Interfaces and Polymorphism - Java Concepts - page 543

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

Thus, when you have an object variable of type Measurable , you don't actually
know the exact type of the object to which x refers. All you know is that the object
has a getMeasure method.

However, you cannot convert between unrelated types:

Measurable x = new Rectangle(5, 10, 20, 30); // Error

That assignment is an error, because the Rectangle class doesn't implement the
Measurable interface.

Occasionally, it happens that you convert an object to an interface reference and you
need to convert it back. This happens in the getMaximum method of the DataSet
class. The DataSet stores the object with the largest measure, as a Measurable
reference.

DataSet coinData = new DataSet();
coinData.add(new Coin(0.25, "quarter"));
coinData.add(new Coin(0.1, "dime"));
coinData.add(new Coin(0.05, "nickel"));
Measurable max = coinData.getMaximum();

Now what can you do with the max reference? You know it refers to a Coin object,
but the compiler doesn't. For example, you cannot call the getName method:

String coinName = max.getName(); // Error

That call is an error, because the Measurable type has no getName method.

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However, as long as you are absolutely sure that max refers to a Coin object, you
can use the cast notation to convert it back:

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

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