2 rd class Department of Programming. OOP with Java Programming

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1 1. Structured Programming and Object-Oriented Programming During the 1970s and into the 80s, the primary software engineering methodology was structured programming. The structured programming approach to program design was based on the following method: To solve a large problem, break the problem into several pieces and work on each piece separately; to solve each piece, treat it as a new problem that can itself be broken down into smaller problems; repeat the process with each new piece until each can be solved directly, without further decomposition. in the early 1980s. The concept has been developed since then into a central platform of software engineering called object-oriented programming, often abbreviated as OOP. Java A programming language specifies the words and symbols that we can use to write a program A programming language employs a set of rules that dictate how the words and symbols can be put together to form valid program statements The Java programming language was created by Sun Microsystems, Inc. It was introduced in 1995 and it's popularity has grown quickly. Java is an object-oriented programming language developed by Sun Microsystems. Originally the developers of Java intended to use C++ for their software development. But they needed a language that could execute on different sets of computer chips to accommodate the everchanging consumer electronics market. So they decided to design their own language which would be independent of the underlying hardware. It allows a user to receive software from a remote system and execute it on a local system, regardless of the underlying hardware or operating system. An interpreter and runtime are called the Java Virtual Machine which insulates the software from the underlying hardware. Unlike more traditional languages, Java source code does not get translated into the machine instructions for a particular computer platform. Instead, Java source code (.java) is Page 1

2 compiled into an intermediate form called bytecodes which are stored in a.class file. These bytecodes can be executed on any computer system that implements a Java Virtual Machine (JVM). This portability is perhaps one of the most compelling features of the Java language, from a commercial perspective. In the current era of cross-platform application development, any tool that allows programmers to write code once and execute it on many platforms is going to get attention. 2. Java Language Characteristics The portable, interpreted nature of Java impacts its performance. While the performance of interpreted Java code is better than scripting languages and fast enough for interactive applications, it is slower than traditional languages whose source code is compiled directly into the machine code for a particular machine. To improve performance, Just- In-Time compilers (JITs) have been developed. A JIT compiler runs concurrently with the Java Virtual Machine and determines what pieces of Java code are called most often. The bytecode portability is what enables Java to be transported across a network and executed on any target computer system. Java applets are small Java programs designed to be included in an HTML (HyperText Markup Language) Web document. HTML tags specify the name of the Java applet +and its Uniform Resource Locator (URL). The URL is the location on the Internet at which the applet bytecodes reside. When a Javaenabled Web browser displays an HTML document containing an applet tag, the Java bytecodes are downloaded from the specified location and the Java Virtual Machine interprets or executes the bytecodes. Java applets are what enable Web pages to contain animated graphics and interactive content. Because Java applets can be downloaded from any system, security mechanisms exist within the Java Virtual Machine to protect against malicious or errant applets. Java is an object-oriented programming language, borrowing heavily from Smalltalk, Objective C, and C++. It is characterized by many as a better, safer C++. Java uses C++ syntax and is readily accessible to the large existing C++ development community. Page 2

3 Java, however, does not drag along the legacy of C. It does not allow global variables, functions, or procedures. With the exception of a few primitive data types like integers or floating-point numbers, everything in Java is an object. Object references are not pointers, and pointer manipulation is not allowed. This contributes to the general robustness of Java programs since pointer operations tend to be particularly nasty and bug-prone. Java also manages memory itself, thereby avoiding problems with allocation and deallocation of objects. It does not allow multiple inheritance like C++ does, but supports another type of reuse through the use of formal interface definitions. Java is similar enough to C and C++ that it already feels familiar to most of the existing programming community. But it is different enough in important ways (memory management and cross-platform portability) that it is worth it for programmers to switch to a new language. Java Program Structure In the Java programming language: A program is made up of one or more classes A class contains one or more methods A method contains program statements These terms will be explored in detail throughout the course A Java application always contains a method called main public class first program public static void main (String[] args) System.out.println ("Whatever you are, be a good one."); Comments Page 3

4 Comments in a program are called inline documentation They should be included to explain the purpose of the program and describe processing steps They do not affect how a program works Java comments can take three forms: // this comment runs to the end of the line /* this comment runs to the terminating symbol, even across line breaks */ Identifiers Identifiers are the words a programmer uses in a program An identifier can be made up of letters, digits, the underscore character ( _ ), and the dollar sign Identifiers cannot begin with a digit Java is case sensitive - Total, total, and TOTAL are different identifiers By convention, programmers use different case styles for different types of identifiers, such as title case for class names - Lincoln upper case for constants - MAXIMUM Reserved Words The table below lists all the words that are reserved: abstract assert boolean break byte case catch char class const* continue default double do else enum extends false final finally float for goto* if implements import instanceof int interface long native new null package private protected public return short static strictfp super switch synchronized this throw throws transient true try void volatile while Page 4

