---

The string data type String

---

• String (in general)
  • A string is a sequence of characters enclosed between the double quotes "..."

    Example:

    "abc123" "Hello World" "Hello, what is your name ?"

    ---

  • Each character in a string is of the type char and uses the Unicode as encoding method

  ---

  ---

• The String data type in Java
  • The String data type is the first data type that we learn that is not built-in into the programming labugae Java

    ---

  • Primitive data types of built-in data types of Java:

    double float byte short int long char boolean (we will learn this soon)

    ---

  • The Java language (and most programming languages) does not provide the String data type

    However: the Java language does provide the char data type

    The String data type is a class that is constructed using the char data type:

    A data type has: information operations (Example: the integer data type contains a value (= information) and operations such as: +, -, etc.) Implementing the String data type in Java: The information of the String data type is stored using a number of char typed variables The operations on String data are implemented by a number of methods (= computer programs) The documentation of the class String (you can find a list of the methods) can be found here: click here

    ---

  • Note:

    At this moment, we do not have enough knowledge on Java to explain how the String data type is constructed (It uses advanced program language knowledge) We will learn about this later in the course.

    ---

  • For now, we will only learn how to use the String data type

  ---

  ---

  ---

  ---

  ---

  ---

• String literals (constants)
  • We write string literals between double quotes "..." (as discussed above)

    Example:

    "Hello World"

    ---

  • Escape character:

    Escape character = a special character that allow Java to change the meaning of the next character \   (backslash) = the escape character for strings The escape character is usually used to express "unprintable" characters

    ---

  • Escape sequence:

    Escape sequence = the escape character \ followed by one character Example: \n A escape sequence denotes one character The character denoted by an escape sequence is usually one that you cannot type in with the keyboard

    ---

    ---

  • Commonly used escape sequences:

    Escape sequence	Denoted character	Example
    \t	Tab	"\t" (string with a TAB character)
    \n	New line (NL)	"\n" (string with a NL character)
    \\	Backslash (\)	"\\" (string with a \)
    \"	Double quote (")	"\"" (string with a ")

    ---

  • Example:

    public class Escape01 { public static void main(String[] args) { System.out.println("He said \t \"Hello\" \nI said \" \\ \" \n"); } }

    Output:

    He said "Hello" I said " \ "

  ---

  ---

• Defining String typed variables
  • Syntax to define a String typed variable:

    String NameOfVariable ;

    Notes:

    The class String announces the variable definition clause The NameOfVariable is an identifier which is the name of the variable. The variable definition clause is must be ended with a semi-colon ";" A String typed variable can store the location (address) of a sequence characters (string)

    ---

  • Example: defining and using a String typed variable

    public class String01 { public static void main(String[] args) { String a; a = "Hello World"; // a = location of "Hello World" System.out.println(a); // Print the string at a } }

    ---

  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   javac String01.java To run:          java String01

  ---

  ---

  ---

  ---

  ---

  ---

• How strings are stored inside the computer
  • Challenge to store strings:

    Strings have variable lengths !!! Example: "Hello" contains 5 characters "Good-bye" contains 8 characters Conclusion: You cannot store a string in a fixed sized variable !!!

    ---

  • There are 2 techniques in Computer Science for storing strings:

    The location + length method The location + sentinel method

  ---

  ---

• The location + length method to store strings
  • The location + length method:

    The characters of string is stored (using Unicode encoding) somewhere consecutively inside the RAM memory A string variable contains: the location of the first character of the string the length of the string (total number of characters in the string)

    ---

  • Example: storing strings "abc" and "12" using location + length

    Explanation:

    The string "abc" is stored beginning at memory location 984 in RAM memory The length of this string is 3 characters The string variable that helps us find the string "abc" contains: The address 984 The length 3 Another string variable that helps us find the string "12" contains: The address 520 which is the first location of the string "12" The length 2

  ---

  ---

• The location + sentinel method to store strings
  • Sentinel

    Sentinel = a special character (symbol) that denotes the end of a string The sentinel used in programming languages to denote the end of a string is usually the NULL character with Unicode value 0.

    ---

  • The location + sentinel method:

    The characters of string is stored (using Unicode encoding) somewhere consecutively inside the RAM memory The sentinel (NULL character) is added after the last character of the string A string variable contains: the location of the first character of the string (The end of the string is marked by the sentinel)

    ---

  • Example: storing strings "abc" and "12" using location + sentinel

    Explanation:

    The string "abc" is stored beginning at memory location 984 in RAM memory The sentinel NULL (with Unicode value 0) is added after the string The string variable that helps us find the string "abc" contains: The address 984 (We trace down the memory until we find the sentinel to find the end of the string) Another string variable that helps us find the string "12" contains: The address 520 which is the first location of the string "12" (We trace down the memory until we find the sentinel to find the end of this string)

  ---

  ---

• Which of the 2 methods is better ?
  • According to an article, the location + length method is much more efficient

  • Article: click here

  ---

  ---

  ---

  ---

  ---

  ---

• Strings in Java
  • String representation in Java:

    Java uses the location + length method to store String typed data

    ---

  • Obtaining the length information of strings in Java:

    Let a be a String typed variable The expression a.length() returns the length of the string stored in the string variable a

    ---

  • Example:

    public class String02 { public static void main(String[] args) { String a; a = "abc"; // a = location + length of "abc" System.out.println(a); // Print the string at a System.out.println(a.length()); // Print the length of the string at a } }

    ---

  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   javac String02.java To run:          java String02

  ---

  ---

  ---

  ---

  ---

  ---

• Attention: numbers and numeric strings

  • Numeric string:

    Numeric string = a string containing only numbers in Unicode that represent digits Numeric string uses the Unicode encoding method !

