Numbers vs. numerical strings  

  • The "plus" operation + can be used on:

    1. numbers (such as int, float, etc)

    2. String

  • The "plus" operation + produces a different outcome on numbers and String:

    • "plus" operation + on number data type means addition: 

        Example:   12 + 34 = 46

    • "plus" operation + on the String data type means concatenation: 

        Example:  "12" + "34" = "1234"

DEMO: demo/04-Math+String/07-string-to-num/Demo1.java     (Show the Unicode of the String with BlueJ)

  Numerical strings  

  • Numerical string:

    • Numerical string = a string that only contains digits and optionally a decimal point

    Examples: 

     "123"
 "3.14159"

  • Important fact:

    • A numeric string is a string

    I.e.:   you cannot perform computations with a numercial string: 

     "123"     + 1 = "1231"       (because + is string concatenation !)
 "3.14159" + 1 = "3.141591"   (because + is string concatenation !)

DEMO: demo/04-Math+String/07-string-to-num/Demo2.java

How to convert numerical strings to numbers

  • Numerical strings revisited:

    • A numerical string can be used to represent a number

      Example: "123" can represent the number 123

  • Performing calculation with numbers that are represented as numerical string:

    • You must first convert the numerical string into a number

  • You can convert a number string to an int using Integer.parseInt( ): 

      int i = Integer.parseInt("123");  // i = 123 (integer)

  • You can convert a number string to an double using Double.parseDouble( ): 

     double x = Double.parseDouble("3.14159"); // x = 3.14159 (double)

DEMO: demo/04-Math+String/07-string-to-num/ConvertStrToNum.java

Programming trick: converting int and double to a (number) string

  • Sometimes, you may need to convert a number (int or double) into a (numerical) string...

  • You can convert an integer, double or float into a numerical string by:

    • Concatenating the number with the empty string:   ""

    Examples: 

    
  int i = 123;
  double x = 3.14;
  String s;

  s = "" + i ;    // s = "123"
  s = "" + x ;    // s = "3.14159"

DEMO: demo/04-Math+String/07-string-to-num/ConvertNumToStr.java