Java 对文件进行读写操作

一．在 JDK 1.0 中，通常是用 InputStream & OutputStream 这两个基类来进行读写操作的。InputStream 中的 FileInputStream 类似一个文件句柄，通过它来对文件进行操作，类似的，在OutputStream 中我们有 FileOutputStream 这个对象。
用FileInputStream 来读取数据的常用方法是：FileInputStream fstream = new FileInputStream(args[0]);DataInputStream in = new DataInputStream(fstream);
用 in.readLine() 来得到数据，然后用 in.close() 关闭输入流。完整代码见 Example 1。
用FileOutputStream 来写入数据的常用方法是：FileOutputStream out out = new FileOutputStream(myfile.txt);PrintStream p = new PrintStream( out );
用 p.println() 来写入数据，然后用 p.close() 关闭输入。完整代码见 Example 2。

二．在 JDK 1.1中，支持两个新的对象 Reader & Writer, 它们只能用来对文本文件进行操作，而JDK1.1中的 InputStream & OutputStream 可以对文本文件或二进制文件进行操作。
用FileReader 来读取文件的常用方法是：FileReader fr = new FileReader(mydata.txt);BufferedReader br = new BufferedReader(fr);
用 br.readLing() 来读出数据，然后用br.close() 关闭缓存，用fr.close() 关闭文件。完整代码见 Example 3。
用 FileWriter 来写入文件的常用方法是：FileWriter fw = new FileWriter(mydata.txt);PrintWriter out = new PrintWriter(fw);
在用out.print 或 out.println 来往文件中写入数据，out.print 和 out.println的唯一区别是后者写入数据或会自动开一新行。写完后要记得 用out.close() 关闭输出，用fw.close() 关闭文件。完整代码见 Example 4。


Example 1:// FileInputDemo
// Demonstrates FileInputStream and DataInputStream
import java.io.*;

class FileInputDemo {
public static void main(String args[]) {
// args.length is equivalent to argc in C
if (args.length == 1) {
try {
// Open the file that is the first command line parameter
FileInputStream fstream = new FileInputStream(args[0]);
// Convert our input stream to a DataInputStream
DataInputStream in = new DataInputStream(fstream);
// Continue to read lines while there are still some left to read
while (in.available() !=0) {
// Print file line to screen
System.out.println (in.readLine());
}
in.close();
} catch (Exception e) {
System.err.println(File input error);
}
}
else
System.out.println(Invalid parameters);
}
}

Example 2:
// FileOutputDemo
// Demonstration of FileOutputStream and PrintStream classes
import java.io.*;

class FileOutputDemo
{
public static void main(String args[]) {
FileOutputStream out; // declare a file output object
PrintStream p; // declare a print stream object

try {
// connected to myfile.txt
out = new FileOutputStream(myfile.txt);
// Connect print stream to the output stream
p = new PrintStream( out );
p.println (This is written to a file);
p.close();
} catch (Exception e) {
System.err.println (Error writing to file);
}
}
}

Example 3:
// FileReadTest.java
// User FileReader in JDK1.1 to read a file
import java.io.*;

class FileReadTest {
public static void main (String[] args) {
FileReadTest t = new FileReadTest();
t.readMyFile();
}

void readMyFile() {
String record = null;
int recCount = 0;
try {
FileReader fr = new FileReader(mydata.txt);
BufferedReader br = new BufferedReader(fr);
record = new String();
while ((record = br.readLine()) != null) {
recCount++;
System.out.println(recCount + : + record);
}
br.close();
fr.close();
} catch (IOException e) {
System.out.println(Uh oh, got an IOException error!);
e.printStackTrace();
}
}

}

Example 4:
// FileWriteTest.java
// User FileWriter in JDK1.1 to writer a file
import java.io.*;

class FileWriteTest {
public static void main (String[] args) {
FileWriteTest t = new FileWriteTest();
t.WriteMyFile();
}

void WriteMyFile() {
try {
FileWriter fw = new FileWriter(mydata.txt);
PrintWriter out = new PrintWriter(fw);
out.print(“hi,this will be wirte into the file!”);
out.close();
fw.close();
} catch (IOException e) {
System.out.println(Uh oh, got an IOException error!);
e.printStackTrace();
}
}

}