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

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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]);

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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.