java第14周实验（IO流）

0. 字节流与二进制文件

我的代码

public class WriterStu {
    public static void main(String[] args) {
        DataOutputStream dos = null;
        Student[] stus = new Student[100];
        Student d1 = new Student(1, "x", 18, 99.5);
        Student d2 = new Student(2, "x", 19, 100.0);
        Student d3 = new Student(3, "x", 20, 59.5);

        try (FileOutputStream fos = new FileOutputStream(new File("e:/Student.data"))) {
            dos = new DataOutputStream(fos);
            for (Student student : stus) {
                dos.writeInt(student.getId());
                dos.writeUTF(student.getName());
                dos.writeInt(student.getAge());
                dos.writeDouble(student.getGrade());
            }
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        try (DataInputStream dis = new DataInputStream(new FileInputStream("e:/Student.data"))) {
            int id = dis.readInt();
            String name = dis.readUTF();
            int age = dis.readInt();
            double grade = dis.readDouble();
            Student stu=new Student(id,name,age,grade);
            System.out.println(stu);
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}     

我的总结

文本文件中的数据都是以字符的形式进行组织，通常可以逐行或全部读取到一个字符串变量中。二进制文件可以存储int/double/char等基本数据类型。

1. 字符流与文本文件：使用 PrintWriter(写)，BufferedReader(读)

我的代码

4.

String fileName1="e:/data.txt";
try(
            FileOutputStream fos=new FileOutputStream(fileName1);
            ObjectOutputStream oos=new ObjectOutputStream(fos))
        {
            Student s=new Student(5,"l",12,85);
            oos.writeObject(s);
        }
        catch (Exception e) {
      
            e.printStackTrace();
        }
        try(
            FileInputStream fis=new FileInputStream(fileName1);
            ObjectInputStream ois=new ObjectInputStream(fis))
        {
            Student newStudent =(Student)ois.readObject();
            System.out.println(newStudent);
        } catch (IOException e) {
            e.printStackTrace();
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }

我的总结

将对象流写在txt中会发生乱码，改为dat格式则不会发生乱码。

2. 缓冲流(结合使用JUint进行测试)

我的代码

public class PrintWriterTest {
    public static void main(String[] args) throws FileNotFoundException {
        File file = new File("e:/data.txt");
        if(file.exists())
        {
            System.out.println("文件已存在！");
            System.exit(0);
        }
        PrintWriter pw=new PrintWriter(file);
        Random r=new Random(100);
        for(int i=0;i<1000_0000;i++) {
            int j=r.nextInt(11);
            pw.println(j);
        }
        pw.close();
    }
}

JunitTest

public class JunitTest {

    @Test
    public void testBufferedReader() {
        BufferedReader br = null;
        int n = 0, sum = 0;
        double average = 0;
        try {
            br = new BufferedReader(new FileReader("e:/data.txt"));
            String line = null;

            try {
                while ((line = br.readLine()) != null) {
                    int num = Integer.parseInt(line);
                    n++;
                    sum += num;
                }
                average = sum / n;
            } catch (IOException e) {
                e.printStackTrace();
            }
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } finally {
            System.out.format("%d %d %.5f", n, sum, average);
            System.out.println();
            if (br != null) {
                try {
                    br.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }

    @Test
    public void testScanner() {
        File file = new File("e:/data.txt");
        Scanner sc = null;
        int n = 0, sum = 0;
        double average = 0;
        try {
            sc = new Scanner(file);
            while (sc.hasNext()) {
                int num = sc.nextInt();
                sum+=num;
                n++;
            }
            average = sum / n;
            System.out.format("%d %d %.5f", n, sum, average);
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }

        

    }
}

我的总结

使用BufferedReader更快，BufferedReader使用缓冲可以减少IO次数，因为IO操作很耗时间，所以读取速度变快。

3. 字节流之对象流

我的代码

public static void writeStudent(List<Student> stuList) {
        String fileName = "e:/data.txt";
        try (FileOutputStream fos = new FileOutputStream(fileName);
                ObjectOutputStream ois = new ObjectOutputStream(fos)) {
            ois.writeObject(stuList);

        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e1) {
            e1.printStackTrace();
        }
    }

    public static List<Student> readStudents(String fileName) {
        List<Student> stuList = new ArrayList<>();
        try (FileInputStream fis = new FileInputStream(fileName); ObjectInputStream ois = new ObjectInputStream(fis)) {
            stuList = (List<Student>) ois.readObject();
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e1) {
            e1.printStackTrace();
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
        return stuList;
    }

我的总结

ObjectOutputStream 和 ObjectInputStream 分别与 FileOutputStream 和 FileInputStream 一起使用时，可以为应用程序提供对对象图形的持久存储。

5. 文件操作

我的代码

import java.io.File;
import java.nio.file.Path;
import java.nio.file.Paths;

public class Files {
    Path dir=Paths.get("e:/data.txt");
    public static void findFile(Path dir,String fileName) {
        File file =dir.toFile();
        File[] files =file.listFiles();
        for(File file1:files) {
            if(file1.isDirectory()) {
                findFile(file1.toPath(),fileName);
            }else {
                if(file1.equals(new File(fileName))) {
                    System.out.println("Found:");
                    System.out.println(file1.getAbsolutePath());
                }
            }
        }
    }
}

我的总结

File类和Path类可以相互转换，Paths类可以直接获得Path对象，不需要new Path。

6. 正则表达式

判断一个给定的字符串是否是10进制数字格式

我的代码

import java.util.regex.Pattern;

public class testIsDigit {
    public static void main(String[] args) {
        System.out.println(isDigit("123a"));
        System.out.println(isDigit("-123"));
        System.out.println(isDigit("a123a"));
        System.out.println(isDigit("123"));
        System.out.println(isDigit("-"));
    }

    public static boolean isDigit(String s) {
        return Pattern.matches("^-?\\d+$", s);
    }
}

我的总结

熟悉正则表达式的语法可以帮助我们更加简便得编写代码。