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Critical thinking, Object-Arranged Configuration and Java

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  1. Problem Solving, Object-Oriented Design and Java B.Ramamurthy B.Ramamurthy

  2. Topics for Discussion • OO Design Principles • Java Virtual Machine • Java Application Structure • Class and objects • Methods and Variables • Access/Visibility Modifiers • Summary B.Ramamurthy

  3. Object-Oriented Principles OOP Polymorphism -- Many forms of same function -- Abstract Methods -- Abstract Classes Inheritance -- Hierarchy -- Reusability -- Extensibility -- Expressive power -- Reflects many real-world problems Encapsulation (class) -- Information Hiding -- Interface and Implementations -- Standardization -- Access Control mechanisms (private /public etc.) B.Ramamurthy

  4. Mac Hardware Mac Compiler Mac Compiler C++source PC Hardware C++source PC Compiler Sun Hardware C++source Sun Compiler Conventional Compiled Languages B.Ramamurthy

  5. Java Virtual Machine JVM Java compiler :javac Mac Hardware Byte code Mac Compiler Java source Mac interpreter JVM PC Hardware Byte code Java source PC Interpreter JVM Sun Hardware Byte code Java source Sun Interpreter B.Ramamurthy

  6. On any machine when you compile Java source code using javac byte code equivalent to the source is generated. Byte code is machine-independent. This enables the “run-anywhere” capability. You invoke java command which will feed the byte code to the machine-dependent interpreter. “Run-anywhere” Capability B.Ramamurthy

  7. Java Application Program Interface (Java API) (JAVA API) Package of related classes : java.awt Random java.util Date Dictionary java.io, java.beans,.. Etc.. package class B.Ramamurthy

  8. Java API : A Simplistic View API packages classes methods and data declarations B.Ramamurthy

  9. Java API Classes • Unlike many other languages, you will referring to the classes in the API. • Where is the API? • Docuemntation for the packages are in: http://java.sun.com/j2se/1.4/docs/api/ • Open in it in your browser and traverse through the javadoc tree B.Ramamurthy

  10. Java is fully object-oriented. Every “function” has to be attached to a class. You will mainly deal with three types of programs: class: methods and data (variables + constants) describing a collection (type) of object application: class that has a main method: represents a our regular program applet: class that is meant for execution using a appletviewer/browser Types of Programs B.Ramamurthy

  11. Problem Solving Using Java OO Design and Progamming in Java Write an applet class Identify classes needed Write an application class Reuse API classes Reuse your classes Design new classes Create and use objects B.Ramamurthy

  12. What is an Object? • Object-oriented programming supports the view that programs are composed of objects that interact with one another. • How would you describe an object? • Using its characteristics (has a ----?) and its behaviors (can do ----?) • Object must have unique identity (name) : Basketball, Blue ball • Consider a ball: • Color and diameter are characteristics (Data Declarations) • throw, bounce, roll are behaviors (Methods) B.Ramamurthy

  13. Classes are Blueprints • A class defines the general nature of a collection of objects of the same type. • The process creating an object from a class is called instantiation. • Every object is an instance of a particular class. • There can be many instances of objects from the same class possible with different values for data. B.Ramamurthy

  14. Example objects Object References redRose class Rose blueRose class B.Ramamurthy

  15. Instantiation : Examples • class FordCar ---- defines a class name FordCar • FordCar windstar; ---- defines a Object reference windStar • windstar = new FordCar(); ---- instantiates a windstar Object • class HousePlan1 { color…. • HousePlan1 blueHouse; • blueHouse = new HousePlan1(BLUE); • HousePlan1 greenHouse = new HousePlan1(GREEN); B.Ramamurthy

  16. Operator new and “dot” • new operator creates a object and returns a reference to that object. • After an object has been instantiated, you can use dot operator to access its methods and data declarations (if you have access permissions). • EX: redRose.bloom(); greenHouse.color B.Ramamurthy

  17. Elements of a Class class data declarations (variables, constants) methods header body header statements modifiers, type, name variables, constants parameters repetition others selection assignment B.Ramamurthy

  18. Class Structure class variables constants methods B.Ramamurthy

  19. Defining Classes • Syntax: • class class_name { • data-declarations • constructors • methods } • Constructors are special methods used for instantiating (or creating) objects from a class. • Data declarations are implemented using variable and constant declarations. B.Ramamurthy

  20. Constants: All characters in uppercase, words in the identifier separated by underscore: EX: MAX_NUM Variables, objects, methods: First word all lowercase, subsequent words start with uppercase. EX: nextInt, myPen, readInt() Instance variable begin with an _ Classes: Begin with an uppercase letter. EX: Tree, Car, System , Math Naming Convention B.Ramamurthy

  21. Compile-time Errors : Usually typos or syntax errors Run-time Errors : Occurs during execution. Example: divide by zero . Logic Errors: Software will compile and execute with no problem, but will not produce expected results. (Solution: testing, and debugging) Debugging and Testing B.Ramamurthy