  • (Integer) Numbers:

    Numbers uses the binary number system as encoding method !!!

    ---

  • Important fact:

    Integer numbers and numeric strings look visually alike in a computer program: What you see in a Java program What's stored inside computer -------------------------------- ------------------------------------- 12 (integer number) 00000000 00000000 00000000 00001100 (12) "12" (numeric string) 00000000 00110001 00000000 00110010 ^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^^^ Unicode for '1' Unicode for '2' Notice that: What is stored inside the computer are complete different (because they use different representation (encoding) techniques)

    ---

    ---

  • Quiz for class:

    Do you still remember how the computer decide what encoding method it must use to interpret a number ?

    Answer:

    Based on the data type information The data type is the necessary context information that the computer need to decide which encoding method it will use.

  ---

  ---

  ---

  ---

  ---

  ---

• Converting integer and floating point values to strings
  • We saw that the integer 12 is stored using Binary number system as the following binary number:

    000...00001100 || |+--------------> 22 = 4 +---------------> 23 = 8 ----- 12

    On the other hand, the string "12" is stored using the Unicode scheme as:

    00000000 00110001 (code for '1') 00000000 00110010 (code for '2')

    Conclusion:

    We need a conversion mechanism to change the representation between numbers and strings

    ---

  • Converting an integer representation to the string representation:

    Converting numbers into strings is an advanced programming problem The algorithm to convert an integer representation to the string representation will be discussed in CS255 Here, I will only show you how to do this in Java:

    ---

    ---

    ---

  • How to convert:   int ⇒ String

    String a; int x; a = Integer.toString(x) ; // Returns the String representation // for the integer value in variable x

    ---

  • How to convert:   double ⇒ String

    String a; double x; a = Double.toString(x) ; // Returns the String representation // for the double value in variable x

    ---

  • Converting:   float ⇒ String

    String a; float x; a = Float.toString(x) ; // Returns the String representation // for the float value in variable x

    ---

  • Example:

    public class String04 { public static void main(String[] args) { String s1, s2; // Strings int x = 12; double y = 3.1415; System.out.print("x + y = "); System.out.println( x + y ); // Add numbers s1 = Integer.toString(x); s2 = Double.toString(y); System.out.print("s1 + s2 = "); System.out.println( s1 + s2 ); // + on String is concatenation } }

    Output:

    x + y = 15.1415 s1 + s2 = 123.1415 (Concatenation of Strings !)

    ---

  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   javac String04.java To run:          java String04

  ---

  ---

  ---

  ---

  ---

  ---

• Operators on the String data type
  • There is only one operator (the + operator) defined for the String data type:

    string1 + string2 = concatenate the strings string1 and string2

    Example:

    "abc" + "12" returns the string: "abc12"

    ---

  • Example Java program with 4 string concatenation operations:

    public class String03 { public static void main(String[] args) { String a, b, c; a = "abc"; b = "12"; c = a + b; // String concatenation System.out.println("a = " + a); // String concatenation !! System.out.println("b = " + b); // String concatenation !! System.out.println("c = " + c); // String concatenation !! } }

    ---

  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   javac String03.java To run:          java String03

  ---

  ---

  ---

  ---

  ---

  ---

• Converting integer and floating point values to strings
  • Recall that the integer 12 is stored using Binary number system as the following binary number:

    00001100 || |+--------------> 22 = 4 +---------------> 23 = 8 ----- 12

    On the other hand, the string "12" is stored using the Unicode scheme as 2 binary numbers:

    00110001 (code for '1') 00110010 (code for '2')

    ---

  • Converting an integer representation to the string representation:

    Converting numbers into strings is an advanced programming problem The algorithm to convert an integer representation to the string representation will be discussed in CS255 Here, I will only show you how to do this in Java:

    ---

    ---

    ---

  • Converting:   int ⇔ String

    String s; int x; s = Integer.toString(x) ; // Returns the String representation // for the integer value in variable x x = Integer.parseInt(s) ; // Returns the binary number representation // for the numeric string in variable s

    ---

  • Converting:   double ⇔ String

    String s; double x; s = Double.toString(x) ; // Returns the String representation // for the double value in variable x x = Double.parseDouble(s) ; // Returns the IEEE 754 double // precision number representation // for the numeric string in variable s

    ---

  • Converting:   float ⇔ String

    String a; float x; s = Float.toString(x) ; // Returns the String representation // for the float value in variable x x = Float.parseFloat(s) ; // Returns the IEEE 754 single // precision number representation // for the numeric string in variable s

    ---

  • Example:

    public class String04 { public static void main(String[] args) { String s1, s2; // Strings int x = 12; double y = 3.1415; System.out.print("x + y = "); System.out.println( x + y ); // Add numbers s1 = Integer.toString(x); s2 = Double.toString(y); System.out.print("s1 + s2 = "); System.out.println( s1 + s2 ); // + on String is concatenation } }

    Output:

    x + y = 15.1415 s1 + s2 = 123.1415 (Concatenation of Strings !)

