Java An Introduction. Outline. Introduction

Transcription

1 Java An Introduction References: Internet Course notes by E.Burris Computing Fundamentals with Java, by Rick Mercer Beginning Java Objects - From Concepts to Codeby Jacquie Barker Object-Oriented Design & Patterns, Cay Horstmann Sun white papers on Java 8/27/ Outline Introduction The Java Virtual Machine Types of Java Programs Java vs C++ Java essential language features 8/27/ Introduction Java is a general-purpose objectoriented programming language. Its syntax is similar to C and C++, but it omits many of the features that make C and C++ complex, confusing and unsafe. 8/27/

2 Introduction Java allows you to write platform independent programs. The Java virtual machine (JVM) enables portability as well as security. Compile-time and runtime checks allow the language to be secure. 8/27/ Java Virtual Machine C++ programs are compiled to produce binary object files. The binary code of these files is specific to the hardware architecture (underlying mach. lang.) Java is an interpreted language. You compile a Java source file (.java) into intermediate byte code (.class file) which is platform independent. The byte codes are interpreted at runtime by the Java Virtual Machine (JVM). 8/27/ Java Virtual Machine It's called a virtual machine because most implementations of the virtual machine will be in software on top of another hardware machine platform. The JVM runs under the OS and the Java program runs under the JVM. 8/27/

3 Java Virtual Machine A Java program can run on any computer that contains the Java Virtual Machine. The byte code format of a program and the JVM allow the language to be small and architecture neutral. 8/27/ Applications Types of Java Programs Stand-alone Java programs. No graphical user interface (GUI) Servlets Platform-independent server-side programs/components Servlets function as CGI programs on the server. Servlets run on the server and interact with clients through a web server. Servlets are real Java classes that get called and may be loaded and cached at the web server. 8/27/ Applets Types of Java Programs Java programs that execute within a browser such as Internet Explorer, Netscape Navigator, and HotJava. (HotJava was the first browser to provide support for executable content.) The applet viewer that comes with the Java Development Kit (JDK) has very few restrictions (the only restriction is you can't delete a file). Applets running in web browsers have many more restrictions (called keeping applet within a sandbox ). 8/27/

4 Java Objects & Methods Java is an object-oriented language Except for simple, native data types, all data in Java are objects All GUI building blocks (windows, buttons, etc) are objects All functions are called methods The main program is a main method that is contained within a class - not stand alone 8/27/ Structure of a Java Program Introductory /** Comments FirstProgram.java A trivial program to illustrate Java programming elements. */ Class wrapper public class FirstProgram{ Main method signature public static void main(string[] args) { System.out.println( Hello World! ); 8/27/ Java vs C++ 8/27/

5 Java vs C++ Java's syntax and semantics are like those of C++ Java doesn't have any implementation-dependent behaviors. All primitive types are guaranteed to be a specific size. For example, in C++ an int may be 16-bits or 32-bits. In Java an int is always 32-bits. C++ allows stand-alone or global functions. In Java all functions must be the member of some class. Static member functions don't require the instance of a class to execute. 8/27/ Java vs C++ In C++ you have to declare a function (provide a prototype) before you can use it. Java doesn't have this requirement because the compiler makes multiple passes. In C++ objects can be created at compile time or at run time. In Java all objects are allocated dynamically from the heap(i.e. at run time). In Java there is no way to directly manipulate memory locations by address. No pointer arithmetic with arrays 8/27/ Pointers vs Object References Java always uses pointers.they re called references An object variable is a reference to an object -- a value that describes the location of the object. Java C++ C++ Bank b; Bank *b; (dynamic) Bank b; (static) b = new Bank(); b = new Bank(); In Java, only the dot operator is used for dereference (no arrows or *s). Java C++ C++ b.setbalance(100); b->setbalance(100); (*b).setbalance(100); b.setbalance(100); 8/27/

6 Object References Multiple variables can store references to the same object. For example, after the following statements: Bank b = new Bank(); Bank anotherbank = b; both object variables refer to the same object 8/27/ Java References Pointers in Java are safe You cannot create invalid pointers The garbage collector automatically reclaims unused objects -- you can't have a memory leak. 8/27/ Implicit Parameter this this refers to the object on which the method was invoked. Assume class Bank has the method public void setname (String name) { this.name = name; then bank.setname( First National ) sets the name field of the Bank object to the value of the explicit parameter which is also called name. Use of this resolves ambiguity between the name field and name parameter 8/27/

