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Trail: Learning the Java Language
Lesson: Numbers and Strings
Section: Strings
Numbers and Strings
Numbers
The Numbers Classes
Formatting Numeric Print Output
Beyond Basic Arithmetic
Summary of Numbers
Questions and Exercises
Characters
Strings
Converting Between Numbers and Strings
Manipulating Characters in a String
Comparing Strings and Portions of Strings
The StringBuilder Class
Summary of Characters and Strings
Questions and Exercises
Home Page > Learning the Java Language > Numbers and Strings
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Converting Between Numbers and Strings

> ### Converting Strings to Numbers

> Frequently, a program ends up with numeric data in a string object—a value entered by the user, for example.

> The `Number` subclasses that wrap primitive numeric types ( `Byte`, `Integer`, `Double`, `Float`, `Long`, and `Short`) each provide a class method named `valueOf` that converts a string to an object of that type. Here is an example, `ValueOfDemo` , that gets two strings from the command line, converts them to numbers, and performs arithmetic operations on the values:

> >     public class ValueOfDemo {
> >         public static void main(String[] args) {

> >     	    //this program requires two arguments on the command line
> >             if (args.length == 2) {
> >     			//convert strings to numbers
> >                 float a = (Float.valueOf(args[0]) ).floatValue();
> >                 float b = (Float.valueOf(args[1]) ).floatValue();

> >                 //do some arithmetic
> >                 System.out.println("a + b = " + (a + b) );
> >                 System.out.println("a - b = " + (a - b) );
> >                 System.out.println("a * b = " + (a * b) );
> >                 System.out.println("a / b = " + (a / b) );
> >                 System.out.println("a % b = " + (a % b) );
> >             } else {
> >                System.out.println("This program requires two command-line arguments.");
> >             }
> >         }
> >     }

> The following is the output from the program when you use `4.5` and `87.2` for the command-line arguments:

> >     a + b = 91.7
> >     a - b = -82.7
> >     a * b = 392.4
> >     a / b = 0.0516055
> >     a % b = 4.5

> > Note: Each of the `Number` subclasses that wrap primitive numeric types also provides a `parseXXXX()` method (for example, `parseFloat()`) that can be used to convert strings to primitive numbers. Since a primitive type is returned instead of an object, the `parseFloat()` method is more direct than the `valueOf()` method. For example, in the `ValueOfDemo` program, we could use:

> > >     float a = Float.parseFloat(args[0]);
> > >     float b = Float.parseFloat(args[1]);

> ### Converting Numbers to Strings

> Sometimes you need to convert a number to a string because you need to operate on the value in its string form. There are several easy ways to convert a number to a string:

> >     int i;
> >     String s1 = "" + i; //Concatenate "i" with an empty string;
> >                         //conversion is handled for you.

> or

> >     String s2 = String.valueOf(i);  //The valueOf class method.

> Each of the `Number` subclasses includes a class method, `toString()`, that will convert its primitive type to a string. For example:

> >     int i;
> >     double d;
> >     String s3 = Integer.toString(i);
> >     String s4 = Double.toString(d);

> The `ToStringDemo` example uses the `toString` method to convert a number to a string. The program then uses some string methods to compute the number of digits before and after the decimal point:

> >     public class ToStringDemo {

> >         public static void main(String[] args) {
> >             double d = 858.48;
> >             String s = Double.toString(d);

> >             int dot = s.indexOf('.');

> >             System.out.println(dot + " digits before decimal point.");
> >             System.out.println( (s.length() - dot - 1) +
> >     		" digits after decimal point.");
> >         }
> >     }

> The output of this program is:

> >     3 digits before decimal point.
> >     2 digits after decimal point.

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