Wrapper classes

Introduction

Java is an object-oriented language and can view everything as an object. A simple file can be treated as an object (with java.io.File), an address of a system can be seen as an object (with java.util.URL), an image can be treated as an object (with java.awt.Image) and a simple data type can be converted into an object (with wrapper classes). This tutorial discusses wrapper classes. Wrapper classes are used to convert any data type into an object.

The primitive data types are not objects; they do not belong to any class; they are defined in the language itself. Sometimes, it is required to convert data types into objects in Java language. For example, upto JDK1.4, the data structures accept only objects to store. A data type is to be converted into an object and then added to a Stack or Vector etc. For this conversion, the designers introduced wrapper classes.

What are Wrapper classes?

As the name says, a wrapper class wraps (encloses) around a data type and gives it an object appearance. Wherever, the data type is required as an object, this object can be used. Wrapper classes include methods to unwrap the object and give back the data type. It can be compared with a chocolate. The manufacturer wraps the chocolate with some foil or paper to prevent from pollution. The user takes the chocolate, removes and throws the wrapper and eats it.

Observe the following conversion.

int k = 100;
Integer it1 = new Integer(k);

The int data type k is converted into an object, it1 using Integer class. The it1 object can be used in Java programming wherever k is required an object.

The following code can be used to unwrap (getting back int from Integer object) the object it1.

int m = it1.intValue();
System.out.println(m*m); // prints 10000

intValue() is a method of Integer class that returns an int data type.

List of Wrapper classes

In the above code, Integer class is known as a wrapper class (because it wraps around int data type to give it an impression of object). To wrap (or to convert) each primitive data type, there comes a wrapper class. Eight wrapper classes exist in java.lang package that represent 8 data types. Following list gives.

Primitive data type	Wrapper class
byte	Byte
short	Short
int	Integer
long	Long
float	Float
double	Double
char	Character
boolean	Boolean

Following is the hierarchy of the above classes.

All the 8 wrapper classes are placed in java.lang package so that they are implicitly imported and made available to the programmer. As you can observe in the above hierarchy, the super class of all numeric wrapper classes is Number and the super class for Character and Boolean is Object. All the wrapper classes are defined as final and thus designers prevented them from inheritance.

Importance of Wrapper classes

There are mainly two uses with wrapper classes.

1. To convert simple data types into objects, that is, to give object form to a data type; here constructors are used.
2. To convert strings into data types (known as parsing operations), here methods of type parseXXX() are used.

The following program expresses the style of converting data type into an object and at the same time retrieving the data type from the object.

public class WrappingUnwrapping
{
  public static void main(String args[])
  {				                  //  data types
    byte grade = 2;
    int marks = 50;
    float price = 8.6f;       		          // observe a suffix of f for float
    double rate = 50.5;
 		                                  // data types to objects		
    Byte g1 = new Byte(grade);                    // wrapping  
    Integer m1 = new Integer(marks);
    Float f1 = new Float(price);
    Double r1 = new Double(rate);
                                                                    // let us print the values from objects	
    System.out.println("Values of Wrapper objects (printing as objects)");
    System.out.println("Byte object g1:  " + g1);
    System.out.println("Integer object m1:  " + m1);
    System.out.println("Float object f1:  " + f1);
    System.out.println("Double object r1:  " + r1);
	        // objects to data types (retrieving data types from objects)
    byte bv = g1.byteValue();                 // unwrapping 
    int iv = m1.intValue();
    float fv = f1.floatValue();
    double dv = r1.doubleValue();
                                                                    // let us print the values from data types	
    System.out.println("Unwrapped values (printing as data types)");
    System.out.println("byte value, bv: " + bv);
    System.out.println("int value, iv: " + iv);
    System.out.println("float value, fv: " + fv);
    System.out.println("double value, dv: " + dv);
  }
}

As you can observe from the screenshot, constructors of wrapper classes are used to convert data types into objects and the methods of the form XXXValue() are used to retrieve back the data type from the object.

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