example read 2 file

import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.BufferedReader;
import java.io.ByteArrayOutputStream;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStreamReader;

public class TestDrive {

  public static void main(String[] args) throws IOException {
    BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(System.in));
    String fullFileName1 = bufferedReader.readLine();
    String fullFileName2 = bufferedReader.readLine();

    BufferedInputStream bis1 = new BufferedInputStream(new FileInputStream(fullFileName1));
    BufferedInputStream bis2 = new BufferedInputStream(new FileInputStream(fullFileName2));

    ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream(
        bis1.available() + bis2.available());
    while (bis2.available() > 0) {
      byteArrayOutputStream.write(bis2.read());
    }
    while (bis1.available() > 0) {
      byteArrayOutputStream.write(bis1.read());
    }

    BufferedOutputStream bos = new BufferedOutputStream(new FileOutputStream(fullFileName1));

    bos.write(byteArrayOutputStream.toByteArray());
    bis1.close();
    bis2.close();
    bos.close();
  }

}