java第14周实验（IO流）

Posted 2021-03-24 舟渡

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。