Converting Between Numbers and Strings
Frequently, a program ends up with numeric data in a string objecta 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; // Concatenate "i" with an empty string; conversion is handled for you. String s1 = "" + i;
// 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.