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.
