Programmi di utilità
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- Mervyn Scott
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1 Programmi di utilità La classe SystemData per il calcolo dei tempi e della occupazione di memoria Per calcolare i tempi e la occupazione di memoria è necessario interagire con il sistema operativo, questo può essere fatto in modo indipendente dalla piattaforma attraverso le API di Java. La seguente classe fornisce alcuni metodi utili in proposito. public class SystemData { private static Runtime R = Runtime.getRuntime(); private static double time0; public static void inittime() { time0 = System.currentTimeMillis(); public static double asktime() { return (System.currentTimeMillis() - time0) / 1000.; public static double askmemory() { R.gc(); return (R.totalMemory()-R.freeMemory())/1024./ 1024.;
2 Input/Output su files Scrittura di un array di interi su un file di tipo testo public void exportint(int[] x, String filename){ try { PrintWriter out = new PrintWriter( new BufferedWriter(new FileWriter(fileName))); for(int i=0; i<x.length; i++) out.println(x[i]); out.close(); catch(ioexception e) {e.printstacktrace(); Scrittura di un array di interi su un file binario. public void saveint(int[] x, String filename){ try { DataOutputStream out = new DataOutputStream( new BufferedOutputStream(new FileOutputStream(fileName))); for(int i=0; i<x.length; i++) out.writeint(x[i]); out.close(); catch(ioexception e) {e.printstacktrace();
3 Lettura di interi da un file di tipo testo in un array public int[] readint(string filename){ try {BufferedReader in = new BufferedReader(new FileReader(fileName)); //determinazione del numero di elementi int n=0; String line = in.readline(); while(line!=null){ n++; line = in.readline(); in.close(); //lettura in = new BufferedReader(new FileReader(fileName)); int[] V = new int[n]; for(int i=0; i<v.length; i++) V[i]=Integer.parseInt(in.readLine().trim()); in.close(); return V; catch(ioexception e) {e.printstacktrace(); return null;
4 Lettura di interi da un file binario in un array public int[] loadint(string filename){ try { File file = new File(fileName); int n = (int) file.length()/4; int[] array = new int[n]; DataInputStream in = new DataInputStream( new BufferedInputStream( new FileInputStream(file))); for(int i=0; i<n; i++)array[i]=in.readint(); in.close(); return array; catch(ioexception e) {e.printstacktrace(); return null;
5 Scansione di un Directory La classe ScanDirectory che segue permette di trattare tutti i file di un directory che hanno una estensione data. import java.io.*; public class ScanDirectory implements FilenameFilter { String[] list; String ends; public ScanDirectory(String dirname, String ends){ this.ends=ends.tolowercase(); this.list = (new File(dirname)).list(this); // lista dei nomi dei file public String[] list() {return list; // realizzazione del FilenameFilter public boolean accept(file dir, String s){ return s.tolowercase().endswith(ends);
6 Vediamo come usare ScanDirectory per stampare i nomi di tutti i file.txt del directory pippo con un for semplificato. ScanDirectory sd = new ScanDirectory("./pippo", ".txt"); for(string name : sd.list()) System.out.println(name); Formattazione di numeri Java fornisce alcune classi per la stampa di numeri secondo formati a scelta dell utente vediamo come si possno scrivere numeri reali con tre decimali. NumberFormat nf = new DecimalFormat("0.###"); String rd3(double x){return nf.format(x); Per maggiori dettagli e per formattazioni più sofisticate si consultino le API di java.
7 Il programma di prova Ecco un programma che prova le due classi (i programmi di I/O sono raccolti in una classe IOfiles). import java.text.*; import java.util.*; public class TestUtil { public static void main(string[] args) { System.out.println("prova SystemData"); SystemData.initTime(); double[] A = new double[ ]; for (int j=0; j<a.length; j++) A[j]=Math.cos(Math.sqrt(j)); System.out.println( "time = "+SystemData.askTime() + " sec. "+ ", memoria = " + rd3(systemdata.askmemory())+ "M"); System.out.println(); System.out.println( "prova ScanDirectory: elenco dei file java"); ScanDirectory sd = new ScanDirectory("./src", "java"); for(string name : sd.list()) System.out.println(name);
8 System.out.println(); System.out.println("prova lettura e scrittura su file"); IOfiles io = new IOfiles(); int[] B = io.readint("./file1.txt"); //lettura testo io.saveint(b, "./file2"); //scrittura binaria int[] C = io.loadint("./file2"); //lettura binaria io.exportint(c, "./file3.txt"); //scrittura testo C = io.readint("./file3.txt"); //lettura testo boolean OK = true; for (int j=0; j<b.length; j++) if(b[j]!= C[j]) OK = false; if(ok) System.out.println(" lettura OK "); else System.out.println(" ERRORE in lettura"); static NumberFormat nf = new DecimalFormat("0.###"); static String rd3(double x){return nf.format(x);
9 Ecco i risultati prova SystemData time = sec., memoria = 8.886M prova ScanDirectory: elenco dei file java IOfiles.java ScanDirectory.java SystemData.java TestUtil.java prova lettura e scrittura su file lettura OK
10 Una classe di logging per l output. La classe EchoWriter permette di dirigere l output sia sulla console che su un file di tipo.txt. Si usa semplicemente chiamando i suoi metodi invece che quelli di System.out. import java.io.*; public class EchoWriter { private PrintWriter out = null; /** * Constructor from a filename filename the string representing the file to write onto * if filename==null no mirroring is done and the output * is written on System.out only IOException in case of I/O error. */ public EchoWriter(String filename) { try{if(filename!=null)out = new PrintWriter( new BufferedWriter(new FileWriter(fileName))); catch(ioexception e) {e.printstacktrace();
11 /** * print a String and then finish the line line the String to print */ public void println(string line){ System.out.println(line); if(out!=null) out.println(line); /** * finish the line line the String to print */ public void println(){ System.out.println(); if(out!=null) out.println(); /** * print a String line the String to print */
12 public void print(string line){ System.out.print(line); if(out!=null) out.print(line); /** * print an integer i the int to print */ public void println(int i){ System.out.println(i); if(out!=null) out.println(i); /** * close the file */ public void close(){ if(out!=null) out.close(); out = null;
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