CS1021 - File IO

name: inverse
layout: true
class: taylor msoe
---
class: center, middle

.title[
  # File IO
]

???

* **P** speaker view
* **C** clone slideshow
* **?** help

---
# File IO

* `java.nio.file.Path` is an interface used to locate a file/directory in a file system

--
* `Paths.get(String)` — Returns a `Path` object

--
* `java.nio.file.Files` is a class with a number of useful class methods for reading/writing small files:

--
    + `Files.readAllLines(Path)` — Returns `List<String>` of all lines in the file
    + `Files.write(Path, Iterable<CharSequence>)` — Writes a `List<String>` containing lines of `String`s to the file

---
# Checked Exceptions

* When working with files, must handle checked exceptions (`IOException`)

--
* Two ways to handle a checked exception:
    + Catch the exception
    + Explictly declare that your method potentially throws the checked exception

---
# Input/Output Streams

* [`InputStream`](http://javadoc.taylorial.com/java.base/io/InputStream.html)s and [`OutputStream`](http://javadoc.taylorial.com/java.base/io/OutputStream.html)s can be used to move data into and out of a Java program.

--
* Abstract classes with one abstract method:
    + [`read()`](http://javadoc.taylorial.com/java.base/io/InputStream.html#read%28%29) — Reads the next byte of data from the input stream
    + [`write()`](http://javadoc.taylorial.com/java.base/io/OutputStream.html#write%28int%29) — Writes the specified byte to the output stream
--
* Subclasses for different data sources:
    + `AudioInputStream`
    + `ByteArrayInputStream`
    + `FileInputStream`
    + `FilterInputStream`
    + `ObjectInputStream`

---
# Streams with Files
* `Files` class provides methods for obtaining streams attached to files:
    + `Files.newInputStream(Path)` — Returns an `InputStream` attached to the file
    + `Files.newOutputStream(Path)` — Returns an `OutputStream` attached to the file

---
# Write Raw Bytes to Binary File
```
public static void writeByte(Path path, int data) throws IOException {
    try (OutputStream out = Files.newOutputStream(path)) {
        out.write(data);
    }
}
public static byte readByte(Path path) throws IOException {
    try (InputStream in = Files.newInputStream(path)) {
        return (byte)in.read();
    }
}
```

--
* `writeByte(Paths.get("stuff.bin"), 72)`; creates file with `0b01001000` in it

--
* Using try-with-resources causes file to be closed when `try` block completes

--
* File closed even if exception propogates up to calling method

---
# Open Options
* Can specify zero or more open options when requesting a new stream: `newOutputStream(Path, OpenOption...)`

--
* `...` after `OpenOption` indicates zero or more OpenOption objects may be passed

--
* Appears as an array in the method implementation

--
* `Files.newOutputStream(path);` — overwrite existing file, create if doesn't exist

--
* `Files.newOutputStream(path, APPEND);` — append to existing file, fail if doesn't exist

--
* `Files.newOutputStream(path, CREATE, APPEND);` — append to existing file, create if doesn't exist

--
* `Files.newOutputStream(path, CREATE_NEW);` — fail if existing file, create if doesn't exist

---
# `OpenOption` Interface
* Empty interface - relies on implementing classes

--
* `StandardOpenOption` — Enumerates standard opening options: `APPEND`, `CREATE`, `CREATE_NEW`, `READ`, `TRUNCATE_EXISTING`, `WRITE`, etc...

--
* Must use static imports to reference open options directly:

```
import static java.nio.file.StandardOpenOption.APPEND;

public static void writeAnotherByte(Path path, int data) throws NoSuchFileException, IOException {
    try (OutputStream out = Files.newOutputStream(path, APPEND)) {
        out.write(data);
    }
}
```

---
# Decorator Classes
* Decorator classes encapsulate a stream within an provide additional functionality
    + `DataInputStream` — Read primitive types from binary file
    + `CipherInputStream` — Read from an encrypted file
    + `ZipInputStream` — Read from a `.zip` compressed file
    + `GZIPInputStream` — Read from a `.gzip` compressed file
--
* Text input/output
    + `Scanner` — Read text from input stream
    + `PrintWriter` — Write text to output stream

---
# High-level Binary I/O
* Decorate/wrap `InputStream` in a `DataInputStream`
* Decorate/wrap `OutputStream` in a `DataOutputStream`

--
* Gain access to read/write methods for primitives
    + `readBoolean()`, `readByte()`, `readChar()`, `readDouble()`, `readFloat()`, `readInt()`, etc...
    + `writeBoolean(boolean)`, `writeByte(int)`, `writeChar(int)`, etc...

--
```
public static void writeDouble(Path path, double data) throws IOException {
    try (OutputStream out = Files.newOutputStream(path);
         DataOutputStream dOut = new DataOutputStream(out)) {
        dOut.writeDouble(data);
    }
}
public static double readDouble(Path path) throws IOException {
    try (DataInputStream in = new DataInputStream(Files.newInputStream(path))) {
        return in.readDouble();
    }
}
```

---
# Text I/O
* Much like using `System.in` and `System.out`

```
public static void writeLine(Path path, String line) throws IOException {
    try (PrintWriter out = new PrintWriter(Files.newOutputStream(path))) {
        out.println(line);
    }
}

public static String readLine(Path path) throws IOException {
    try (Scanner in = new Scanner(Files.newInputStream(path))) {
        return in.nextLine();
    }
}
```

---
# Object I/O
* `ObjectOutputStream` and `ObjectInputStream` classes provide the ability to send entire objects as blobs of data via a stream
* The object to be written/read to/from the stream must implement the `Serializable` interface
* Care should be taken when reading/writing objects since it introduces security concerns

```
public static void writeDate(Path path, Date date) throws IOException {
    try (ObjectOutputStream out = new ObjectOutputStream(Files.newOutputStream(path))) {
        out.writeObject(date);
    }
}

public static Date readDate(Path path) throws ClassNotFoundException, IOException {
    try (ObjectInputStream in = new ObjectInputStream(Files.newInputStream(path))) {
        return (Date)in.readObject();
    }
}
```

---
# Buffered Reader/Writer
* `Files` class also provides methods for obtaining a buffered reader or writer attached to files:
    + `Files.newBufferedReader(Path)` — Returns a `BufferedReader` attached to the file
    + `Files.newBufferedWriter(Path)` — Returns a `BufferedWriter` attached to the file
--
* `BufferedReader` and `BufferedWriter` read/write text from a buffered character stream.

--
* Without buffering, each invocation of `read()` could cause bytes to be read from the file (requiring interaction with the OS)

--
* OS interaction requires significant overhead.

--
* Buffered I/O can be significantly more efficient.