Java 基础 IO

2023-04-11, by alamide

Java 基础语法 IO，学习内容来自《Java 编程的逻辑》

1.InputStream、OutputStream

读写一个文件

try(FileInputStream is = new FileInputStream("data.txt")) {
    byte[] bytes = new byte[1024];
    ByteArrayOutputStream baos = new ByteArrayOutputStream();
    int read = 0;
    while ((read = is.read(bytes)) != -1){
        baos.write(bytes, 0, read);
    }
    System.out.println(baos.toString(StandardCharsets.UTF_8));
}catch (Exception e){
    e.printStackTrace();
}

try (OutputStream out = new FileOutputStream("data.txt", true)) {
    out.write("\n".getBytes(StandardCharsets.UTF_8));
    out.write("Hello World".getBytes(StandardCharsets.UTF_8));
} catch (Exception e) {
    e.printStackTrace();
}

复制文件

try (FileInputStream is = new FileInputStream("data.txt");OutputStream out = new FileOutputStream("data_backup.txt", true)) {
    is.transferTo(out);
} catch (Exception e) {
    e.printStackTrace();
}

2.ByteArrayInputStream、ByteArrayOutputStream

ByteArrayInputStream byteArrayInputStream = new ByteArrayInputStream(new byte[]{});

3.DataInputStream、DataOutputStream

基本数据读取

@Test
public void readStudents() {
    try (DataInputStream dis = new DataInputStream(new FileInputStream("object.txt"))) {
        final int size = dis.readInt();
        System.out.println(size);
        List<Student> students = new ArrayList<>(size);
        for (int i = 0; i < size; i++) {
            Student student = new Student();
            student.setName(dis.readUTF());
            student.setId(dis.readUTF());
            student.setAge(dis.readInt());
            int length = dis.readInt();
            String[] hobbies = new String[length];
            for (int j = 0; j < length; j++) {
                hobbies[j] = dis.readUTF();
            }
            student.setHobbies(hobbies);
            student.setHeight(dis.readDouble());
            student.setWeight(dis.readDouble());
            students.add(student);
        }
        students.forEach(System.out::println);
    } catch (Exception e) {
        e.printStackTrace();
    }
}

@Test
public void saveStudents() {
    try (DataOutputStream dos = new DataOutputStream(new FileOutputStream("object.txt"))) {
        final List<Student> students = IntStream.rangeClosed(1, 10)
                .boxed()
                .map(i -> new Student("stud" + i, "10000" + i, 16 + i % 3, new String[]{"football", "game"}, 178.3, 65.3))
                .toList();
        dos.writeInt(students.size());
        for (Student student : students) {
            dos.writeUTF(student.getName());
            dos.writeUTF(student.getId());
            dos.writeInt(student.getAge());
            dos.writeInt(student.getHobbies().length);
            for (String hobby : student.getHobbies()) {
                dos.writeUTF(hobby);
            }
            dos.writeDouble(student.getHeight());
            dos.writeDouble(student.getWeight());
        }

        dos.flush();
    } catch (Exception e) {
        e.printStackTrace();
    }
}

4.BufferedInputStream、BufferedOutputStream

InputStream 的包装类，可以提高效率

ByteArrayOutputStream baos = new ByteArrayOutputStream();

try (FileInputStream is = new FileInputStream("data.txt")) {
    byte[] bytes = new byte[1024];
    int read = 0;
    while ((read = is.read(bytes)) != -1) {
        baos.write(bytes, 0, read);
    }
} catch (Exception e) {
    e.printStackTrace();
}

long start = System.currentTimeMillis();

try (FileOutputStream out = new FileOutputStream("data.txt")) {
    byte[] bytes = baos.toByteArray();
    for (byte b : bytes) {
        out.write(b);
    }
} catch (IOException e) {
    throw new RuntimeException(e);
}
log.info("FileOutputStream cost time: {}ms", (System.currentTimeMillis() - start));

start = System.currentTimeMillis();
try (BufferedOutputStream bos = new BufferedOutputStream(new FileOutputStream("data.txt"))) {
    byte[] bytes = baos.toByteArray();
    for (byte b : bytes) {
        bos.write(b);
    }
    bos.flush();
} catch (IOException e) {
    throw new RuntimeException(e);
}
log.info("BufferedOutputStream cost time: {}ms", (System.currentTimeMillis() - start));

output:

08:25:08.711 [main] INFO com.alamide.base.FileTest - FileOutputStream cost time: 9ms
08:25:08.718 [main] INFO com.alamide.base.FileTest - BufferedOutputStream cost time: 1ms

可以看到同样的数据，在按字节写时，使用 Buffer 的效率要高出很多倍

5.InputStreamReader、OutputStreamWriter

可以以字符串的形式写

@Test
public void testStreamReader() {
    try (Writer writer = new OutputStreamWriter(new FileOutputStream("data.txt", true))) {
        writer.write("\ntestStreamReader");
    } catch (IOException e) {
        throw new RuntimeException(e);
    }

    try (Reader reader = new InputStreamReader(new FileInputStream("data.txt"))) {
        char[] chars = new char[1024];
        int count = 0;
        StringBuilder stringBuilder = new StringBuilder();
        while ((count = reader.read(chars)) != -1){
            stringBuilder.append(Arrays.copyOfRange(chars, 0, count));
        }
        System.out.println(stringBuilder.toString());
    } catch (IOException e) {
        throw new RuntimeException(e);
    }
}

try (Writer writer = new FileWriter("data.txt", true)) {
    writer.write("\ntestStreamReader");
} catch (IOException e) {
    throw new RuntimeException(e);
}

try (Reader reader = new FileReader("data.txt")) {
    char[] chars = new char[1024];
    int count = 0;
    StringBuilder stringBuilder = new StringBuilder();
    while ((count = reader.read(chars)) != -1){
        stringBuilder.append(Arrays.copyOfRange(chars, 0, count));
    }
    System.out.println(stringBuilder.toString());
} catch (IOException e) {
    throw new RuntimeException(e);
}

