Java核心技术：随机访问文件

import java.io.*;
import java.time.LocalDate;
import java.util.Scanner;

public class Test {
    public static void main(String[] args){
//        TextFileTest.test();
        RandomAccessTest.test();
    }
}

class Employee {
    private String name;
    private double salary;
    private LocalDate hireDay;
    public static final int NAME_SIZE = 30;
    public static final int RECORD_SIZE = 50;

    public Employee(String name, double salary, LocalDate hireDay) {
        this.name = name;
        this.salary = salary;
        this.hireDay = hireDay;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public double getSalary() {
        return salary;
    }

    public void setSalary(double salary) {
        this.salary = salary;
    }

    public LocalDate getHireDay() {
        return hireDay;
    }

    public void setHireDay(LocalDate hireDay) {
        this.hireDay = hireDay;
    }

    @Override
    public String toString() {
        return "Employee{" +
                "name=‘" + name + ‘\‘‘ +
                ", salary=" + salary +
                ", hireDay=" + hireDay +
                ‘}‘;
    }
}

class TextFileTest {

    public static void test() {
        Employee[] staff = new Employee[]{
                new Employee("A", 10, LocalDate.now()),
                new Employee("B", 20, LocalDate.now()),
                new Employee("C", 30, LocalDate.now())
        };

        try(PrintWriter out = new PrintWriter("employee.dat","UTF-8")) {

            writeData(staff,out);

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

        try (Scanner in = new Scanner(new FileInputStream("employee.dat"))) {

            Employee[] newStaff = readData(in);

            for (Employee e : newStaff) {
                System.out.println(e);
            }

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

    }

    private static void writeData(Employee[] employees, PrintWriter out) {
        out.println(employees.length);
        for (Employee e : employees) {
            writeEmployee(out,e);
        }
    }

    private static void writeEmployee(PrintWriter out, Employee employee) {
        out.println(employee.getName()+"|"+employee.getSalary()+"|"+ employee.getHireDay());
    }

    private static Employee[] readData(Scanner in) {
        int n = in.nextInt();
        in.nextLine();

        Employee[] employees = new Employee[n];
        for (int i = 0; i < n; i++) {
            employees[i] = readEmployee(in);
        }
        return employees;
    }

    private static Employee readEmployee(Scanner in) {
        String line = in.nextLine();
        String[] tokens = line.split("\\|");

        String name = tokens[0];
        double salary = Double.parseDouble(tokens[1]);
        LocalDate hire = LocalDate.parse(tokens[2]);

        return new Employee(name, salary, hire);
    }
}

/*
    写了大半天，突然发现这个实验好像不是太严谨：
        1.RandomAccessFile算长度时，应该是根据字节数算出来的
        2.写字符串时，我们只是指定了码元数量，我们写的是固定码元数量的字符串
        3.这样的化，我们记录的Employee.RECORD_SIZE根本就代表不了一条记录的长度
        4.但是我们最后又通过RandomAccessFile的长度和Employee.RECORD_SIZE来计算记录数量
        5.我觉得这个实验有问题，以后研究吧
 */

class DataIO {
    //Java核心技术 卷二 第十版 2.3.2节
    //写出从字符串开头开始的指定数量的码元，如果码元过少，该方法会用‘0’来补齐字符串
    public static void writeFixedString(String s, int size, DataOutput output) throws IOException {
        for (int i = 0; i < size; i++) {
            char ch =0;
            if(i<s.length())
                ch = s.charAt(i);
            output.write(ch);
        }
    }

    //Java核心技术 卷二 第十版 2.3.2节
    //从输入流中读入字符，直至读入size个码元，或者直至遇到具有0值的字符串，然后跳出输入字段中剩余的0值。
    public static String readFixedString1(int size, DataInput in) throws IOException {
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < size; i++) {
            char c;
            if ((c = in.readChar()) != 0) {
                sb.append(c);
            }
        }
        return sb.toString();
    }

    //功能和上一个方法是一样的，但是这个效率会高那么一点点
    public static String readFixedString2(int size, DataInput in) throws IOException {
        StringBuilder sb = new StringBuilder();
        /*
        int i;
        for (i = 0; i < size; i++) {
            char c;
            if ((c = in.readChar()) == 0) {
                break;
            }
            sb.append(c);
        }
        in.skipBytes(2*(size-i));   //这个地方不是太严谨
        */

        //用书中代码测试一下
        int i =0;
        boolean more = true;
        while (more && i < size) {
            char ch = in.readChar();
            i++;
            if (ch == 0) {
                more = false;
            } else {
                sb.append(ch);
            }
        }
        in.skipBytes(2 * (size - i));

        return sb.toString();
    }
}

class RandomAccessTest {
    public static void test() {
        Employee[] staff = new Employee[]{
                new Employee("A", 10, LocalDate.now()),
                new Employee("B", 20, LocalDate.now()),
                new Employee("C", 30, LocalDate.now())
        };

        //写入
        try(DataOutputStream out = new DataOutputStream(new FileOutputStream("employee1.dat"))) {

            for (Employee e : staff) {
                writeData(out, e);
            }

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

        //读取

        try(RandomAccessFile in = new RandomAccessFile("employee1.dat","r")) {
            int n = (int) (in.length() / Employee.RECORD_SIZE);
            Employee[] newStaff = new Employee[n];

            for (int i = n - 1; i >= 0; i--) {
                in.seek(i*Employee.RECORD_SIZE);
                newStaff[i] = readDate(in);
            }

            for (Employee e : newStaff) {
                System.out.println(e);
            }
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

    }

    private static void writeData(DataOutput out, Employee employee) throws IOException {
        DataIO.writeFixedString(employee.getName(), Employee.NAME_SIZE, out);
        out.writeDouble(employee.getSalary());

        LocalDate hireDay = employee.getHireDay();
        out.writeInt(hireDay.getYear());
        out.writeInt(hireDay.getMonthValue());
        out.writeInt(hireDay.getDayOfMonth());
    }

    private static Employee readDate(DataInput input) throws IOException {
        String name = DataIO.readFixedString2(Employee.NAME_SIZE, input);
        double salary = input.readDouble();
        int
                y= input.readInt(),
                m= input.readInt(),
                d= input.readInt();
        return new Employee(name, salary, LocalDate.of(y, m, d));
    }
}

原文地址：https://www.cnblogs.com/junjie2019/p/10543843.html