What is a wrapper class?

What is a wrapper class?
How are such classes useful?

In general, a wrapper class is any class which "wraps" or "encapsulates" the functionality of another class or component. These are useful by providing a level of abstraction from the implementation of the underlying class or component; for example, wrapper classes that wrap COM components can manage the process of invoking the COM component without bothering the calling code with it. They can also simplify the use of the underlying object by reducing the number interface points involved; frequently, this makes for more secure use of underlying components.

Just what it sounds like: a class that "wraps" the functionality of another class or API in a simpler or merely different API.

See: Adapter pattern, Facade pattern

Wrapper classes provide a way to use primitive types as objects. For each primitive , we have a wrapper class such as,

int Integer
byte Byte 

Integer and Byte are the wrapper classes of primitive int and byte. There are times/restrictions when you need to use the primitives as objects so wrapper classes provide a mechanism called as boxing/unboxing.

Concept can be well understood by the following example as

double d = 135.0 d;
Double doubleWrapper = new Double(d);

int integerValue = doubleWrapper.intValue();
byte byteValue = doubleWrapper.byteValue();
string stringValue = doubleWrapper.stringValue();

so this is the way , we can use wrapper class type to convert into other primitive types as well. This type of conversion is used when you need to convert a primitive type to object and use them to get other primitives as well.Though for this approach , you need to write a big code . However, the same can be achieved with the simple casting technique as code snippet can be achieved as below

double d = 135.0;
int integerValue = (int) d ;

Though double value is explicitly converted to integer value also called as downcasting.