6.FileReader、FileWriter

try (Writer writer = new FileWriter("data.txt", true)) {
    writer.write("\ntestStreamReader");
} catch (IOException e) {
    throw new RuntimeException(e);
}

try (Reader reader = new FileReader("data.txt")) {
    char[] chars = new char[1024];
    int count = 0;
    StringBuilder stringBuilder = new StringBuilder();
    while ((count = reader.read(chars)) != -1){
        stringBuilder.append(Arrays.copyOfRange(chars, 0, count));
    }
    System.out.println(stringBuilder.toString());
} catch (IOException e) {
    throw new RuntimeException(e);
}

7.CharArrayReader、CharArrayWriter

类似于 ByteArrayInputStream 、ByteArrayOutputStream

try (Reader reader = new FileReader("data.txt")) {
    char[] chars = new char[1024];
    int count = 0;
    CharArrayWriter charArrayWriter = new CharArrayWriter();
    while ((count = reader.read(chars)) != -1){
        charArrayWriter.write(chars, 0, count);
    }
    System.out.println(charArrayWriter.toString());
} catch (IOException e) {
    throw new RuntimeException(e);
}

8.StringReader、StringWriter

StringWriter stringWriter = new StringWriter();
stringWriter.write("Hello world!");
System.out.println(stringWriter);

StringReader stringReader = new StringReader("Hello world");

9.BufferedReader、BufferedWriter

可以按行存取数据

@Test
public void testBufferedWriter(){
    try(BufferedWriter bufferedWriter = new BufferedWriter(new FileWriter("data.txt"))){
        for (int integer = 1; integer <= 10; integer++) {
            bufferedWriter.newLine();
            bufferedWriter.write("Hello World" +integer);
        }
        bufferedWriter.flush();
    } catch (IOException e) {
        throw new RuntimeException(e);
    }

    try(BufferedReader bufferedReader = new BufferedReader(new FileReader("data.txt"))){
        String line;
        while ((line = bufferedReader.readLine()) != null){
            System.out.println(line);
        }
    } catch (IOException e) {
        throw new RuntimeException(e);
    }
}

10.PrintWriter

@Test
public void testPrintWriter(){
    try(PrintWriter printWriter = new PrintWriter(new FileWriter("data.txt", true))) {
        printWriter.printf("this is a %s", "test");
        printWriter.println("testPrintWriter");
    } catch (IOException e) {
        throw new RuntimeException(e);
    }
}

11.Scanner

Scanner是一个单独的类，它是一个简单的文本扫描器，能够分析基本类型和字符串，它需要一个分隔符来将不同数据区分开来，默认是使用空白符，可以通过useDelimiter()方法进行指定。

@Test
public void testScanner(){
    try(BufferedReader bufferedReader = new BufferedReader(new StringReader("2,suzhou,china,38.55,false"))){
        String line;
        while ((line = bufferedReader.readLine()) != null){
            Scanner scanner = new Scanner(line).useDelimiter(",");
            System.out.println(scanner.nextInt());
            System.out.println(scanner.next());
            System.out.println(scanner.next());
            System.out.println(scanner.nextDouble());
            System.out.println(scanner.nextBoolean());
        }
    } catch (IOException e) {
        throw new RuntimeException(e);
    }
}

从控制台读取数据

Scanner scanner = new Scanner(System.in);
int nextInt = scanner.nextInt();
System.out.println(nextInt);
String next = scanner.next();
System.out.println(next);

标准流重定向

@Test
public void testStreamRedirect() throws FileNotFoundException {
    System.setIn(new FileInputStream("data.txt"));
    System.setOut(new PrintStream("out.txt"));
    System.setErr(new PrintStream("error.txt"));
    try {
        Scanner scanner = new Scanner(System.in);
        System.out.println(scanner.nextLine());
        System.out.println(scanner.nextLine());
        System.out.println(scanner.nextLine());
    }catch (Exception e){
        System.err.println(e.getMessage());
    }
}

12.Properties

文件不能直接处理中文，非 ASCII 字符要使用 Unicode 编码

Properties properties = new Properties();
properties.load(getClass().getClassLoader().getResourceAsStream("config.properties"));
final String property = properties.getProperty("jdbc.username");
System.out.println(property);

13.Serializable

@Test
public void testSerializable(){
    try (ObjectOutputStream dos = new ObjectOutputStream(new FileOutputStream("object.txt"))) {
        final List<Student> students = IntStream.rangeClosed(1, 10)
                .boxed()
                .map(i -> new Student("stud" + i, "10000" + i, 16 + i % 3, new String[]{"football", "game"}, 178.3, 65.3))
                .toList();
        dos.writeObject(students);
    } catch (Exception e) {
        e.printStackTrace();
    }

    try (ObjectInputStream objectInputStream = new ObjectInputStream(new FileInputStream("object.txt"))){
        List<Student> students = (List<Student>) objectInputStream.readObject();
        students.forEach(System.out::println);
    } catch (IOException | ClassNotFoundException e) {
        throw new RuntimeException(e);
    }
}

Tags: Java - IO

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