7 Another example using this public class Bank { /** sets another bank s name to this bank s other a reference to the other Bank */ public void copynameto(bank other) other.name = this.name; Bank b1 = new Bank( National Bank ); Bank b2 = new Bank( Commerce ); b1.copynameto(b2); A method can never update the contents of a variable that is passed as a parameter other.name is changed; after return, b2.name is changed Contents of b2 is same object reference 8/27/2003 before and after the call 19 Java super Method In Java the keyword super refers to the superclass or parent class of a class. super is a reference you can use to call a method in the parent class. public void paintcomponent(graphics g) { super.paintcomponent(g); 8/27/ Java super Method The reference super is also used to call a specific constructor in a parent class. This Class s Constructor public class TypeSafeVector extends Vector { TypeSafeVector(int size, Object elementtype) {... super(size); Vector class constructor 8/27/

8 Java vs C++ In C++ the integer 0 can be exchanged for the bool value false and any non-zero integer can be exchanged for the value true. In Java you can't interchange integer and boolean values. In Java both primitive types and reference types are passed by value. There is no pass by reference (&). 8/27/ Java vs C++ Java objects don't have destructors. Java doesn't have a pre -processor. Java has no typedefs, #defines or structs There is no goto statement in Java. (goto is a reserved word, but it isn't used.) But you can add a label to a loop and use that label as an argument on a break or continue statement. 8/27/ Java vs C++ Java has no enum types. To get the same functionality as an enum, you can declare a class. For example: public class Rainbow{ private Rainbow(String name) {this.name=name; public static final Rainbow YELLOW = new Rainbow( YELLOW ); public static final Rainbow BLUE = new Rainbow( BLUE ); public static final Rainbow RED = new Rainbow( RED ); public static final Rainbow GREEN = new Rainbow( GREEN ); public static final Rainbow PURPLE = new Rainbow( PURPLE ); To refer to the BLUE constant, use the notation Rainbow color = Rainbow.BLUE; Static fields belong to the class, not the objects 8/27/

9 Java vs C++ Java has no arithmetic operator overloading. Effects of operator overloading can be achieved by declaring a class, appropriate instance variables, and appropriate methods to manipulate those variables. 8/27/ Java vs C++ : Exceptions Exception handling is optional in C++; it is mandatory in Java. If you do something that causes ( throws ) an exception, then the program must handle ( catch ) it. 8/27/ Exception Handling The exception handling mechanism in Java also might be considered a form of control flow. When an exception occurs, the runtime system takes control and looks up the call stack for an exception handler. If it finds one, it transfers control to the exception handler. If it doesn't find one, the default behavior is to terminate the program. 8/27/

10 Java vs C++ No multiple inheritance. Java uses interfaces to implement multiple inheritance. An interface is a definition of a set of methods that one or more classes will implement. An important issue of interfaces is that they declare only methods and constants. Variables may not be defined in interfaces. (Sun) 8/27/ Java Language Features 8/27/ Data Types Two categories of data types in Java: 1. primitive 2. reference A primitive data type represents a single value of a certain type and size recognizable by the compiler. For example, int i = 5; defines a 4-byte storage area that will hold integer values in the two's complement data format. A reference data type represents a reference to an instance of a class. 8/27/

11 Data Types There are 8 primitive data types in Java: Integers Data type Length Format byte 1 byte Two's Complement short 2 bytes Two's Complement int 4 bytes Two's Complement long 8 bytes Two's Complement 8/27/ Data Types Floating Point Data type Length Format float 4 bytes IEEE 754 double 8 bytes IEEE 754 Other Data type Length Format char 2 bytes Unicode boolean N/A true/false Note: no unsigned int type 8/27/ public class test { Data Types - Example public static void main(string[] args) { // Integer types byte b; // 1 byte short s; // 2 bytes int i; // 4 bytes long l; // 8 bytes // Floating point types follow IEEE 754 format float f; // 4 bytes (6-7) significant decimal digits double d; // 8 bytes (15) significant decimal digits 8/27/

12 Data Types - Example // Character type // Characters are represented in Unicode char c; // 2 byte Unicode // Boolean -- note, this is different from C++(bool) boolean bl; // can be true or false i = 1; s = 2; l = 3L; b = 128; // Why is this an error? 8/27/ Data Types - Example f = 1.2; // Why is this an error? (see next slide) f = 1.3F; d = 1.4; // Why is this NOT an error? c = 'c'; // How is this different from c = "c"; c = '\u2122'; // \u2122 is the tm symbol bl = true; 8/27/ Data Types Decimal constants (such as 1.2) are by default of type double. You can't directly assign or convert a double to a float because there might be a loss of data.. You specify a decimal constant of type float by following the value with an F. For example: 1.2F. boolean and int are not interchangeable in Java 8/27/

