database.java

JAVA实现的数据结构

import java.io.*;

public class Database {
    private RandomAccessFile database;
    private String fName = new String();
    java.util.Scanner kb = new java.util.Scanner(System.in);
    Database() {
    }
    private void add(DbObject d) throws IOException {
        database = new RandomAccessFile(fName,"rw");
        database.seek(database.length());
        d.writeToFile(database);
        database.close();
    }
    private void modify(DbObject d) throws IOException {
        DbObject[] tmp = new DbObject[1];
        d.copy(tmp);
        database = new RandomAccessFile(fName,"rw");
        while (database.getFilePointer() < database.length()) {
            tmp[0].readFromFile(database);
            if (tmp[0].equals(d)) {
                 tmp[0].readFromConsole();
                 database.seek(database.getFilePointer()-d.size());
                 tmp[0].writeToFile(database);
                 database.close();
                 return;
            }
        }
        database.close();
        System.out.println("The record to be modified is not in the database");
    }
    private boolean find(DbObject d) throws IOException {
        DbObject[] tmp = new DbObject[1];
        d.copy(tmp);
        database = new RandomAccessFile(fName,"r");
        while (database.getFilePointer() < database.length()) {
            tmp[0].readFromFile(database);
            if (tmp[0].equals(d)) {
                database.close();
                return true;
            }
        }
        database.close();
        return false;
    }
    private void printDb(DbObject d) throws IOException {
        database = new RandomAccessFile(fName,"r");
        while (database.getFilePointer() < database.length()) {
            d.readFromFile(database);
            d.writeLegibly();
            System.out.println();
        }
        database.close();
    }
    public void run(DbObject rec) throws IOException {
        String option;
        System.out.print("File name: ");
        fName = kb.next();
        System.out.println("1. Add 2. Find 3. Modify a record; 4. Exit");
        System.out.print("Enter an option: ");
        option = kb.next();
        while (true) {
            if (option.charAt(0) == '1') {
                 rec.readFromConsole();
                 add(rec);
            }
            else if (option.charAt(0) == '2') {
                 rec.readKey();
                 System.out.print("The record is ");
                 if (find(rec) == false)
                     System.out.print("not ");
                 System.out.println("in the database");
            }
            else if (option.charAt(0) == '3') {
                 rec.readKey();
                 modify(rec);
            }
            else if (option.charAt(0) != '4')
                 System.out.println("Wrong option");
            else return;
            printDb(rec);
            System.out.println("1. Add 2. Find 3. Modify a record; 4. Exit");
            System.out.print("Enter an option: ");
            option = kb.next();
        }
    }
    static public void main(String[] a) throws IOException {
//      (new Database()).run(new Personal());
        (new Database()).run(new Student());
    }
}