Object-oriented programming in Java (3)
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1 Programming Languages Week 14 Object-oriented programming in Java (3) College of Information Science and Engineering Ritsumeikan University
2 plan last week basic syntax how it differs from Python this week review: the main method, basic syntax review: classes, inheritance review: exceptions files, file output streams next week review: interfaces, generic classes, polymorphism boxes for primitive types encapsulation, visibility, constants file input streams 2
3 all code in Java is written in methods normal (instance) methods, or static (class) methods review: the main method every Java program must have a well-defined starting point a static (class) method called main returns void (no value) has a String[] (string array) parameter containing command-line arguments must be publically visible every Java program therefore includes something like this class Main { public static void main(string[] commandlinearguments) { // program execution begins here 3
4 primitive types char // unicode character byte // 8-bit integer short // 16-bit integer int // 32-bit integer long // 64-bit integer review: primitive types float double // 32-bit floating-point // 64-bit floating-point examples x // char unicode character 100 // byte 8-bit integer // short 16-bit integer // int 32-bit integer // long 64-bit integer // float 32-bit floating-point // double 64-bit floating-point 4
5 variable declaration syntax typename variable; typename variable = value; examples review: declarations and assignment double d; int i = 42; // with initialiser String s; String t = "hello"; // with initialiser assignment syntax (these are all expressions) variable = value object.property = value object.property.property = value // etc... examples i = 42 apoint.x = 100 arectangle.origin.y = 200 5
6 expression statement review: statements expression; // any expression can be a statement (e.g., assignment) examples i = 42; compound statement { statements... examples { i = 42; { apoint.x = 100; apoint.y = 200; 6
7 conditional statement review: conditionals if ( expression ) statement if ( expression ) statement1 else statement2 examples if (i < 100) i = i + 1; if (j > 0) { j = j - i; i = i * 2; if (c == 0 ) c = 1 ; else c = 0 ; if (c == 0 ) { c = 1 ; numzeros += 1; else { c = 0 ; numones += 1; // += is the same as in Python 7
8 while loops review: loops while ( expression ) statement do { statements... while ( expression ); examples while (i < 100) { j += i; i += 1; do { j += i; i -= 1; while (i > 0); for loops for (initialisation; expression; update) statement this is identical to initialisation; while (expression) { statement update; example for (int i = 1; i <= 10; i += 1) j *= i; 8
9 review: arrays array type syntax elementtype [] // the type array of elementtype example byte [] bytearray; int [] integers = { 1, 2, 3, 4, 5 ; // declare array of bytes // initialise array of ints iteration over a collection for (Type variable : collectionoftype) statement example int [] integers = { 1, 2, 3, 4, 5 ; for (int i : integers) System.out.println(i); 9
10 exception handling syntax review: exceptions try { dangerous statements... // potential for disaster catch (ExceptionType1 variable) { recovery statements... // run only if type 1 disaster occurs // catch more than one exception type, if you have to... catch (ExceptionType2 variable) { recovery statements... // run only if type 2 disaster occurs // optional final statement finally { cleanup statements... // always run, regardless of disasters example try { FileInputStream in = new FileInputStream("data.txt"); try { for (int i; (i = in.read()) > -1;) System.out.write(i); finally { in.close(); catch (Exception e) { e.printstacktrace(); 10
11 class and interface declaration syntax review: classes and interfaces interface InterfaceName { method declarations... class ClassName { method and class variable declarations... class ClassName extends SuperclassName { declarations... class ClassName implements InterfaceNames... { declarations... examples interface Magnitude { double magnitude(); class Point implements Magnitude { int x, y; Point(int _x, int _y) { x = _x; y = _y; // constructor public double magnitude() { return Math.sqrt(x*x + y*y); class Point3D extends Point implements Magnitude { int z; Point(int _x, int _y, int _z) { super(_x, _y); z = _z; // constructor public double magnitude() { return Math.sqrt(x*x + y*y + z*z); 11