13 Unicode The Unicode Standard is a universal character encoding standard used for the representation and processing of text. Unicode provides a unique number for every character, for any platform, for any program, for any language Unicode is a 16-bit encoding standard with enough room to represent the characters and symbols included in most of the languages of the world. For more information about Unicode check out the Unicode web site at 8/27/ Variables A variable is an instance of a primitive or reference type. In the diagram, i is primitive. References act like pointers. Java doesn't call them pointers, though, because references don't support all of the features associated with pointers as they are used in other languages such as C++ (e.g. pointer arithmetic). 8/27/ Variables Variable names are represented in Unicode. Local variables must have a value before they are used. The length of a variable name is essentially unlimited. Variable names are formed with lower/uppercase letters and optional _ 8/27/

14 Initializing Variables By convention, the following values are used to initialize variables of built-in types: boolean answer = false; //note spelling int i=0; double rate = 0.0; char ch = ; //blank space String name = null; 8/27/ Naming conventions Variable names are lowercase if one word. If compound words, the first letter of second (third ) word is capitalized. item productnumber nameofthiscustomer 8/27/ Constants The key word final denotes a constant: final int MAX_VALUE = 3; You can assign a value to a final variable only once. You don't have to assign a value in the definition, but it's good programming style. Final variables are all capitals. 8/27/

15 Operators Operators are defined and behave like the operators in C++ Relational (<, >, ) Arithmetic Conditional (and, or, ) Assignment Shift (>>, <<) Bitwise 8/27/ Other Operators Shortcut if-then-else (? : ) If the expression before the '?' is true, the expression before the ':' is returned; otherwise the expression after the ': is returned. "You " + (score > 70? "passed" : "failed") + " the test" Array declaration/access ([ ]) new (create new object or array) String [] names = new String[3]; 8/27/ Dot operator (.) Other Operators The only operator used to access the methods or member data of an object. instanceof Tests whether a reference is of a particular type or is to a class that implements a particular interface Object obj = e.nextelement(); if (obj instanceofshape) { Shape s = (Shape) obj; 8/27/

16 Other Operators cast operator (type). Converts from one type to another. The example here casts a reference to an Object to a reference to a Shape. Shape s = (Shape) e.nextelement(); Precedence chart - use of parentheses overrides All operations (except assignment) of equal precedence are performed left to right. The assignment operator is evaluated right to left. 8/27/ Type Conversions Automatic type conversions are done only if no precision will be lost in the conversion (C++ will convert and truncate if necessary) int x; double y = 3.5; x = y; //in Java, will be an error Explicit type conversion can be done with a cast to override this decision: x = (int) y; //cast the double to an int 8/27/ Blocks Statements within {'s form a block. A block can be used anywhere a statement can be used. Blocks define the scope of variables. The variables defined within a block are not known outside of the block they are defined in 8/27/

17 Blocks public class test { public static void main(string[] args) { if (args.length > 1) { int x = 0; // x is only known within this block else {... 8/27/ Control Conditional, loops and branching are defined and behave as in C++ Note, that although i isn't physically declared within the block of statements that make up the for loop below, it's scope is the for loop expression and block of statements. This allows you to reuse for-loop index variables. For example: for(int i=0;i<5;i++){ for(int i=0;i<20;i++){ 8/27/ Branching There is no goto statement in Java, but Java does include the break, continue, and return statements. These statements provide a more structured way of branching. The break statement transfers control to the end of the immediate loop or switch statement which contains it. 8/27/

18 Break statement If there are nested loops, the break statement breaks out of the most closely nested loop, not the most closely nested block. A non-labeled break statement must appear within a loop or switch statement. A labeled break may appear within any block. 8/27/ Other Language Features 8/27/ String class Strings are Java programming language objects The reserved word begins with capital S since it is a class name A length() accessor method is provided to obtain the number of characters in the string. The + operator indicates string concatenation. 8/27/

