A data structure is a way of storing data in an organised but accessible format, and typically different data structures lend themselves to different contexts.

An abstract data type is a purely conceptual model of how this data is stored and operations that be done on them.

Arrays

An array is a sequential list of data, where each item of data is called an element. These can have more than one dimension e.g. for a 2D image, you would use a two-dimension array to store the pixel values. In Java, we use the syntax:

// when you have values to assign
int[] ages = {6, 7, 3, 23};
float[] weights = {60.5, 23.1, 223.4, 40.2};

// when you just want zeroes
int[][] pixels = new int[640][480];

In this example, ages is an array of ints because that is the type before the [] which indicate that the type is an array. Similarly, weights is an array of floats, and pixels is an array of int[]s.

Files

Files are split into two categories: binary and text files.

Binary files

These start with a bit sequence known as a header which indicate what kind of file this is, and only consist of bits. Some common formats are:

• JPEG, PNG and BMP files for storing images.
• ZIP files for compressing files.

Reading from a binary file

In Java, we use the DataInputStream class to read binary data from a file:

try {
    DataInputStream output = new DataInputStream(new FileInputStream("numbers.dat"));
    System.out.println(input.readUTF());
    System.out.println("answer to life = " + input.readInt());
    System.out.println("pi = " + input.readFloat());
    input.close();
} catch(FileNotFound ex) {
    System.err.println("Failed to open file");
}

Writing to a binary file

In Java, we use the DataOutputStream class to write binary data to a file:

try {
    DataOutputStream output = new DataOutputStream(new FileOutputStream("numbers.dat"));
    output.writeUTF("Hello, world!");
    output.writeInt(42);
    output.writeFloat(3.1415);
    output.close();
} catch(IOException ex) {
    System.err.println("Failed to open file");
}

Text files

These may have a header, to indicate the file’s structure, and an End-Of-File marker so the program can know when to stop reading in from the file. This is a file that consists of lines of text, with a given encoding (usually UTF-8). Each line is known as a record. Some common formats are:

• HTML files for detailing the contents of a web page.
• CSV files for storing databases and tables. Each value between commas is known as a field.
• TXT files for storing plain text.

Reading from a text file

In Java, we use the Scanner class combined with the File class to read textual data from a file:

try {
    Scanner scanner = new Scanner(new File("greetings.txt"));
    ArrayList<String> greetings = new ArrayList();
    while(scanner.hasNext())
        greetings.add(scanner.next());
    scanner.close();
} catch(FileNotFoundException ex) {
    System.err.println("Failed to open file");
}

Writing to a text file

In Java, we use the FileWriter class to write textual data to a file:

try {
    FileWriter writer = new FileWriter("greetings.txt");
    writer.write("Hello\n");
    writer.write("Hola\n");
    writer.write("Bonjour\n");
    writer.write("Ohio\n");
    writer.close();
} catch(IOException ex) {
    System.err.println("Failed to open file");
}