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README.md

🖨️ Chapter 8 - IO - Revision Notes ✍️

🟥 8.1 Files and Directories

  • You can instantiate a file using either a String representing the location or using another File instance which is the parent directory
File parent = new File("/home/smith");
File child = new File(parent, "data/zoo.txt");

  • We have methods to make directories, confirm existence, delete, rename and list files.

  • We do NOT have methods to MOVE/COPY

  • Here are the methods available to File class:

boolean exists();
String getName(); // gets the name of file or directory. E.g. zoo.txt
boolean isDirectory();
boolean isFile();
boolean delete(); // deletes file OR directory if empty
boolean renameTo(File file);
boolean mkdir(File directoryFile); // makes directory file denoted by file
boolean mkdirs(File file); // makes directory AND file even for dirs which don't exist
String getParent(); // returns absolute filepath of parent directory
File[] listFiles(); // returns a File[] denoting files in directory
long lastModified(); // returns no. of ms since the epoch the file was last modified

🟥 8.2 Streams

🟡 Input/Output Stream Vs Reader/Writer

  • There four abstract classes in java.io:
  1. Reader 📖
  2. Writer ✍️
  3. InputStream 👉0️⃣
  4. OutputStream 1️⃣👉
  • These classes can be divided into two types
  1. Classes which have InputStream/OutputStream in their name
  • Used for reading/writing BINARY DATA
  1. Classes which have Reader/Writer in their name
  • Used for reading/writing CHARACTER/STRING DATA

🟡 Low-Level Streams

  • LOW LEVEL streams connect directly to source of data
Class Description
FileInputStream This is an InputStream and reads binary data from file
FileOutputStream This is an OutputStream and writes binary data to file
FileReader This is a Reader and reads character data from file
FileWriter This is a Writer and writes character data from file
  • HIGH LEVEL streams are used to wrap another stream (the final 2 are not in the exam)
Class Description
BufferedReader This is a Reader and takes in a low level FileReader
BufferedWriter This is a Writer and takes in a low level stream FileWriter
BufferedInputStream This is a InputStream which takes a low level FileInputStream
BufferedOutputStream This is a OutputStream which takes a low level FileOutputStream
ObjectInputStream This deserializer is an InputStream and takes either FileInputStream OR BufferedInputStream
ObjectOutputStream This serializer is an OutputStream and takes either FileOutputStream OR BufferedOutputStream
PrintStream This is a OutputStream which gives useful methods for writing and formatting data
PrintWriter This is a Writer which gives useful methods for writing and formatting data
InputStreamReader This is a Reader which takes an InputStream (e.g. FileInputStream, ObjectInputStream)
OutputStreamWriter This is a Writer which takes an OutputStream (e.g. FileOutputStream)

🟡 Common Stream Operations

  • Streams should be closed via close() method to prevent resouce leaks and the program deadlocking. You can also use the try-with-resources syntax to do this automatically💡
  • You should use the flush() method to ensure all data is written to disk when writing data with OutputStream
  • Only some streams support marking and resetting. Check that it is supported using boolean markSupported() otherwise you can encounter an UnsupportedOperationException⚠️

🟥 8.3 Working With Streams

🟡 FileInputStream and FileOutputStream Classes

  • These are LOW LEVEL InputStream/OutputStream classes which read/write binary data to files
try (InputStream fileInputStream = new FileInputStream(new File("data.txt")) {
	int b;
	while((b=fileInputStream()) != -1) {
		System.out.print((char)b);
	}
}

🟢 BufferedInputStream and BufferedOutputStream Classes

  • This are HIGH LEVEL InputStream/OutputStream classes which take in low level FileInputStream/FileOutputStream classes!
BufferedInputStream bufferedInputStream
	= new BufferedInputStream(
		new FileInputStream(
			new File(alphabetFile)));
int b;
while ((b=bufferedInputStream.read()) != -1) {
	System.out.println((char)b);
}

🟡 FileReader and FileWriter Classes

  • These classes offer AUTOMATIC CHARACTER ENCODING💡
  • These are LOW LEVEL Reader/Writer classes which read and write String data
  • We have a int read() method for the reader ,and a void write(String) method for the writer
FileReader fileReader = new FileReader(alphabetFile);
int b;
while((b=fileReader.read()) != -1) {
	System.out.println((char)b);
}

🟢 BufferedReader and BufferedWriter

  • These classes are HIGH LEVEL wrappers, there is also a ReadLine() method which is useful!
try (BufferedWriter writer = new BufferedWriter(new FileWriter(alphabetFile));
	 BufferedReader reader = new BufferedReader(new FileReader(alphabetFile));) {
	writer.write("abcd\nefghi");
	String line;
	while((line=reader.readLine())!=null) {
		System.out.println(line);
	}
}

🟡 ObjectInputStream and ObjectOutputStream Classes

  • These classes allow for SERIALIZATION and DESERIALIZATION of Java objects:
  • The ObjectInputStream takes in an InputStream in the form of a FileInputStream or BufferedInputStream
ObjectOutputStream writer = new ObjectOutputStream(
	new FileOutputStream(outputFile));
writer.writeObject("hello world");

ObjectInputStream reader = new ObjectInputStream(
	new FleInputStream(outputFile));
// reader.read(); // returns -1
reader.readObject(); // hello world is returned

🟢 Serializable Interface

  • Any abstract, concrete or final class can be marked as serializable
  • Attempting to serialize a non-serializable instance will not cause compilation errors but WILL THROW NotSerializableException:
objectOutputStream.writeObject(new NotSerializable()); // THROWS NotSerializableException

🟢 How Objects are Created When Deserializing

  • Static and transient variables are ignored during serialization/deserialization
    • Transient variables will be the default value java gives to the instance type (e.g. null for Strings)
    • Static variables will have the value of the last value it was assigned to in the program
  • When an object is deserialized, the constructor of the class is NOT called
    • java will call the first no-argument constructor for the first nonserializable parent class

🟡 PrintStream and PrintWriter Classes

  • These classes offer nice methods like format(String, args) and printf(String, args) which are functionally the same
  • PrintStream is an OutputStream
  • PrintWriter is a Writer

🟥 8.4 Interacting With Users

🟡 Old Way: Using Buffered Reader

  • System.out returns a PrintStream
  • System.in returns an InputStream.
  • We use InputStreamReader to capture input from the user:
Reader reader = InputStreamReader(System.in);
System.out.println("Enter a character");
		int input = reader.read();
		System.out.println("You entered: "+(char)input);
  • BufferedReader enables us to read String terminated by enter key by the user
BufferedReader bufferedReader = 
	new BufferedReader(new InputStreamReader(System.in));
System.out.println("Please enter a string:");
String input = bufferedReader.readLine();
System.out.printf("You entered: %s", input);

🟡 New Way: Using Console

  • The Console is a singleton which is accessible using Sytem.console()
  • There is a risk the console can be null⚠️
  • The console has the format()/printf() that PrintStream also has
  • The console has the following methods:
    • String readLine()
    • char[] readPassword() - a secure way of reading password without invoking String pool!
    • PrintWriter writer() - gives a writer instance for methods like print()
  • Example program:
Console console = System.console();
console.printf("Hi, %s! Please enter some text:", "shiv");
String input = console.readLine();
console.writer().println("You typed: "+input);