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Working with ASCII code in Java

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• Characters in a programming language
  • Characters:

    Characters (a.k.a. character constants) are denoted using single quotes: 'A' 'B' 'C' '1' '2' ... The type of a character is: char The value of a character is: The value of the binary number ASCII code that corresponds to the character

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  • Example:

    'A': The data type of the character constant A is: char the value of the character constant A is: 6510 (or 01000001(2) in binary) See the ASCII code table on this webpage: click here

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  • Quiz 1:

    Consider the following program: char c; c = 'A'; Question: What is the bit pattern stored in the variable c ?       Answer: 01000001

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  • Quiz 2:

    Consider the following program: byte c; c = 65; Question: What is the bit pattern stored in the variable c ?       Answer: same as quiz 1 !!! 01000001

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  • Note:

    The difference between: The character constant H that is represented by the binary pattern 01001000 The unsigned value 7210 that is represented by the same binary pattern 01001000 is: The data type !!! (Remember the Java compiler use the data type as context to interpret (= decode) the values !!!)

    Example that show you the same binary number is "interpreted" differently:

    char c; byte b; c = 65; b = 65; System.out.println("char c = " + c); // Prints "A" System.out.println("byte b = " + b); // Prints "65" System.out.println(); System.out.println("How to change 'interpretation' of a binary number"); System.out.println("char c casted by (byte) = " + (byte)c); System.out.println("byte b casted by (char) = " + (char)b);

    Output:

    char c = A byte b = 65 How to change 'interpretation' of binary number char c casted by (byte) = 65 byte b casted by (char) = A --- You can see that variables c and b contains the SAME binary number !!! --- By putting a casting operation, the number 65 is printed different (because a different println( ) function is called using different data type - through the overloading feature in Java

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/atoi/CastingChar1.java

    How to run the program:

    To compile:   javac CastingChar1.java          To run: java CastingChar1

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• Strings
  • String:

    String = a sequence of characters (I.e.: A string is an array of characters (= array of ASCII codes !!))

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• Performing computations on ASCII codes
  • Prelude to the ASCII to Integer (and Integer to ASCII) conversion algorithm:

    The conversion algorithms require the use of arithmetic computation on ASCII code as numbers Fortunately: Java does allow arithemtic operations on char data type !!!

    Example:

    int x = 'C' - 'A'; // Subtract the ASCII codes as binary numbers.... // x = 2 (in binary)

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/atoi/CastingChar2.java

    How to run the program:

    To compile:   javac CastingChar2.java          To run: java CastingChar2

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• Casting (= conversion) between character typed value and integer typed value
  • Sometimes, we need to convert between char and another data type

  • In Java, we can cast between char and byte (or short or int) data types:

    byte b; char c; b = (byte) c; // Casts character value in c to byte type c = (char) b; // Casts byte value in b to char type

    Why do you want to do this:

    We will need to String concentation operations on the binary ASCII code as a character In that case, we need to cast the binary number (used for computation) to char so Java will understand that we want to concatenate characters Example: we want to concatenate the letter 'A' to a string String s; s = s + 65; <--- Concatenates "65" to a string s = s + (char)65; <---- Concatenates 'A' to a string // This is the same as: s = s + 'A' !!!

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/atoi/CastingChar3.java

    How to run the program:

    To compile:   javac CastingChar3.java          To run: java CastingChar3

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  • Quiz:

    What is the output of: char c; c = 'H'; c = (char) (c+1); System.out.println( c ); // ???????

    Answer:

    I

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/atoi/CastingCharQuiz.java

    How to run the program:

    To compile:   javac CastingCharQuiz.java          To run: java CastingCharQuiz