from string to numeric, and vice versa

Java primitive types: boolean, byte, char, short, int, long, float, and double. Java primitive wrappers: Boolean, Byte, Character, Short, Integer, Long, Float, and Double. The pairs follow auto-boxing principle. Wrapper classes offer certain amount of static methods, for instance, converting number in string into numeric values.

/*
 * Class wrappers of primitive types offer static service methods. 
 * converting digit numbers from String to numeric primitive type.
 * String class offers another way around, converting number to string.
 */
package NumberWraper;

/**
 *
 * @author YNZ
 */
public class UserNumbers {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        double d = Double.parseDouble("20.456");
        double a = Double.parseDouble("10.46");
        
        double sum = a + d;
        System.out.printf("%f + %f = %f \n ", d, a, sum);

        //converting from numeric to string. 
        String aStr = String.valueOf(d);
        String dStr = String.valueOf(a);
        
        //valueOf srevice returns wrapper type
        Double e = Double.valueOf(dStr);
        
    }
}

You need watch out the number type even in the String format.

   
        double ee = Double.parseDouble("20"); //1
         int ff = Integer.parseInt("121.3");  //2

//1 is fine, for double may hold a int type without losing. 

//2 is not valid for "121.3" is not integer.