JAVA Chapter 13 Flashcards

An abstract method cannot exist in a nonabstract class; a subclass of an abstract class must implement all abstract methods unless it is defined as abstract itself.
Abstract classes cannot be instantiated directly with the new operator, but they can have constructors, which are called by their subclasses.
It is possible to define an abstract class without any abstract methods; however, instances of such classes cannot be created with the new operator.

A subclass may be abstract even if its superclass is concrete; for example, GeometricObject can be an abstract subclass of the concrete Object class.
A concrete method can be overridden to become abstract in a subclass if its implementation becomes invalid, necessitating the subclass to be defined as abstract.

Abstract classes can be used as data types even though instances cannot be created with them; for instance, an array can be declared with an abstract class type.

The java.util.Calendar class is an abstract class for extracting detailed date and time information and can have subclasses like GregorianCalendar.
java.util.GregorianCalendar supports the Gregorian calendar system and can be initialized with the current time or specific date values (note: month is 0-based).

The get(int field) method in the Calendar class extracts date and time information, with fields being predefined constants.

An interface is a class-like construct that contains only constants and abstract methods, used to specify common behaviors across objects.
Unlike abstract classes, interfaces focus on behavior specification, allowing for comparisons and functional definitions like edible and cloneable.

Interfaces are defined with constant declarations and abstract method signatures.

A typical interface declaration follows the structure:

public interface InterfaceName { 
    // constants and method signatures
}

Interfaces are treated as separate classes in Java, compiled into their own bytecode files.
Similar to abstract classes, interfaces cannot have instances created via the new operator, but they can be utilized for variables and type casting.

Classes can implement interfaces to indicate a shared characteristic, such as being edible, using the implements keyword.
Example: Classes like Chicken and Fruit can implement the Edible interface to denote their edibility.

All fields in an interface are implicitly public, final, and static; methods are implicitly public and abstract, requiring no explicit modifiers to be stated.