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Wrapper Classes

int, float, long, char, double etc. are primitive data types. Primitive data types may be converted into object types, by using the wrapper classes contained in the java.lang package.  Given below is a table, which shows the simple data types and their corresponding wrapper class types:

Simple TypeWrapper Class
boolean   Boolean 
charCharacter 
double Double 
floatFloat 
intInteger
longLong 


Given below is a table, which shows how Constructor methods are used for converting primitive numbers to object numbers:

Constructor CallConversion Action
Integer IntVal = new Integer(i);Primitive integer to Integer object
Float FloatVal = new Float(f);Primitive float to Float object
Double DoubleVal = new Double(d);Primitive double to Double object
Long LongVal = new Long(l);Primitive long to Long object 

NOTE: i, f, d and l are primitive data values denoting int, float, double and long data types.  They may be constants or variables.

Given below is a table, which lists the methods used for converting Object numbers to Primitive numbers using typeValue() method:

Method CallConversion Action
int i = IntVal.intValue();Object to primitive integer
float f = FloatVal.floatValue();Object to primitive float
double d = DoubleVal.doubleValue();Object to primitive double
long l = LongVal.longValue();Object to primitive long

Given below is a table, which shows how numbers are converted to Strings using toString() method:

Method CallConversion Action
str = Integer.toString(i);Primitive integer to String
str = Float.toString(f);Primitive float to String
str = Double.toString(d);Primitive double to String
str = Long.toString(l);Primitive long to String

Given below is a table, which shows how string objects can be converted to numeric objects using the static method valueOf():

Method CallConversion Action
DoubleVal = Double.valueOf(str);String to Double object
FloatVal = Float.valueOf(str);String to Float object
LongVal = Long.valueOf(str);String to Long object
IntVal = Integer.valueOf(str);String to Integer object

Given below is a table, which is used for converting numeric strings to primitive numbers using Parsing methods:

Method CallConversion Action
int i = Integer.parseInt(str);Convert String to primitive integer
float f = Float.parseFloat(str);Convert String to primitive float
double d = Double.parseDouble(str);Convert String to primitive double
long l = Long.parseLong(str);Convert String to primitive long

NOTE: parseType( ) methods throw a NumberFormatException if the value of the str does not represent the respective primitive type. 

Given below is an example, which uses some of most commonly used wrapper class methods,

    import java.io.*;
    
    class Invest
    {
        public static void main(String args[])
        {
            Float principalAmount = new Float(0);
            Float interestRate = new Float(0);
            int numYears=0;
            try
            {
                DataInputStream in = new DataInputStream(System.in);
                System.out.println("Enter principal amount : ");
                System.out.flush();
                String principalString = in.readLine();
                principalAmount = Float.valueOf(principalString);
                System.out.println("Enter interest rate : ");
                System.out.flush();
                String interestString = in.readLine();
                interestRate = Float.valueOf(interestString);
                System.out.println("Enter Number of years : ");
                System.out.flush();
                String yearsString = in.readLine();
                numYears = Integer.parseInt(yearsString);
            }
            catch(IOException e)
            {
                System.out.println("I/O Exception ");
                System.exit(1);
            }
            float value = loan(principalAmount, floatValue(), interestRate, 
                               floatValue(), numYears);
            printline();
            System.out.println("Final value = "+value);
            printline();
        }
        static float loan(float p,float r, int n)
        {
            int year=1;
            float sum = p;
            while(year <= n)
            {
                sum = sum * (1 + r);
                year = year + 1;
            }
            return sum;
        }
        static void printline()
        {
            for(int i = 1; i <= 30; i++)
            {
                System.out.print("=");
            }
            System.out.println(" ");
        }
    }

Output:

Enter principal amount:
20000
Enter interest rate:
.12
Enter number of years:
2
==============================
Final Value = 25088.0
==============================

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