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Java statements that manipulate character and strings

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• Java statements and methods that manipulate characters and strings
  • The charAt( ) method:

    Let s be a string (constant or variable) Then: s.charAt(i) = the character at position i in string s

    Examples:

    "abc".charAt(0) is 'a' "abc".charAt(1) is 'b' "abc".charAt(2) is 'c'

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  • The + (string concatenation) operator on String data type:

    Let s1 and s2 be 2 strings Then: s1 + s2 = the concatenation of strings s1 and s2 String + char is also a concatenation operation: String s; char c; s + c = the concatenation of string s and character c

    Example:

    String s1 = "abc"; // = an array of 3 ASCII codes (for 'a', 'b', 'c') String s2 = "123"; // = an array of 3 ASCII codes (for '1', '2', '3') char c = 'H'; s1 + s2 = "abc123" // an array of 6 ASCII codes s1 + c = "abcH" // an array of 4 ASCII codes

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• I just have to emphasize this fact over and over again....
  • Students often fail to understand this material because they missed this very important fact:

    Every (decimal) number that you write in a Java program will be converted into a 2's complement binary number by the Java compiler

  • Example:

    int x; x = 0; // Java compiler generates instructions that store // 00000000000000000000000000000000 into variable x x = 4; // Java compiler generates instructions that store // 00000000000000000000000000000100 into variable x x = x + 1 // Java compiler generates instructions that // add binary number in x with 00000000000000000000000000000001 x = -x; // Java compiler generates instructions that // inverts the binary number in x and store result in x

    A concrete example:

    int x; x = 0; // variable x contains: 00000000000000000000000000000000 x = 4; // variable x contains: 00000000000000000000000000000100 x = x + 1; // variable x contains: 00000000000000000000000000000101 ( 52's compl ) x = -x; // variable x contains: 11111111111111111111111111111011 ( -52's compl )