    ---

  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   javac String04.java To run:          java String04

  ---

  ---

  ---

  ---

  ---

  ---

• The + operator on number typed and String typed data
  • Java provides an automatic conversion feature when the + operator is used between:

    a number typed data and a string typed data

    Java's automatic conversion rule for number ⇒ string:

    When the + operator is used between a number and a string Then: the number is automatically converted to a string The + operator is then applied on 2 strings (i.e., the + operator is a concatenation !)

    ---

  • Example:

    public class String05 { public static void main(String[] args) { String a, b; int x; a = "abc"; x = -12; b = a + x; System.out.println(b); // Prints "abc-12" b = x + a; System.out.println(b); // Prints "-12abc" } }

    Explanation:

    a = "abc"; x = -12; b = a + x; is executed as follows: a + x = "abc" + (-12) convert number to string = "abc" + "-12" concatenate 2 strings = "abc-12" b = x + a; is executed as follows: x + a = (-12) + "abc" convert number to string = "-12" + "abc" concatenate 2 strings = "-12abc"

    ---

  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   javac String05.java To run:          java String05

  ---

  ---

  ---

  ---

  ---

  ---

• Class Quiz.... (think deeply !)
  • What is printed by the following Java program:

    public class String06 { public static void main(String[] args) { System.out.println( "a" + 1 + 2 ); System.out.println( 1 + 2 + "a" ); } }

    Solution:

    "a" + 1 + 2 = "a" + 1 + 2 (+ is left associative !) = "a" + "1" + 2 (convert 1 to "1") = "a1" + 2 = "a1" + "2" (convert 2 to "2") = "a12" 1 + 2 + "a" = 1 + 2 + "a" (+ is left associative !) = 3 + "a" (This + is an integer addition !) = "3" + "a" = "3a"

    ---

  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   javac String06.java To run:          java String06

  ---

  ---

  ---

  ---

• Other operations on Strings
  • There are many other operations defined on the String data type.

    However, these operations are available as instance methods (that we will learn later)

    Here, I will just give you a little taste of these operations without explaining the details

    ---

  • charAt(i): return the character at position i in the string.

    Example:

    01234567890 <---- Character position String s = "Hello World"; char x; x = s.charAt(0); // x = 'H'; x = s.charAt(1); // x = 'e'; x = s.charAt(2); // x = 'l';

    ---

  • substring(i, j): return the substring consisting of the characters between positions i and j-1 in the string.

    Example:

    01234567890 <---- Character position String s = "Hello World"; char x; y = s.substring(0,1); // y = "H" y = s.substring(2,4); // y = "ll"

    ---

  • Jave program showing the effects:

    public class String07 { public static void main(String[] args) { String s = "Hello World"; char x; String y; x = s.charAt(0); System.out.println("s.charAt(0) = " + x); x = s.charAt(2); System.out.println("s.charAt(2) = " + x); y = s.substring(0,1); System.out.println("s.substring(0,1) = " + y); y = s.substring(2,4); System.out.println("s.substring(2,4) = " + y); } }

    Output:

    s.charAt(0) = H s.charAt(2) = l s.substring(0,1) = H s.substring(2,4) = ll

    ---

  • Example Program: (Run it yourself !)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   javac String07.java To run:          java String07

  ---

  ---

  ---

  ---

  ---

  ---

• Reading a string from the keyboard
  • Previously discussed:

    Procedure to read in a floating point number from keyboard: The procedure to read other types of inputs from the keyboard is similar to the one above. The only different is that we need to use a different method in the Scanner class that read the correct type of data.

    ---

  • Reading a string (word) from the keyboard: use next()

    Note: in this case, you need to use a String typed variable to store a string !!!

  ---

  ---

• Class quiz
  • Can you replace the highlighted statements by one statement:

    import java.util.Scanner; public class Quiz1 { public static void main(String[] args) { Scanner in = new Scanner(System.in); int i; String s; s = in.next(); i = Integer.parseInt(); } }

    What effect does these 2 highlighted statements have ?

    What have you learned that does the same thing ?

    ---

  • Answer:

    s = in.next(); // Read in a string i = Integer.parseInt(); // Converts to binary number (integer) // representation Total effect: read in a "integer numeric string" as a binary number Replaced by: i = in.nextInt();