Converting Between Class and Interface Types

• You can convert from a class type to an interface type, provided
   the class implements the interface

BankAccount account = new BankAccount(10000);
 Measurable x = account; // OK

Coin dime = new Coin(0.1, "dime");
 Measurable x = dime; // Also OK

• Cannot convert between unrelated types
   Measurable x = new Rectangle(5, 10, 20, 30); // ERROR

Because Rectangle doesn't implement Measurable

Casts

• Add coin objects to DataSet

DataSet coinData = new DataSet();
coinData.add(new Coin(0.25, "quarter"));
coinData.add(new Coin(0.1, "dime"));
. . .
Measurable max = coinData.getMaximum(); // Get the largest coin

• What can you do with it? It's not of type Coin

String name = max.getName(); // ERROR

• You need a cast to convert from an interface type to a class type

• You know it's a coin, but the compiler doesn't. Apply a cast:

Coin maxCoin = (Coin) max;
String name = maxCoin.getName();

• If you are wrong and max isn't a coin, the compiler throws an exception

• Difference with casting numbers:
  When casting number types you agree to the information loss
  When casting object types you agree to that risk of causing an exception

Can you use a cast (BankAccount) x to convert a Measurable variable x to a BankAccount reference?

   Answer: Only if x actually refers to a BankAccount object.

If both BankAccount and Coin implement the Measurable interface, can a Coin reference be converted to a BankAccount reference?

   Answer: No – a Coin reference can be converted to a Measurable reference,
                  but if you attempt to cast that reference to a BankAccount, an exception occurs.