Programming by Delegation

Transcription

1 Chapter 2 a Programming by Delegation I. Scott MacKenzie a These slides are mostly based on the course text: Java by abstraction: A client-view approach (4 th edition), H. Roumani (2015). 1 Topics What is delegation Application development Software engineering 2 1

2 What Delegation Is Not import java.lang.system; public class Area { public static void main(string[] args) { int width; width = 8; int height = 3; int area = width * height; System.out.println(area); } } The code inside the rectangle computes the area of a rectangle. It handles both storage (of data) and computation (of area). This is easy here because there are just a few variables and the computation is trivial. This is not an example of delegation. 3 Delegating to a Static Method (using the core Java API) Keep storage but delegate computation to a class double radius = 10.5; double pi = ; double area = pi * Math.pow(radius, 2.0); pow is a static method of the Math class. View API for details. 4 2

3 Delegating to a Static Method (using JBA s type package) Keep storage but delegate computation to a class double weight = 170.0; String height = "5'9"; double bmi = ToolBox.getBMI(weight, height); getbmi is a static method of the ToolBox class. View API for details. 5 Notes About (Static) Methods A method belongs to a class. It performs an action, and hence, its name is usually indicative of the action (e.g., pow, sqrt, etc.) Method name is followed by a pair of parentheses containing any parameters needed The method name together with the types of its parameters make up the method signature. It is unique per class. A method's action culminates in a return. It can be void. Invocation syntax: ClassName.method( ) (see previous slide) 6 3

4 Delegating to an Object (using JBA's type package) Delegate both storage and computation Rectangle r = new Rectangle(3, 4); Rectangle s = new Rectangle(2, 5); System.out.println(r.getArea()); System.out.println(s.getCircumference()); getarea getcircumference Notes About Delegating to an Object Create an instance (aka, an object) of a class that can handle storage and computation and work with the instance The instance has a name, r, known as the object reference Methods are invoked on the instance, not on the class (e.g., getarea in the previous slide) Think of the object (or instance) as a copy of the original class Each instance can store different values in its attributes; these values are known as the state of the object A class has attributes and methods. An object has, in addition, state, and identity 8 4

5 Topics What is delegation Application development Software engineering 9 Application Architecture (1) A Java application consists of several cooperating classes One of the classes starts the application, and is known as the main class (execution begins in a method called "main") The other classes are known as helpers or components 10 5

6 Application Architecture (2) The main class for a desktop application (as opposed to an applet or servlet) is known as an app. It must have a method with the following header: public static void main(string[] args) The main class delegates to components; as more ready-made components become available, application development reduces to developing the main class. 11 The Client View (1) The client is the developer of the main class The implementer is the developer of a component The client understands the big picture, the purpose of the application. The implementer focuses only on the inner details of one component. The client knows how to shop for components and how to read their specs; i.e., it knows what each one does but not how it does it focuses on being a client. It prepares you to write applications using components that are already available. Separation of concerns means the client and the implementer share info on a need-to-know basis. 12 6

7 The Client View (2) Given a component, the client does not care what is inside it, only what it does. This is the component's interface or API (application programming interface). The class of a component thus encapsulates it. An attempt to look inside is breaking the encapsulation. CLIENT Interface Interface IMPLEMENTER Interface Interface 13 The Client View (3) A class is made up of features. A feature is an attribute or a method. The class of a component classifies each feature as either public or private depending, respectively, on whether the client needs or does not need to know about it. The API (interface) of a component lists only the headers of its public methods and the declarations of its public attributes (a.k.a. fields). 14 7

8 Runtime Errors Pre-packaged components are syntactically correct (hopefully!). This means they are free of compile-time errors (aka syntax errors) As well as compile-time errors, there is the possibility of a runtime error Consider the following syntactically-correct statement: int y = 5 / x; If the user entered 0 for x, the VM will protest by throwing an exception. The following message is output: java.lang.arithmeticexception: / by zero This is an example of a runtime error 15 Logic Errors The most difficult to correct The program is Free of compile-time errors Free of runtime errors (i.e., runs without crashing or throwing an exception) But... the output is wrong! E.g., int area = width + height; The programmer used + instead of *. There is no compile error and no runtime error, but the output is wrong 16 8

9 Runtime Errors vs. Compile-time errors runtime and logic errors EDIT create or edit save the file COMPILE read source file Java to bytecode RUN launch main class interact with user compile-time errors 17 Directory structure: Case Study: The JDK bin jdkx.y.z_n lib jre bin lib ext 18 9

10 JDK Top-level Packages (1 of 2) 19 JDK Top-level Packages (2 of 2) 20 10