5 Program Development The mechanics of developing a program include several activities writing the program in a specific programming language (such as Java) translating the program into a form that the computer can execute investigating and fixing various types of errors that can occur Programming Languages Each type of CPU executes only a particular machine language A program must be translated into machine language before it can be executed A compiler is a software tool which translates source code into a specific target language Often, that target language is the machine language for a particular CPU type The Java approach is somewhat different Syntax and Semantics Errors The syntax rules of a language define how we can put together symbols, reserved words, and identifiers to make a valid program The semantics of a program statement define what that statement means (its purpose or role in a program) A program that is syntactically correct is not necessarily logically (semantically) correct A program will always do what we tell it to do, not what we meant to tell it to do A program can have three types of errors The compiler will find syntax errors and other basic problems (compile-time errors) If compile-time errors exist, an executable version of the program is not created A problem can occur during program execution, such as trying to divide by zero, which causes a program to terminate abnormally (run-time errors) A program may run, but produce incorrect results, perhaps using an incorrect formula (logical errors) Page 5

6 Problem Solving The purpose of writing a program is to solve a problem Solving a problem consists of multiple activities: Understand the problem Design a solution Consider alternatives and refine the solution Implement the solution Test the solution These activities are not purely linear they overlap and interact The key to designing a solution is breaking it down into manageable pieces When writing software, we design separate pieces that are responsible for certain parts of the solution An object-oriented approach lends itself to this kind of solution decomposition Object-Oriented Programming We will dissect our solutions into pieces called objects and classes Java is an object-oriented programming language As the term implies, an object is a fundamental entity in a Java program Objects can be used effectively to represent real-world entities For instance, an object might represent a particular employee in a company Each employee object handles the processing and data management related to that employee Objects Classes An object has: state - descriptive characteristics behaviors - what it can do (or what can be done to it) The state of a bank account includes its account number and its current balance The behaviors associated with a bank account include the ability to make deposits and withdrawals Note that the behavior of an object might change its state An object is defined by a class A class is the blueprint of an object Page 6

7 The class uses methods to define the behaviors of the object The class that contains the main method of a Java program represents the entire program A class represents a concept, and an object represents the embodiment of that concept Multiple objects can be created from the same class Character Strings A string of characters can be represented as a string literal by putting double quotes around the text: Examples: "This is a string literal." "123 Main Street" "X" Every character string is an object in Java, defined by the String class Every string literal represents a String object The println Method The System.out object provides another service as well The print method is similar to the println method, except that it does not advance to the next line Therefore anything printed after a print statement will appear on the same line String Concatenation The string concatenation operator (+) is used to append one string to the end of another "Peanut butter " + "and jelly" It can also be used to append a number to a string A string literal cannot be broken across two lines in a program public class Facts public static void main (String[] args) // Strings can be concatenated into one long string Page 7

8 System.out.println ("We present the following facts for your " + "extracurricular edification:"); System.out.println (); // A string can contain numeric digits System.out.println ("Letters in the Hawaiian alphabet: 12"); // A numeric value can be concatenated to a string System.out.println ("Dialing code for Antarctica: " + 672); System.out.println ("Year in which Leonardo da Vinci invented " + "the parachute: " ); System.out.println ("Speed of ketchup: " " km per year"); The + operator is also used for arithmetic addition The function that it performs depends on the type of the information on which it operates If both operands are strings, or if one is a string and one is a number, it performs string concatenation If both operands are numeric, it adds them The + operator is evaluated left to right, but parentheses can be used to force the order public class Addition // // Concatenates and adds two numbers and prints the results. // public static void main (String[] args) System.out.println ("24 and 45 concatenated: " ); System.out.println ("24 and 45 added: " + ( )); Escape Sequences What if we wanted to print a the quote character? The following line would confuse the compiler because it would interpret the second quote as the end of the string Page 8

9 System.out.println ("I said "Hello" to you."); An escape sequence is a series of characters that represents a special character An escape sequence begins with a backslash character (\) System.out.println ("I said \"Hello\" to you."); Escape Sequence Description \t Insert a tab in the text at this point. \b Insert a backspace in the text at this point. \n Insert a newline in the text at this point. \' Insert a single quote character in the text at this point. \" Insert a double quote character in the text at this point. \\ Insert a backslash character in the text at this point. Variable A variable is a name for a location in memory A variable must be declared by specifying the variable's name and the type of information that it will hold int total; int count, temp, result; Variable Initialization A variable can be given an initial value in the declaration int sum = 0; int base = 32, max = 149; public class PianoKeys // Page 9

10 // Prints the number of keys on a piano. // public static void main (String[] args) int keys = 88; System.out.println ("A piano has " + keys + " keys."); Some Built-In Types Of Variables In Java Type byte short int long float Description 8 bit signed integer 16 but signed integer 32 bit signed integer 64 bit signed integer 32 bit signed real number Page 11

11 double char boolean 64 bit signed real number 16 bit Unicode character (ASCII and beyond) 1 bit true or false value String A sequence of characters between double quotes ("") Assignment An assignment statement changes the value of a variable The assignment operator is the = sign total = 55; The expression on the right is evaluated and the result is stored in the variable on the left The value that was in total is overwritten You can only assign a value to a variable that is consistent with the variable's declared type public class Geometry public static void main (String[] args) int sides = 7; // declaration with initialization System.out.println ("A heptagon has " + sides + " sides."); sides = 10; // assignment statement System.out.println ("A decagon has " + sides + " sides."); sides = 12; System.out.println ("A dodecagon has " + sides + " sides."); Constants Page 11

12 A constant is an identifier that is similar to a variable except that it holds the same value during its entire existence As the name implies, it is constant, not variable The compiler will issue an error if you try to change the value of a constant In Java, we use the final modifier to declare a constant final int MIN_HEIGHT = 69; Page 12