复制一个已经存在的文本文件，生成一个新文件。 2、用PrintWriter流将九九乘法表写入到文件99.txt中。 3、将斐波那契数列的前10项写入到随机存取文件fei.dat中。 4、编程读取fei.dat文件中所有数据，计算其累加和，并添加到原有数据的后面。 5、将fei.dat文件的最后修改时间修改为前一天。

以下是Java代码实现所有要求：

1. 复制文件

import java.io.*;

public class FileCopy {
    public static void main(String[] args) throws IOException {
        File file1 = new File("file1.txt");
        File file2 = new File("file2.txt");
        FileInputStream fis = new FileInputStream(file1);
        FileOutputStream fos = new FileOutputStream(file2);
        byte[] buffer = new byte[1024];
        int len;
        while ((len = fis.read(buffer)) != -1) {
            fos.write(buffer, 0, len);
        }
        fis.close();
        fos.close();
    }
}

2. 写入九九乘法表

import java.io.*;

public class WriteMultiplicationTableToFile {
    public static void main(String[] args) throws IOException {
        File file = new File("99.txt");
        PrintWriter pw = new PrintWriter(file);
        for (int i = 1; i <= 9; i++) {
            for (int j = 1; j <= i; j++) {
                pw.print(j + "*" + i + "=" + (j * i) + "\t");
            }
            pw.println();
        }
        pw.close();
    }
}

3. 写入斐波那契数列

import java.io.*;

public class WriteFibonacciSequenceToFile {
    public static void main(String[] args) throws IOException {
        File file = new File("fei.dat");
        RandomAccessFile raf = new RandomAccessFile(file, "rw");
        int[] fibonacci = new int[]{1, 1, 2, 3, 5, 8, 13, 21, 34, 55};
        for (int i = 0; i < fibonacci.length; i++) {
            raf.writeInt(fibonacci[i]);
        }
        raf.close();
    }
}

4. 读取斐波那契数列并计算累加和

import java.io.*;

public class ReadFibonacciSequenceFromFile {
    public static void main(String[] args) throws IOException {
        File file = new File("fei.dat");
        RandomAccessFile raf = new RandomAccessFile(file, "rw");
        int sum = 0;
        int length = (int) (raf.length() / 4);
        for (int i = 0; i < length; i++) {
            sum += raf.readInt();
        }
        raf.seek(raf.length());
        raf.writeInt(sum);
        raf.close();
    }
}

5. 修改文件最后修改时间

import java.io.*;
import java.util.Date;

public class ModifyFileLastModifiedTime {
    public static void main(String[] args) {
        File file = new File("fei.dat");
        long lastModified = file.lastModified();
        long oneDay = 24 * 60 * 60 * 1000L;
        file.setLastModified(lastModified - oneDay);
    }
}