Why does it not work?

Question about the task

Under discussion

I do know what is a problem with this solution. I think it can be problem with 40 line.

Read 2 file names from the console.
Write the contents of the second file to the beginning of the first file so that the files are combined.
Close the streams.

Requirements:

The program should read a file name twice from the console.
Create an input stream for the first file and read its contents.
Then create an output stream for the first file. And an input stream for the second file.
The contents of the first and second files must be combined in the first file.
The contents of the second file should go first, then the contents of the first file.
The file streams must be closed.

package com.codegym.task.task18.task1819; /* Combining files */ import java.io.FileInputStream; import java.io.FileNotFoundException; import java.io.FileOutputStream; import java.io.IOException; import java.util.Scanner; public class Solution { public static void main(String[] args) throws IOException { Scanner sc=new Scanner(System.in); String file1=sc.nextLine(); String file2=sc.nextLine(); try( FileInputStream fileInputStreamFile1=new FileInputStream(file1); FileOutputStream fileOutputStreamFile1=new FileOutputStream(file1); FileInputStream fileInputStreamFile2=new FileInputStream(file2); ) { byte[] bytes = new byte[fileInputStreamFile1.available()]; int count = fileInputStreamFile1.read(bytes); byte[] bytes2=new byte[fileInputStreamFile2.available()]; int count2=fileInputStreamFile2.read(bytes2); fileOutputStreamFile1.write(bytes2, 0, bytes2.length); fileOutputStreamFile1.write(bytes,count2+1,bytes.length); } } }

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Krystian C

Level 27 , San Francisco, United States

26 January 2020, 19:02

Ok i passed this. Is this good solution? ByteArrayOutputStream baos = new ByteArrayOutputStream(); baos.write(bytes2); baos.write(bytes); byte[] c = baos.toByteArray(); fileOutputStreamFile1.write(c);

Guadalupe Gagnon

Level 37 , Tampa, United States

27 January 2020, 17:01

With your original code, the main problem is that when you create an OuputStream, without marking appendas true, It creates a new file and will replace any files that exist already. So by creating the OutputStream before reading in the data you effectively erased all the data beforehand. Next, The read() method of Inputstreams return how many bytes were read, which is useful information when using a buffer where you do not read all the bytes in. The way that you would use this is in a loop where you read a chunk of data and write it using an OutputStream. OutputStreams write in sequential order from the bytes passed into the write() method. I think that you are trying to use the counts to set the place in the file where you want the write to occur, but what you are doing is setting the start point of the buffer array you are passing in. For example: Lets say you have a file that is length 5:

public static void main(String[] args) throws IOException {
        String file1 = "5 byte file";
        try (
                FileInputStream f1 = new FileInputStream(file1);
        ) {
            byte[] bytes = new byte[f1.available()];
            int count = f1.read(bytes); // count = 5

            try (FileOutputStream fileOutputStreamFile1 = new FileOutputStream(file1)) {
                fileOutputStreamFile1.write(bytes, count, bytes.length);
            }
        }
    }

Line 9 is attempting to write the bytes from the array starting at index 5, which will throw an IndexOutOfBoundsException. In your code line 39 is acceptable, but line 40 has the problem detailed above. Since the writes are sequential, you can just do this:

fileOutputStreamFile1.write(bytes2);
fileOutputStreamFile1.write(bytes);

Guadalupe Gagnon

Level 37 , Tampa, United States

27 January 2020, 17:04

Here is an example of using a buffer where you are reading in chunks at a time:

public static void main(String[] args) throws IOException {
        String file1 = "file that is 134 bytes long";
        String file2 = "some other file";

        try (
                FileInputStream f1 = new FileInputStream(file1);
                FileOutputStream os = new FileOutputStream(file2)
        ) {

            byte[] bytes = new byte[5];
            while(f1.available() > 0) {
                int count = f1.read(bytes);
                os.write(bytes, 0, count);
            }
        }
    }

In this example the buffer is only 5 bytes in size while the file is 134 bytes. It will read in chunks of 5 bytes at a time as long as there is still data to read. On the last read it will only read in 4 bytes instead of 5, so using the count variable will stop the write method from writing a full 5 bytes.

Guadalupe Gagnon

Level 37 , Tampa, United States

27 January 2020, 17:16

useful

I use ByteArrayOutputStreams as a way to store data. A quick example:

public static void main(String[] args) throws IOException {
        Scanner sc=new Scanner(System.in);
        ArrayList<String> files = new ArrayList<>();
        while(true){
            String file = sc.nextLine();
            if(file.equals("exit")) break;
            files.add(file);

        } // infinite files possible
        ByteArrayOutputStream bos = new ByteArrayOutputStream();

        for(String file : files){
            try(FileInputStream fis = new FileInputStream(file)) {
                byte[] data = new byte[fis.available()];
                fis.read(data);
                bos.write(data); // saves all the data from files
            }
        }
        byte[] data = bos.toByteArray();

        // now you can do what you want with ALL the data
    }

This is useful for stitching together a single file that has been split into parts, or reading from a zip file.