Object Declaration v Every object used must be declared v Syntax: <class name> <object name>; • <class name>: the name of the class to which the object belongs • <object name>: the name of the object (any valid identifier) v Identifier: any sequence of letters, digits, underscores, and dollar signs with the following limitations: • Must begin with a letter • Cannot contain any spaces or other “white space” • Cannot be a Java “reserved” word (aka “keyword”) v Java Convention on Identifiers: • First letter lowercase • First letter of subsequent words uppercase v Reserved Word: an Identifier that is used for a specific purpose and cannot be used for any other purpose. • Example of some of Java’s Reserved Words: public private protected import class new static void byte short int long float double boolean final return while if do forObject Creation v No objects are actually created by a declaration (with declaration, only an Identifier used to refer to an object is created) v Use the ‘new’ command. Syntax: <object name> = new <class name> (<arguments>) • <object name>: the name of the declared object • <class name>: the name of the class to which the object belongs • <arguments>: sequence of values passed to the method Examples of Object Declaration: Student dave; Noisemaker clapper; Ship battleship; Examples of Object Creation: dave = new Student (4.0, 1234); clapper = new Noisemaker (); battleship = new Battleship (numPegs, xPos, yPos, dir);Message Sending v Once an object has been created, messages can be sent to it v Syntax: <object name>.<method name> (<arguments>); • <object name>: name of a declared object • ‘.’ : the “dot notation” gives relation to the items on either side of the dot • <method name>: name of a method of the object • <arguments>: sequence of values passed to the method Examples of Message Sending: dave.setGPA (2.5); clapper.makeNoise (decibelLevel); battleship.insertHit ();Program Components Three (3) main parts: 1. Comments 2. Import statements 3. Class declarations Comments v Uses: 1. State the purpose of the program 2. Explain the meaning of code 3. Give other explanations to help programmers understand the program v Syntax: /* ANY text between slash-asterisk and asterisk-slash */ OR // ANY text following two slashes to the end of the line v All programs should contain a Header Comment containing the following information: 1. Program Title 2. Author 3. Course (including section number) 4. Date Written (or Due Date) 5. Description of Program v Comments are NOT required to run a program. However, they are indispensable in writing easy to understand code. (You will lose points if your programs do NOT contain adequate comments.) v Excessive comments can hurt more than help in understanding code.Program Components Three (3) main parts: 1. Comments 2. Import statements 3. Class declarations Import Statements v Classes are grouped into “Packages” v To use a class from a Package, the class must be “imported” into the program. Syntax: import <package name>.<class name>; • import: a reserved word indicating a class is to be imported • <package name>: the name of the package to which the class belongs • <class name>: the name of the class to be imported v With subclasses, use multiple dot notations For example: import java.awt.image.ColorModel; v To import more than one class from a package, use the asterisk notation. Syntax: import <package name>.*; v When the asterisk notation is used, ALL of the classes (or subclasses) of a particular package (or super class) will be imported. v Java Convention: all package names are lowercase.Program Components Three (3) main parts: 1. Comments 2. Import statements 3. Class declarations Class Declaration v Syntax: class <class name> { <class member declarations> } • class: a reserved word indication the declaration of a class • <class name>: the name of a class (any valid identifier) Java convention: class names start with a capital letter and each subsequent word in the class name also has a capital letter • <class member declarations>: a sequence of class member declarations class member: a data value or a method v A program can (and usually does) have more than one class, but only one class will be designated the “main” class. v Typically, the application and the main class have the same name v The main class must define a method called main. This method is executed FIRST when the Java application is executed.Method Declaration v Syntax: <modifiers> <return type> <method name> (<arguments>) { <method body> } • <modifiers>: sequence of terms designating different kinds of methods • <return type>: type of data value returned by a method • <method name>: name of the method (any valid identifier) • <arguments>: sequence of values passed to a method • <method body>: sequence of instructions public static void main (String args []) modifier modifier return type method name parameterEdit-Compile-Run Cycle Three (3) steps: 1. Step 1: Type in the program using an editor and save it 2. Step 2: Compile the source file 3. Step 3: Execute the bytecode file using an Interpreter Step 1: Type in the program using an editor and save it v Examples of editors: • Code Warrior • JavaWorks • vi • emacs v Save the entered code with the following filename syntax: <name of main class>.java v The resultant is a source file written in a “high level language” (HLL) v Examples of high level languages; • Java • C • C++ • Pascal • BASIC • Fortran v Machines (i.e., computers) can only understand machine language (written in binary). Machine language is a “low level language” (LLL).Edit-Compile-Run Cycle Three (3) steps: 1. Step 1: Type in the program using an editor and save it 2. Step 2: Compile the source file 3. Step 3: Execute the bytecode file using an Interpreter Step 2: Compile the source file v A compiler translates the HLL into a LLL called bytecode (Code Warrior contains a compiler) v The bytecode file that is generated is titled as follows: <name of source file>.class v The whole source file is compiled at once v Compilers can detect Compilation Errors (aka “Syntax Errors”) • Compilation Errors: errors resulting from the