be able to specify some instance variables but not all For example let s say our circle class has a default radius of 1 If the radius is something else users can specify it Point p1 new Point 3 3 Circle c new Circle p creates a circle with radius 1 at 3 3 Point p1 new Point 3 3 Circle c new Circle p 5 creates a circle with radius 5 at 3 3 Department of Computer Science University of San Francisco p 5 4 6 Multiple Constructors public class Circle private Point center private int radius public Circle Point c center c radius 1 public Circle Point c int r center c radius r Department of Computer Science University of San Francisco p 6 4 7 Exercise Modify the book class from last week s lecture Add three constructors Default takes no arguments 1 argument title 2 arguments author and title author should be an instance of name Add a constructor to name that takes two arguments first name and last name Department of Computer Science University of San Francisco p 7 4 8 Designing a Program Knowing the syntax of how to build a class is only the beginning The bigger challenge is figuring out how to fit the pieces together The software development process consists of the following steps Establishing requirements Creating a design Implementing the design Testing Usually this is an iterative repeated process Department of Computer Science University of San Francisco p 8 4 9 Requirements Requirements indicate what a program is supposed to do Expressed as a functional specification What is the input like What must the output look like Are there other programs it must interact with How quickly must the program run Often this comes from a client Usually not as precise as you d like Department of Computer Science University of San Francisco p 9 4 10 Design This is the how part of the program Specifies the classes that are needed and how they interact What methods are called what data is returned Skipping this step can lead to serious unpleasant bugs A good design should make implementation straightforward Department of Computer Science University of San Francisco p 10 4 11 Structure Charts A structure chart is a helpful tool for doing design Helps to divide and conquer Divide the problem into smaller pieces until you reach pieces that can be tackled directly This is called top down design Department of Computer Science University of San Francisco p 11 4 12 Example student database Let s say we ve been hired by USF to build an application for tracking students It should be able to do the following Add new students to the database Delete a student from the database Enter a student s test scores Print out the average of a student s scores These are our requirements Department of Computer Science University of San Francisco p 12 4 13 Top down structure chart main is at the top of the chart Nodes below this represent portsions of the program that are called Arrows indicate input and output of data Nodes at the next level are then decomposed in the same way Eventually we get to methods we know how to implement Department of Computer Science University of San Francisco p 13 4 14 Top down structure chart The structure chart indicates the calling sequence of the program Serves as a template for the actual program Annotate parameters as in out or in out Objects called with methods on them are parameters too Department of Computer Science University of San Francisco p 14 4 15 Choosing Objects Each object should have one well defined responsibility A common mistake is to cram too much into a single object or method Methods are typically 1 2 screens of code Before making a new class does what you need already exist Department of Computer Science University of San Francisco p 15 4 16 Bottom up design To implement the program in our structure chart start at the leaves and work upward Bottom level methods can be implemented directly Methods at the next level up are implemented in terms of those methods Eventually you make it back up to main Department of Computer Science University of San Francisco p 16 4 17 Unit testing As you code each method you should test it in isolation Write a small program that calls this method with all expected inputs Testing each method seems tedious but it s very important A simple bug missed at a lower level can be very difficult to find later on Good programming practice can help limit the amount of time you spend debugging Department of Computer Science University of San Francisco p 17
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