Core Java (四) Java文件操作

下面介绍几个在OCJP中要求的io中的类的使用：

java.io:

 BufferedReader, BufferedWriter, File, FileReader, FileWriter, PrintWriter, and Console.

OCJP要求的几个文件操作类的层次关系：

左边的类是右边类的supercalss

PrintWriter类

创建一个PrintWriter类型的对象，PrintWriter pw = new PrintWriter(new File("G:\\file.txt"));

这个类API中的函数非常多，比较重要的是

pw.println();

pw.printf();

pw.write();等

测试程序，实现将in.txt中的数据一行一行复制到file.txt中，并添加了一些内容。：

package cn.xujin;

import java.io.File;
import java.io.FileNotFoundException;
import java.io.PrintWriter;
import java.util.Scanner;


public class Test {
public static void main(String[] args) throws FileNotFoundException {
	Scanner in = new Scanner(new File("G:\\in.txt"));
	PrintWriter pw = new PrintWriter(new File("G:\\file.txt"));
	String name = "GinSmile";
	pw.write(name);//把字符串添加到文件末尾
	pw.append('w');//直接把字符添加到文件末尾
	pw.println();//仅仅打印一个换行符
	while(in.hasNext()){
		String s = in.nextLine();
		pw.println(s);
	}
	
	pw.println();
	pw.printf("%x", 100);
	pw.println();
	char[] buf = {'a','b','c','d','e','f'};
	pw.write(buf, 0, 4);//把字符数组的一部分添加到文件末尾
	
	pw.close();
}
}

PrintWriter类实现写入操作，write()，与print()两个函数意义相同，最后一定要记得close()这个函数。

close()在JDK的doc中是这样解释的：

• Closes the stream and releases any system resources associated with it. 

注意：可能出现找不到文件的情况，所以要在main上有throws子句标记。

BufferedReader, BufferedWriter, File，FileReader，FIleWriter

BufferedReader

创建一个BufferedReader 类型的对象：

Constructor and Description
BufferedReader(Reader in) Creates a buffering character-input stream that uses a default-sized input buffer.
BufferedReader(Reader in, int sz) Creates a buffering character-input stream that uses an input buffer of the specified size.

例如：BufferedReader br = new BufferedReader(new FileReader(file));

FileReader类继承自Reader类，所以用它可以。

重要的函数：

int	read() Reads a single character.
int	read(char[] cbuf, int off, int len) Reads characters into a portion of an array.
String	readLine() Reads a line of text.

BufferedWriter

创建一个BufferedWriter 类型的对象：

BufferedWriter(Writer out) Creates a buffered character-output stream that uses a default-sized output buffer.

BufferedWriter(Writer out, int sz) Creates a new buffered character-output stream that uses an output buffer of the given size.

例如：BufferedWriter bw = new BufferedWriter(new FileWriter("G:\\file.txt"));

FileWriter类继承自Writer类。

全部函数：

Modifier and Type	Method and Description
void	close() Closes the stream, flushing it first.
void	flush() Flushes the stream.
void	newLine() Writes a line separator.
void	write(char[] cbuf, int off, int len) Writes a portion of an array of characters.
void	write(int c) Writes a single character.
void	write(String s, int off, int len) Writes a portion of a String.

File

创建一个File 类型的对象：

Constructor and Description
File(File parent, String child) Creates a new  File instance from a parent abstract pathname and a child pathname string.
File(String pathname) Creates a new  File instance by converting the given pathname string into an abstract pathname.
File(String parent, String child) Creates a new  File instance from a parent pathname string and a child pathname string.
File(URI uri) Creates a new  File instance by converting the given  file: URI into an abstract pathname.

FileReader

构造函数：

Constructor and Description
FileReader(File file) Creates a new  FileReader, given the  File to read from.
FileReader(FileDescriptor fd) Creates a new  FileReader, given the  FileDescriptor to read from.
FileReader(String fileName) Creates a new  FileReader, given the name of the file to read from.

FileWriter

构造函数：

Constructor and Description
FileWriter(File file) Constructs a FileWriter object given a File object.
FileWriter(File file, boolean append) Constructs a FileWriter object given a File object.
FileWriter(FileDescriptor fd) Constructs a FileWriter object associated with a file descriptor.
FileWriter(String fileName) Constructs a FileWriter object given a file name.
FileWriter(String fileName, boolean append) Constructs a FileWriter object given a file name with a boolean indicating whether or not to append the data written.

测试程序，从一个文件in.txt中读取数据，再把数据写入到file.txt中：

package cn.xujin;

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;

public class Test {
	public static void main(String[] args) throws FileNotFoundException {
		File file = new File("G:\\in.txt");
		String s;
		BufferedReader br = new BufferedReader(new FileReader(file));
		try {
			BufferedWriter bw = new BufferedWriter(new FileWriter("G:\\file.txt"));
			while((s = br.readLine())!= null){
				bw.write(s);
				bw.newLine();
				bw.flush();
				bw.write("GinSmile");
				bw.newLine();
			}			
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		
		
	}
}

Console

这个类用来和控制台打交道，

创建一个Console变量：Console cons = System.console();

读取一个字符串：String username = cons.readLine("%s", "User name:");就是输出一个User name:  在之后输入的内容（String类型）将存到username里面。

读取一个密码：char[] passwd = cons.readPassword("%s", "Password:");就是输出一个Password:  在之后输入的内容（char[]类型）将存到passwd里面。

输出：

public Console printf(String format,
             Object... args)

这个方法和c语言一样，和System.out.printf也一样，就是把内容送到输出流。

测试程序，读入用户名和密码，并显示出来，在读密码的时候不会在屏幕上显示。

package cn.xujin;

import java.io.Console;      

public class TestConsole {      
      
    public static void main(String[] args) {      
       Console cons;
       char[] passwd = null;
       String username = null;
       if((cons = System.console()) != null && 
    		   (username = cons.readLine("%s", "User name:")) != null &&
    		     (passwd = cons.readPassword("%s", "Password:")) != null)
       {
           System.out.println("User name:" + username);
           String password = new String(passwd);
           System.out.println("Password:" + password);
       }
       else System.out.println("The console is unavailable ");
       
       
    }     
     
}     

结果：

创建一个BufferedWriter 类型的对象：

BufferedWriter(Writer out) Creates a buffered character-output stream that uses a default-sized output buffer.

BufferedWriter(Writer out, int sz) Creates a new buffered character-output stream that uses an output buffer of the given size.

例如：BufferedWriter bw = new BufferedWriter(new FileWriter("G:\\file.txt"));

FileWriter类继承自Writer类。