Java Oriented Object Programming II Files II - Binary I/O Lesson 3

Transcription

1 Java Oriented Object Programming II Files II - Binary I/O Lesson 3

2 The objectives in this lesson are: Convert Binary to Hexadecimals (vice versa) Understand Binary Code Use Hex Editors Explain how Streams work Bytes Explain what Endianness is Little/Big Understand File Headers.bmp images

3 Binary & Hexadecimal Conversions Computers manipulate information based on ones(1) and zeros(0) and are represented in bits and bytes. This concept is called the Binary Number system or Base-2. The other major numbering system used by computers is Hexadecimal or Base-16. In this system the numbers are counted from 0-9, then letter A through F. The letters A through F represent 10 through 15. In HTML programming colors can be represented by a 6-digit hexadecimal number like #FFFFFF represents white and # represents black.

4 Binary & Hexadecimal Conversions Binary Hex Step 1: Divide the binary number into sets of 4 digits Step 2: Add all of the numbers where there is a 1 Step 3: Convert the Base 10 number to Hex = =5 =14(e) Hex 5E

5 Binary & Hexadecimal Conversions Hex Binary Step 1: Do the next steps for each hex number individually Step 2: Convert to Base 10 Hex 5D 5 D 5 13 Step 3: Convert the Base 10 number to Hex (Base 2 #s) (Binary #5) is 13 8 is 5 4 is 1 2 is = 5 YES YES NO YES 13-8=5 5-4=1 1-1= Hex 5D = Binary

6 Examining Binary Data Typical computer users never have to deal with binary data, so in order to look at the binary code of a file you need what s called a hex editor. Hex editors have many purposes such as opening files that aren t working properly, debugging etc. Here are some hex editors: Windows: HxD Mac: Hex Fiend

7 Hex Editor There are typically 3 parts to a hex editor. On the left is the address area, in the center is the hexadecimal area and on the right is the character area. Address Hexadecimal Character Once you download a Hex editor based on your operating system, go File Open file to view the raw bits and bytes.

8 Streams Streams work similarly to readers and writers but are intended for dealing directly with bytes. Files are stored as bytes instead of as text because it s more efficient in terms of space and processing time than storing as text. Each character in text is at least a byte in size, including numbers, but numbers themselves are much smaller when stored in binary form. In a file such as an image the pixel data is stored as a big list of numbers. Streams are useful when trying to enhance performance

9 Endianness When thinking of computer data the analogy often used is that of a book. Think of a hard drive as the book. A file as a paragraph and groupings of data in a file as a word. Many terms in dealing with data borrow from terms dealing with books. For example a grouping of bytes used by the processor is called a word. A group of bytes for any purpose is called block and often the term word and block are used interchangeably. Endianness is the order that the bytes of a word or block are written in. You would think that data is always stored in its natural order (ie, counting: 1,2,3,4 or RGBA colors: r,g,b,a) but that would be too easy and make too much sense. There are two main orders used for storing blocks, Big Endian and Little Endian, or forwards and backwards respectively.

10 Endianness Cont d The reason for using one endian or another has to do with how the computer hardware can most efficiently handle the data. Typically big-endian is the norm but occasionally little-endian is used. When writing multiple blocks in big-endian the bytes are ordered one after the other. In little-endian the blocks are in the same order but the bytes in each block are reversed Ex. Block 1: Block 2: Block 3: Block 1 Block 2 Block 3 Big-Endian: Little Endian:

11 File headers There is no universal standard for how files are written but there are standards for how some files are written, for compatibility and conventions that are generally followed. Files that contain large amounts of data usually have the first few bytes (10s or 100s of bytes) describe the file and how it works, these are called the file header. File Header

12 The objectives in this lesson are: Convert Binary to Hexadecimals (vice versa) Understand Binary Code Use Hex Editors Explain how Streams work Bytes Explain what Endianness is Little/Big Understand File Headers.bmp images