Unboxing Because a variable of type object can refer to a boxed copy of a value, it's only reasonable to allow you to get at that boxed value through the variable. You might expect to be able to access the boxed int value that a variable o refers to by using a simple assignment statement such as: int i = o; However, if you try this syntax, you'll get a compile time error. If you think about it, it's pretty sensible that you can't use the int i = o; syntax. After all, o could be referencing absolutely anything and. | Unboxing Because a variable of type object can refer to a boxed copy of a value it s only reasonable to allow you to get at that boxed value through the variable. You might expect to be able to access the boxed int value that a variable o refers to by using a simple assignment statement such as int i o However if you try this syntax you ll get a compile time error. If you think about it it s pretty sensible that you can t use the int i o syntax. After all o could be referencing absolutely anything and not just an int. Consider what would happen in the following code if this statement was allowed Circle c new Circle int i 42 object o o c o refers to a circle i o what is stored in i To obtain the value of the boxed copy you must use what is known as a cast an operation that checks that it is safe to convert one type to another and then does the conversion. You prefix the object variable with the name of the type in parentheses as in this example int i 42 object o i boxes i int o compiles okay The effect of this cast is subtle. The compiler notices that you ve specified the type int in the cast. Next the compiler generates code to check what o actually refers to at runtime. It could be absolutely anything. Just because your cast says o refers to an int that doesn t mean it actually does. If o really does refer to a boxed int and everything matches the cast succeeds and the compiler-generated code extracts the value from the boxed int. In this example the boxed value is then used to initialize i. This is called unboxing. The following diagram shows what is happening However if o does not refer to a boxed int there is a type mismatch causing the cast to fail. The compiler-generated code throws an InvalidCastException at runtime. Here s an example of an unboxing cast that fails Circle c new Circle 42 object o c doesn t box because Circle is a class int i int o compiles okay but throws at runtime NOTE The type you specify in the unboxing cast must exactly match the type .
