Notes

An object cannot be converted to an unrelated type. The Java compiler does not allow you to convert a String to a Point, for example, even if you use a cast operator.

An object can be converted to the type of a superclass. This is a widening conversion, so no cast is required. For example, a String value can be assigned to a variable of type Object or passed to a method where an Object parameter is expected. Note that no conversion is actually performed; the object is simply treated as if it were an instance of the superclass.

An object can be converted to the type of a subclass, but this is a narrowing conversion and requires a cast. The Java compiler provisionally allows this kind of conversion, but the Java interpreter checks at runtime to make sure it is valid. Only cast an object to the type of a subclass if you are sure, based on the logic of your program, that the object is actually an instance of the subclass. If it is not, the interpreter throws a ClassCastException. For example, if we assign a String object to a variable of type Object, we can later cast the value of that variable back to type String:

Object o = "string"; // Widening conversion from String to Object
// Later in the program...
String s = (String) o; // Narrowing conversion from Object to String
All array types are distinct, so an array of one type cannot be converted to an array of another type, even if the individual elements could be converted. For example, although a byte can be widened to an int, a byte[] cannot be converted to an int[], even with an explicit cast.

Arrays do not have a type hierarchy, but all arrays are considered instances of Object, so any array can be converted to an Object value through a widening conversion. A narrowing conversion with a cast can convert such an object value back to an array. For example:

Object o = new int[] {1,2,3}; // Widening conversion from array to Object
// Later in the program...
int[] a = (int[]) o; // Narrowing conversion back to array type