19 String class There are actually two kinds of string objects: 1. The String class is for read-only (immutable) objects. 2. The StringBuffer class is for string objects you wish to modify (mutable string objects). Can create a string in two ways: String stra = stringa ; //preferred method String stra = new String( stringa ); 8/27/ ArrayList Class Class in java.utilthat lets you collect a sequence of objects of any type add method inserts new objects are at the end of the array list ArrayList namelist = new ArrayList(); namelist.add( Hong ); namelist.add( Juan ); namelist.add( Gracie ); Hong Juan Gracie /27/ ArrayList Methods size : returns number elements in array list get : returns element at i th position as anobject for (int i=0; i<namelist.size(); i++) { String name = (String) namelist.get(i); set : lets you overwrite an existing element namelist.set(2, George ); If a nonexistent position is accessed, an IndexOutOfBounds Exception is thrown Must cast returned object 8/27/

20 ArrayList Methods remove : takes out an element from middle of array list namelist.remove(1); add(i,object) : adds element in middle of array list namelist.add(1, Olivia ); These operations are not efficient, since they require that remaining elements be moved up or down Hong Olivia Juan Gracie /27/ LinkedList class A linked list supports insertion and removal of objects efficiently from any location LinkedList class implements the linked list data structure LinkedList namelist = new LinkedList(); add method inserts elements at end of linked list 8/27/ LinkedList class Accessing elements in the middle of the list is done with an iterator Iteratorobjects can access a position anywhere in the list ListIterator iterator = namelist.listiterator(); next() advances iterator to next position of the list and returns element the iterator just passed hasnext() tests if iterator has already passed the last element in the list 8/27/

21 LinkedList class while (iterator.hasnext()) { String name = (String) iterator.next(); To traverse the list again, after reaching the end of the list, define another iterator 8/27/ LinkedList class Adding an element in the middle of the list iterate past insert location and call add: iterator = namelist.listiterator(); iterator.next(); iterator.add( Dolly ); Removing an element from the list call next until you jump over element you want to remove (e.g. third) : iterator = namelist.listiterator(); iterator.next(); iterator.next(); iterator.next(); iterator.remove(); 8/27/ Arrays Can hold primitive types Arrays are first-class objects ; that is, they have an internal runtime representation in memory. Unlike in C++, an exception is generated if the index is outside the bounds of the array Elements are initialized to zero, false or null 8/27/

22 Arrays Ways to declare arrays: int[ ] test = new int[100]; double number [ ] = new double[40000]; // C++ way String[ ] name = new String[15]; BankAccount[ ] customer = new BankAccount[100]; public static void main(string [ ]argv); public static void main(string argv[ ]); 8/27/ Arrays Arrays can also be initialized without using new int numbers = {2,4,6,8,10; The length variable returns the capacity of an array. System.out.println(numbers.length); //returns 5 If size is exceeded, you must construct a new array and move elements from old array to new one 8/27/ Arrays Arrays are passed as parameters like they are in C++ public static void main (String[ ] args) args is an array that holds input strings typed at the keyboard when a program is launched from the command line. For example: java ComputeResults in.data 8/27/

23 Vector class Allows you to store items without having to worry about sizing the container To use, must import java.util.* or java.util.vector Instantiating the Vector class: Vector coursestaken = new Vector(); Adding courses to the vector: Course c1 = new Course ( CS101 ); Course c2 = new Course ( CS191 ); coursestaken.add(c1); coursestaken.add(c1); How many have been added: int courses = coursestaken.size(); 8/27/ Vectors and Casting Vectors hold Object references Object class (in the java.lang package) is the parent class to all other classes. It is the root of the entire Java inheritance hierarchy. When a vector is accessed, it returns a generic Object reference. The programmer must cast the result to the appropriate type of data(i.e. class) that is stored in the vector. 8/27/ Vectors and Casting Assume that we have added 5 Course objects to the vector coursestaken. To retrieve those courses, we can use a for-loop: for (int i=0; i<coursestaken.size(); i++) { Course c = (Course) coursestaken.elementat(i); System.out.println(c.getName()); 8/27/

24 Some(not all) Vector Methods add(object): adds object reference to end of vector add(int, Object) : adds object reference to position in vector indicated by integer argument set(int,object ) : replaces nth object reference with specified object reference elementat(int) : retrieves nth object reference as type Object (requires cast) removeelementat(int) : takes out nth reference 8/27/ Some(not all) Vector Methods remove(object) :removes first occurrence of Object reference indexof(object ) : returns an integer indicating the first position of the object reference in the vector,if found contains(object) : returns true if object reference found in the vector; false otherwise isempty() : returns true if vector is empty; false otherwise clear() : empties the vector 8/27/