- To cast, you must preceed the variable to be casted with the type to which you wish to cast in parenthesis. In other words, to cast a long to an int:
- This casts the long variable l to an int variable, so that it can be assigned to i.
- The same deal goes for objects. To convert a Vehicle to a Hybrid:
- You would only really want to do this if you want to take advantage of fields and/or methods that are declared for the first time in the subclass. In this example, you can use the method setElectricMode() only after casting.
- You must implicitly cast when going from a superclass to a subclass. This is because Java wants you to confirm that you know what you are doing, since subclasses have more functionality than superclasses. Going the other way is no problem, because we know that the superclass has all of the fields and methods of the subclass. Further, we know that with dynamic casting, Java will use the method or field that appears at the lowest level of granularity.
- If you attempt to cast a superclass to a subclass that is not the true subclass of the object, an exception is thrown. We'll get in to exceptions later in this quarter, but let's just say that they can be caught, but they are nasty.
- If you want to confirm that a class is a certain type, use the instanceof operator.
- This works fine. Interstingly, notice that instanceof is an operator, not a method. Also notice that the 'o' in 'of' is lower case. This might get you on a certification exam.
- If the classes are in no way related, (changing a String to an Integer, for instance), the compiler will not allow you to do it. You can cast only within inheritance hierarchy.
- Keep in mind, all classes derive from class Object, so all classes are subclasses of Object.
- In general, casts are bad. Unless you are using something like a Vector that stores Objects types, try to avoid them. If you catch yourself using one, try to think of a better way to program.
long l = 12345;
int i = (int)l;
public void run(Vehicle v) {
Hybrid h = (Hybrid)v;
}
if (v instanceof Hybrid) {
h = (Hybrid)v;
}