  22. Class Components • Class name (starts with uppercase), constants, instance variables, constructors definitions and method definitions. • Constants: public final static double PI = 3.14; • Variables: private double _bonus; public string _name; B.Ramamurthy

  23. Operations • Behaviors, methods or messages to which an object will respond to. • Methods: • Constructors • Set/get methods (mutators/accesors) • Predicate methods (boolean/status indicators; Ex: isEmpty()) • Utility methods (for local use only: ex: internal sort after an insert) • Explicit methods or interface methods that define the interface an object offers to the world. B.Ramamurthy

  24. Method Invocation/Call • Syntax: method_name (values); object_name.method_name(values); classname.method_name(values); Examples: computeSum(); // call to method from within the class where it is located YourRose.paintIt(Red); Math.abs(X); B.Ramamurthy

  25. Defining Methods • A method is group of (related) statements that carry out a specified function. • A method is associated with a particular class and it specifies a behavior or functionality of the class. • A method definition specifies the code to be executed when the method is invoked/activated/called. B.Ramamurthy

  26. Method Definition : Syntax visibilityreturn_type method_name (parameter_list) { statements } B.Ramamurthy

  27. Return Type • can be void, type or class identifier • void indicates that the method called to perform an action in a self-standing way: Example: println • type or class specify the value returned using a return statement inside the method. B.Ramamurthy

  28. Return Statement • Syntax of return statement: return; // for void methods return expression; // for type or class return value // the expression type and return type should be same B.Ramamurthy

  29. Parameter List • Parameter list specified in method header provides a mechanism for sending information to a method. • It is powerful mechanism for specializing an object. • The parameter list that appears in the header of a method • specifies the type and name of each parameter and • is called formal parameter list. • The corresponding parameter list in the method invocation is called an actual parameter list. B.Ramamurthy

  30. Parameter list : Syntax • Formal parameter list: This is like molds or templates (parm_type parm_name, parm_type parm_name, ....) • Actual parameter list: This is like material that fit into the mold or template specified in the formal list: (expression, expression....) B.Ramamurthy

  31. Method Definition : review definition header body Visibility modifiers parameter list return type Name { statements } B.Ramamurthy

  32. Method Definition : Example • Write a method that computes and returns the perimeter of a rectangle class. • Analysis: • Send to the method: Length and Width • Compute inside the method: Perimeter • Return from the method: Perimeter B.Ramamurthy

  33. ...Example (contd.) public int perimeter (int length, int width) { int temp; // local temporary variable temp = 2 * (length + width); // compute perimeter return temp; // return computed value } B.Ramamurthy

  34. What happens when a method is called? • Control is transferred to the method called and execution continues inside the method. • Control is transferred back to the caller when a return statement is executed inside the method. B.Ramamurthy

  35. Method Invocation : semantics Operating System 1. OS to main method 2. Main method execution 3. Invoke area 4. Transfer control to area 5. Execute area method 6. Return control back to main method 7. Resume executing main 8. Exit to OS 8 1 2 Main method Rect.area(….) 3 7 4 8 area method 5 6 B.Ramamurthy

  36. Constructors • A Constructor is used to create or instantiate an object from the class. • Constructor is a special method: • It has the same name as the class. • It has no return type or return statement. • Typically a class has more than one constructor: a default constructor which has no parameters, and other constructors with parameters. (overloading) B.Ramamurthy

  37. Constructors (contd.) • You don’t have to define a constructor if you need only a default constructor. • When you want initializing constructors : 1. you must include a default constructor in this case. 2. You will use initializing constructors when you want the object to start with a specific initial state rather than as default state. 3. Example: Car myCar = new Car(RED); // initializing constructor for Car class with color as parameter B.Ramamurthy

  38. Visibility Modifiers type Method/variable name public protected “nothing” DEFAULT Package Visibility private static “nothing” DEFAULT for class methods and variables for object methods and variables B.Ramamurthy

  39. private : available only within class “nothing” specified : DEFAULT: within class and within package protected : within inherited hierarchy (only to sub classes) public : available to any class. ..Modifiers (contd.) B.Ramamurthy

  40. Many times classes needed are available in a packages other than the Java API. Set the CLASSPATH environment variable of your .cshrc file to point to such packages. One such is located at /projects/bina/CSE116/ setenv CLASSPATH .:/projects/bina/CSE116/other paths You may now import a class from a package in this directory. CLASSPATH B.Ramamurthy

  41. Arrays • Array is a numbered collection of variables all of the same type. • Length attribute gives the capacity of the array • Cells or the individual elements of the array distinguished by an index. • Lets look at an example. • Common error: ArrayIndexOutofBounds B.Ramamurthy

  42. Summary • An overview of OOP, problem solving using OOP and Java language was presented. • Next class we will “review” interface, abstract class, concrete class, polymorphic dispatch and relationship among classes. B.Ramamurthy