How to open files with FileInputStream?

I have a problem, I am programming an app to encrypt files and I get the following error:

  

java.lang.RuntimeException: java.io.FileNotFoundException:   /tmp/File.txt: open failed: ENOENT (No such file or directory)

This is my code:

public void encriptar(String clave)throws IOException, NoSuchAlgorithmException, NoSuchPaddingException, InvalidKeyException {

    //Archivo de entrada
    FileInputStream Entrada = new FileInputStream("/tmp/Archivo.txt");

    //Archivo de salida, encriptado
    FileOutputStream Salida = new FileOutputStream("/tmp/salida.txt");

    // ¡Cuidado al tomar la entrada del usuario! Https://stackoverflow.com/a/3452620/1188357

    //Tamaño de el arhivo  16 bytes

    SecretKey sks = new SecretKeySpec(clave.getBytes(),"AES");

    //Se crea el cipher, se encargan de encriptar los byte

    Cipher cipher = Cipher.getInstance("AES");
    cipher.init(Cipher.ENCRYPT_MODE,sks);

    //stream de salida
    CipherOutputStream cos = new CipherOutputStream(Salida,cipher);

    //escribe bytes

    byte[] d = new byte[8];
    int b = Entrada.read(d);

    while(b != -1){

        cos.write(d,0,b);

        b = Entrada.read(d);
    }

    //cierra streams
    cos.flush();
    cos.close();
    Entrada.close();

}

and I call him from the OnCreate method of the main:

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    final ArrayList<File> Archivos = EncontrarArchivos(Environment.getExternalStorageDirectory());

    Button BotonEncriptar = (Button) findViewById(R.id.boton1);
    Button BotonDesencriptar = (Button) findViewById(R.id.boton2);
    final EditText EntradaClave = (EditText) findViewById(R.id.clavesita);

    BotonDesencriptar.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {

            String clave = EntradaClave.getText().toString();

            try {
                desencriptar(clave);

            } catch (IOException | NoSuchAlgorithmException | NoSuchPaddingException | InvalidKeyException k ) {
                throw new RuntimeException(k );
            }
        }
    });

             BotonEncriptar.setOnClickListener(new View.OnClickListener() {
                 @Override
                 public void onClick(View view) {

                    String clave = EntradaClave.getText().toString();

                     try {

                         encriptar(clave);

                     } catch (IOException | NoSuchAlgorithmException | NoSuchPaddingException | InvalidKeyException k ) {
                         throw new RuntimeException(k );

                     }

                 }
             });
}

but I get the following error:

07-16 01:01:25.562 19145-19145/en1gm4app.user.pruebaencryptararchivos E/AndroidRuntime: FATAL EXCEPTION: main
                                                                                    Process: en1gm4app.user.pruebaencryptararchivos, PID: 19145
                                                                                    java.lang.RuntimeException: java.io.FileNotFoundException: /tmp/Archivo.txt: open failed: ENOENT (No such file or directory)
                                                                                        at en1gm4app.user.pruebaencryptararchivos.MainActivity$2.onClick(MainActivity.java:73)
                                                                                        at android.view.View.performClick(View.java:5201)
                                                                                        at android.view.View$PerformClick.run(View.java:21215)
                                                                                        at android.os.Handler.handleCallback(Handler.java:739)
                                                                                        at android.os.Handler.dispatchMessage(Handler.java:95)
                                                                                        at android.os.Looper.loop(Looper.java:148)
                                                                                        at android.app.ActivityThread.main(ActivityThread.java:5525)
                                                                                        at java.lang.reflect.Method.invoke(Native Method)
                                                                                        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:730)
                                                                                        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:620)