java--File的读取和写入实例

实例来源think in java

读取：

根据需要读取文件返回不同类型的信息进行选择使用。

实例1

使用BufferReader读取。

package io;

// 使用Reader方式进行读取 
import java.io.*;

public class BufferedInputFile {
	// Throw exceptions to console:
	public static String read(String filename) throws IOException {
		// 使用BufferedReader进行缓冲，来提高文件的读取数据
		BufferedReader in = new BufferedReader(new FileReader(filename));
		String s;
		StringBuilder sb = new StringBuilder();
		while ((s = in.readLine()) != null)
			sb.append(s + "\n");
		in.close();
		return sb.toString();
	}

	public static void main(String[] args) throws IOException {
		System.out.print(read("C:\\Users\\zm\\workspace\\Thinking in java\\src\\io\\BufferedInputFile.java"));
	}
}

实例2

使用StringReader类读取，in.read()以int形式返回下一个字节。

package io;

// 从内存中读取
import java.io.*;

public class MemoryInput {
	public static void main(String[] args) throws IOException {
		StringReader in = new StringReader(BufferedInputFile
				.read("C:\\Users\\zm\\workspace\\Thinking in java\\src\\io\\MemoryInput.java"));
		int c;
		while ((c = in.read()) != -1)
			System.out.print((char) c);
	}
}

实例3

使用DataInputStream类读取，in.readByte()返回的byte类型。

package io;

//相对TestEOF.java 此方法不能检测字节是否结束
//面向字节的IO类
import java.io.*;

public class FormattedMemoryInput {
	public static void main(String[] args) throws IOException {
		try {
			DataInputStream in = new DataInputStream(
					new ByteArrayInputStream(BufferedInputFile
						.read("C:\\Users\\zm\\workspace\\Thinking in java\\src\\io\\FormattedMemoryInput.java")
						.getBytes()));
			while (true)
				System.out.print((char) in.readByte());
		} catch (EOFException e) {
			System.err.println("End of stream");
		}
	}
}

另一种用法，差异在第9行，使用in.available() != 0，判断检测字节是否结束。

FormattedMemoryInput例子在运行时 不能检测流中是否还有字节，所以在控制台最后会抛出一个异常：End of stream

下面用法可以避免此问题。

package io;

import java.io.*;

public class TestEOF {
	public static void main(String[] args) throws IOException {
		DataInputStream in = new DataInputStream(new BufferedInputStream(
				new FileInputStream("C:\\Users\\zm\\workspace\\Thinking in java\\src\\io\\TestEOF.java")));
		while (in.available() != 0)
			System.out.print((char) in.readByte());
	}
}

写：

实例1

基本的文件写入

package io;

// 基本的文件输出
import java.io.*;

public class BasicFileOutput {
	static String file = "C:\\Users\\zm\\workspace\\Thinking in java\\src\\io\\BasicFileOutput.out";

	public static void main(String[] args) throws IOException {
		BufferedReader in = new BufferedReader(
				new StringReader(
				BufferedInputFile.read("C:\\Users\\zm\\workspace\\Thinking in java\\src\\io\\BasicFileOutput.java")));
		PrintWriter out = new PrintWriter(new BufferedWriter(new FileWriter(
				file)));
		int lineCount = 1;
		String s;
		while ((s = in.readLine()) != null)
			out.println(lineCount++ + ": " + s);
		out.close();
		// Show the stored file:
		System.out.println(BufferedInputFile.read(file));
	}
}

快捷用法，PrintWriter提供了进行缓存操作构造函数，省去了我们自己进行包装。

package io;

// 输出方式的快捷用法，PrintWriter提供了进行缓存操作构造函数
import java.io.*;

public class FileOutputShortcut {
	static String file = "C:\\Users\\zm\\workspace\\Thinking in java\\src\\io\\FileOutputShortcut.out";

	public static void main(String[] args) throws IOException {
		BufferedReader in = new BufferedReader(new StringReader(
				BufferedInputFile.read("C:\\Users\\zm\\workspace\\Thinking in java\\src\\io\\FileOutputShortcut.java")));
		// Here's the shortcut:
		PrintWriter out = new PrintWriter(file);
		int lineCount = 1;
		String s;
		while ((s = in.readLine()) != null)
			out.println(lineCount++ + ": " + s);
		out.close();
		// Show the stored file:
		System.out.println(BufferedInputFile.read(file));
	}
}

读取和写入工具类

//: net/mindview/util/TextFile.java
// Static functions for reading and writing text files as
// a single string, and treating a file as an ArrayList.
package net.mindview.util;

import java.io.*;
import java.util.*;

public class TextFile extends ArrayList<String> {
	private static String gpfile = "C:\\Users\\zm\\workspace\\Thinking in java\\src\\net\\mindview\\util\\";
	// Read a file as a single string:
	public static String read(String fileName) {
		StringBuilder sb = new StringBuilder();
		try {
			BufferedReader in = new BufferedReader(new FileReader(new File(
					fileName).getAbsoluteFile()));
			try {
				String s;
				while ((s = in.readLine()) != null) {
					sb.append(s);
					sb.append("\n");
				}
			} finally {
				in.close();
			}
		} catch (IOException e) {
			throw new RuntimeException(e);
		}
		return sb.toString();
	}

	// Write a single file in one method call:
	public static void write(String fileName, String text) {
		try {
			PrintWriter out = new PrintWriter(new File(fileName)
					.getAbsoluteFile());
			try {
				out.print(text);
			} finally {
				out.close();
			}
		} catch (IOException e) {
			throw new RuntimeException(e);
		}
	}

	// Read a file, split by any regular expression:
	public TextFile(String fileName, String splitter) {
		super(Arrays.asList(read(fileName).split(splitter)));
		// Regular expression split() often leaves an empty
		// String at the first position:
		if (get(0).equals(""))
			remove(0);
	}

	// Normally read by lines:
	public TextFile(String fileName) {
		this(fileName, "\n");
	}

	public void write(String fileName) {
		try {
			PrintWriter out = new PrintWriter(new File(fileName)
					.getAbsoluteFile());
			try {
				for (String item : this)
					out.println(item);
			} finally {
				out.close();
			}
		} catch (IOException e) {
			throw new RuntimeException(e);
		}
	}

	// Simple test:
	public static void main(String[] args) {
		String file = read(gpfile + "TextFile.java");
		write("d:\\test.txt", file);
		TextFile text = new TextFile("d:\\test.txt");
		text.write("d:\\test2.txt");
		// Break into unique sorted list of words:
		TreeSet<String> words = new TreeSet<String>(new TextFile(
				gpfile + "TextFile.java", "\\W+"));
		// Display the capitalized words:
		System.out.println(words.headSet("a"));
	}
}