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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

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

or

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

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.