12 inheritance subclasses extend their superclass class Point extends Point2D.Double { Point(double x, double y) { super(x, y); // NOT INHERITED Point add(point other) { return new Point(x + other.x, y + other.y); note Java syntax for calling superclass constructor: super(arguments...) in Java, there is no multiple inheritance 12
13 the filesystem path name of directory or file / / root directory /Users Applications/ Library/ Users/ bin/ lib/ tmp/ /Users/jim fred/ jim/ sheila/ home directories /Users/jim/Programming Documents/ Music/ Programming/ sub-directories /Users/jim/Programming/test.java test.py test.java test.class regular files hierarchical files and directories files contain data; directories contain files and sub-directories a path identifies a location with the filesystem path components separated by / characters the root is at the top, a directory with an empty name, called / 13
14 files Java represents locations in the filesystem with instances of File every File has an associated path (a String) naming a filesystem location the location may or may not exist File provides methods to create and access filesystem locations exists(), isfile(), isdirectory() canread(), canwrite(), length() createemptyfile(), mkdir(), renameto(file newname) Files created either from path (a String) or by listing the contents of a directory list() returns a String[] containing all path names in a directory listfiles() returns a File[] containing all files in a directory File temp = new File("/tmp/temporary.dat"); // absolute path File text = new File("results.txt"); // current directory File elog = new File("../logs/event.log"); // relative path if (!elog.exists()) elog.createemptyfile(); 14
15 a stream lets you write (or read) data output streams a FileOutputStream lets you write data to a file File tempfile = new File("/tmp/temporary.dat"); FileOutputStream temp = new FileOutputStream(tempFile); or, if you do not need (or have) the intermediate File... FileOutputStream temp = new FileOutputStream("/tmp/temporary.dat"); FileOutputStreams can be sent byte data temp.write( x ); // write one byte temp.write("hello, world".getbytes()); // write a byte array temp.write(byte[] b, int off, int len); to make sure the data written to a file is really in the file temp.flush(); // ensures all data is actually in the file when you have finished writing data to the file temp.close(): // flushes and then closes the stream 15
16 what does flush() actually do? output streams afileoutputstream afos.write(somebytes) bytes copy internal data buffer bytes copy actual data stored on disk bytes afos.flush() data written to a FileOutputStream might be buffered (in many places) in the stream itself (to reduce operating system calls) in the operating system (to reduce device i/o operations) in the disk drive itself (to make writes complete faster) calling flush() makes sure the data is actually in the device 16
17 print streams a PrintStream converts data into text wrapper around an output stream converts any data type into a text representation writes a human-readable representation to the stream as a sequence of bytes System.out is a PrintStream can print() and println() primitive and object types calls tostring() to get a representation for objects double average = computeaverage(); FileOutputStream temp = new FileOutputStream("/tmp/report.txt"); PrintStream ps = PrintStream(temp); ps.print("the average recorded was "); ps.println(average); ps.close(): // flushes and then closes the stream temp.close(); 17
18 exceptions an exception indicates an error condition file not found when creating a FileInputStream input/output error when reading/writing a file stream etc... some exceptions can be ignored ArithmeticException (e.g., division by zero) others must be explicitly handled, or the compiler complains FileNotFoundException (e.g., when opening a file input stream) 18
19 example of exception that must be caught exceptions FileInputStream in = new FileInputStream("file.txt");... in.close(); error: unreported exception FileNotFoundException FileInputStream in = new FileInputStream("file.txt"); ^ error: unreported exception IOException in.close(); ^ exception catching pattern always looks the same try {... // code that might raise an exception catch (ExceptionType e) { e.printstacktrace(); // deal with the exception here 19
20 exceptions example rewritten to catch exceptions properly FileInputStream in; try { in = new FileInputStream("file.txt"); catch (FileNotFoundException fnfe) { fnfe.printstacktrace();... try { in.close(); catch (IOException ioe) { ioe.printstacktrace(); 20
21 exercises file and output stream examples 21
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