Java Text-IO基础

File Class

public class TestFileClass {  public static void main(String[] args) {    java.io.File file = new java.io.File("image/us.gif");    System.out.println("Does it exist? " + file.exists());    System.out.println("Can it be read? " + file.canRead());    System.out.println("Can it be written? " + file.canWrite());    System.out.println("Is it a directory? " + file.isDirectory());    System.out.println("Is it a file? " + file.isFile());    System.out.println("Is it absolute? " + file.isAbsolute());    System.out.println("Is it hidden? " + file.isHidden());    System.out.println("Absolute path is " +      file.getAbsolutePath());    System.out.println("Last modified on " +      new java.util.Date(file.lastModified()));  }}

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Text-IO

This section introduces how to read/write strings and numeric values from/to a text file using the Scanner and PrintWriter classes.

PrintWriter

import java.io.*;public class WriteData {    public static void main(String[] args) throws Exception{        File file = new File("scores.txt");        if(file.exists()){            System.out.println("File already exists");            System.exit(0);        }        //Create a file        PrintWriter output = new PrintWriter(file);        //Write formatted out put to the file        output.print("John T smith ");        output.print(90);        output.print(" Eric K Jones ");        output.print(85);        //Close the file        output.close();    }}

Scanner

import java.util.*;import java.io.*;public class ReadData {    public static void main(String[] args) throws Exception{        // Create a File instance        File file = new File("scores.txt");        //Create a Scanner for the file        Scanner input = new Scanner(file);        //Read data from a file        while(input.hasNext()){            String firstname = input.next();            String mi = input.next();            String lastname = input.next();            int score = input.nextInt();            System.out.println(firstname + " " + mi + " " + lastname + " " + score);        }        //Close the file        input.close();    }}

Example

import java.io.*;import java.util.*;public class ReplaceText {  public static void main(String[] args) throws Exception {    // Check command line parameter usage    if (args.length != 4) {      System.out.println(        "Usage: java ReplaceText sourceFile targetFile oldStr newStr");      System.exit(0);    }    // Check if source file exists    File sourceFile = new File(args[0]);    if (!sourceFile.exists()) {       System.out.println("Source file " + args[0] + " does not exist");       System.exit(0);    }    // Check if target file exists    File targetFile = new File(args[1]);    if (targetFile.exists()) {      System.out.println("Target file " + args[1] + " already exists");      System.exit(0);    }    // Create input and output files    Scanner input = new Scanner(sourceFile);    PrintWriter output = new PrintWriter(targetFile);    while (input.hasNext()) {      String s1 = input.nextLine();      String s2 = s1.replaceAll(args[2], args[3]);      output.println(s2);    }    input.close();     output.close();  }}