讀檔案用BufferedReader類，寫檔案用BufferedWriter類。

讀檔案

讀檔案需要按順序建立FileInputStream類、InputStreamReader類、BufferedReader類的例項，具體如下：

FileInputStream fileInputStream = new FileInputStream(file);
InputStreamReader inputStreamReader = new InputStreamReader(fileInputStream, "UTF-8");
BufferedReader bufferedReader = new BufferedReader(inputStreamReader);
 

String line = bufferedReader.readLine();

bufferedReader.close();
inputStreamReader.close();
fileInputStream.close();

寫檔案

寫檔案需要按順序建立FileOutputStream類、OutputStreamWriter類、BufferedWriter類的例項，具體如下：

FileOutputStream fileOutputStream = new FileOutputStream(newfile);
OutputStreamWriter outputStreamWriter = new OutputStreamWriter(fileOutputStream, "UTF-8");
BufferedWriter bufferedWriter = new BufferedWriter(outputStreamWriter);

bufferedWriter.write("12345");
 

bufferedWriter.close();
outputStreamWriter.close();
fileOutputStream.close();

程式案例

import java.io.*;

public class FileIO {

	public static void main(String[] args) {
		// 檔案讀取
		File file = new File("file.txt");
		if (file.exists()) {
			try {
				// 建立檔案輸入流(這是一個位元組流)
				FileInputStream fileInputStream = new FileInputStream(file);
				// 建立輸入流reader，將位元組流轉換成字元流，且設定字元編碼為UTF-8(這是一個字元流)
				InputStreamReader inputStreamReader = new InputStreamReader(fileInputStream, "UTF-8");
				// 建立BufferedReader
				BufferedReader bufferedReader = new BufferedReader(inputStreamReader);

				// 逐行讀取
				String line;
				while ((line = bufferedReader.readLine()) != null) {
					System.out.println(line);
				}

				// 按照與建立相反的順序關閉物件
				bufferedReader.close();
				inputStreamReader.close();
				fileInputStream.close();

			} catch (FileNotFoundException e) {
				e.printStackTrace();
			} catch (UnsupportedEncodingException e) {
				e.printStackTrace();
			} catch (IOException e) {
				e.printStackTrace();
			}
		}

		// 檔案寫入
		File newfile = new File("newfile.txt");

		try {
			//建立檔案輸出流 (位元組流)
			FileOutputStream fileOutputStream = new FileOutputStream(newfile);
			//建立檔案輸出流(字元流)
			OutputStreamWriter outputStreamWriter = new OutputStreamWriter(fileOutputStream, "UTF-8");
			//建立檔案BufferedWriter
			BufferedWriter bufferedWriter = new BufferedWriter(outputStreamWriter);
			
			//輸出到檔案
			bufferedWriter.write("12354");

			//按照與建立相反的順序關閉
			bufferedWriter.close();
			outputStreamWriter.close();
			fileOutputStream.close();

		} catch (FileNotFoundException e) {
			e.printStackTrace();
		} catch (UnsupportedEncodingException e) {
			e.printStackTrace();
		} catch (IOException e) {
			e.printStackTrace();
		}

	}

}