码海拾遗 · Java I/O 学习笔记

一、标准输入输出控制台1. 标准输出System.outSystem.out.print()/println()/printf()最常用的控制台输出。javaSystem.out.println(普通输出); System.out.printf(格式化输出%d %d %d%n, 3, 5, 3 5);2. 标准输入System.inSystem.in是字节流InputStream通常用Scanner或BufferedReader包装。方式一Scanner最常用javaimport java.util.Scanner; Scanner sc new Scanner(System.in); System.out.print(请输入姓名); String name sc.nextLine(); System.out.print(请输入年龄); int age sc.nextInt(); System.out.println(name 今年 age 岁); sc.close();方式二BufferedReader效率高需处理异常javaimport java.io.BufferedReader; import java.io.InputStreamReader; BufferedReader br new BufferedReader(new InputStreamReader(System.in)); System.out.print(请输入内容); String line br.readLine(); System.out.println(你输入了 line); br.close();二、文件 I/O 基础Java I/O 分为字节流和字符流顶层抽象类字节流InputStream/OutputStream字符流Reader/Writer1. 字节流读写文件适合图片、视频等二进制文件文件输入流FileInputStreamjavaimport java.io.FileInputStream; import java.io.IOException; try (FileInputStream fis new FileInputStream(test.txt)) { int b; while ((b fis.read()) ! -1) { System.out.print((char) b); } } catch (IOException e) { e.printStackTrace(); }文件输出流FileOutputStreamjavaimport java.io.FileOutputStream; import java.io.IOException; try (FileOutputStream fos new FileOutputStream(output.txt)) { String data Hello, Java I/O!; fos.write(data.getBytes()); } catch (IOException e) { e.printStackTrace(); }拷贝文件字节流一次读取一个字节数组javatry (FileInputStream fis new FileInputStream(src.jpg); FileOutputStream fos new FileOutputStream(dst.jpg)) { byte[] buffer new byte[1024]; int len; while ((len fis.read(buffer)) ! -1) { fos.write(buffer, 0, len); } } catch (IOException e) { e.printStackTrace(); }2. 字符流读写文件适合文本文件FileReader / FileWriterjavaimport java.io.FileReader; import java.io.FileWriter; import java.io.IOException; // 写文本文件 try (FileWriter fw new FileWriter(note.txt)) { fw.write(第一行内容\n); fw.write(第二行内容); } catch (IOException e) { e.printStackTrace(); } // 读文本文件 try (FileReader fr new FileReader(note.txt)) { int ch; while ((ch fr.read()) ! -1) { System.out.print((char) ch); } } catch (IOException e) { e.printStackTrace(); }3. 缓冲流提高性能BufferedInputStream/BufferedOutputStream字节缓冲BufferedReader/BufferedWriter字符缓冲支持按行读写使用 BufferedReader 按行读取javaimport java.io.BufferedReader; import java.io.FileReader; import java.io.IOException; try (BufferedReader br new BufferedReader(new FileReader(data.txt))) { String line; while ((line br.readLine()) ! null) { System.out.println(line); } } catch (IOException e) { e.printStackTrace(); }使用 BufferedWriter 按行写入javaimport java.io.BufferedWriter; import java.io.FileWriter; import java.io.IOException; try (BufferedWriter bw new BufferedWriter(new FileWriter(out.txt))) { bw.write(第一行); bw.newLine(); bw.write(第二行); } catch (IOException e) { e.printStackTrace(); }三、转换流字节 ↔ 字符InputStreamReader字节→字符OutputStreamWriter字符→字节通常用于指定编码如 UTF-8, GBK。javaimport java.io.*; // 以 UTF-8 编码读取文件 try (BufferedReader br new BufferedReader( new InputStreamReader(new FileInputStream(utf8.txt), UTF-8))) { String line; while ((line br.readLine()) ! null) { System.out.println(line); } } catch (IOException e) { e.printStackTrace(); } // 以 GBK 编码写入文件 try (BufferedWriter bw new BufferedWriter( new OutputStreamWriter(new FileOutputStream(gbk.txt), GBK))) { bw.write(中文内容); } catch (IOException e) { e.printStackTrace(); }四、NIONew I/OJava 1.4NIO 提供更高效的非阻塞 I/O核心ChannelBufferSelector。简单示例使用 FileChannel 拷贝文件javaimport java.io.*; import java.nio.channels.FileChannel; try (FileChannel src new FileInputStream(src.txt).getChannel(); FileChannel dest new FileOutputStream(dest.txt).getChannel()) { dest.transferFrom(src, 0, src.size()); } catch (IOException e) { e.printStackTrace(); }使用 Files 工具类Java 7最简洁javaimport java.nio.file.*; // 一次性读取所有行 ListString lines Files.readAllLines(Paths.get(data.txt), StandardCharsets.UTF_8); lines.forEach(System.out::println); // 一次性写入 ListString content Arrays.asList(行1, 行2, 行3); Files.write(Paths.get(out.txt), content, StandardCharsets.UTF_8); // 复制文件 Files.copy(Paths.get(a.txt), Paths.get(b.txt), StandardCopyOption.REPLACE_EXISTING);五、常见问题与最佳实践问题建议中文乱码使用InputStreamReader/OutputStreamWriter指定编码如UTF-8忘记关闭流使用try-with-resourcesJava 7自动关闭效率低使用缓冲流BufferedReader/BufferedWriter大文件读取使用BufferedReader按行读或 NIO 的Files.lines()需要随机访问使用RandomAccessFile或 NIO 的FileChannel六、完整综合示例含异常处理javaimport java.io.*; import java.nio.charset.StandardCharsets; import java.nio.file.*; public class IODemo { public static void main(String[] args) { // 写入 try (BufferedWriter bw Files.newBufferedWriter(Paths.get(demo.txt), StandardCharsets.UTF_8)) { bw.write(Hello, 你好); bw.newLine(); bw.write(第二行); } catch (IOException e) { e.printStackTrace(); } // 读取 try (BufferedReader br Files.newBufferedReader(Paths.get(demo.txt), StandardCharsets.UTF_8)) { String line; while ((line br.readLine()) ! null) { System.out.println(line); } } catch (IOException e) { e.printStackTrace(); } } }