An abstract class, in the context of Java, is a superclass that cannot be instantiated and is used to state or define general characteristics. An object cannot be formed from a Java abstract class; trying to instantiate an abstract class only produces a compiler error. The abstract class is declared using the keyword abstract.
An
abstract class is a class that is declared
abstract—it may or may not include abstract methods. Abstract classes cannot be instantiated, but they can be subclassed.
An
abstract method is a method that is declared without an implementation (without braces, and followed by a semicolon), like this:
abstract void moveTo(double deltaX, double deltaY);
If a class includes abstract methods, then the class itself
must be declared
abstract, as in:
public abstract class GraphicObject {
// declare fields
// declare nonabstract methods
abstract void draw();
}
When an abstract class is subclassed, the subclass usually provides implementations for all of the abstract methods in its parent class. However, if it does not, then the subclass must also be declared
abstract.
Note: Methods in an
interface (see the
Interfaces section) that are not declared as default or static are
implicitly abstract, so the
abstract modifier is not used with interface methods. (It can be used, but it is unnecessary.)
Abstract classes are similar to interfaces. You cannot instantiate them, and they may contain a mix of methods declared with or without an implementation. However, with abstract classes, you can declare fields that are not static and final, and define public, protected, and private concrete methods. With interfaces, all fields are automatically public, static, and final, and all methods that you declare or define (as default methods) are public. In addition, you can extend only one class, whether or not it is abstract, whereas you can implement any number of interfaces.
Which should you use, abstract classes or interfaces?
- Consider using abstract classes if any of these statements apply to your situation:
- You want to share code among several closely related classes.
- You expect that classes that extend your abstract class have many common methods or fields, or require access modifiers other than public (such as protected and private).
- You want to declare non-static or non-final fields. This enables you to define methods that can access and modify the state of the object to which they belong.
- Consider using interfaces if any of these statements apply to your situation:
- You expect that unrelated classes would implement your interface. For example, the interfaces
Comparable and Cloneable are implemented by many unrelated classes.
- You want to specify the behavior of a particular data type, but not concerned about who implements its behavior.
- You want to take advantage of multiple inheritance of type.
An example of an abstract class in the JDK is
AbstractMap, which is part of the Collections Framework. Its subclasses (which include
HashMap,
TreeMap, and
ConcurrentHashMap) share many methods (including
get,
put,
isEmpty,
containsKey, and
containsValue) that
AbstractMap defines.
An example of a class in the JDK that implements several interfaces is
HashMap, which implements the interfaces
Serializable,
Cloneable, and
Map<K, V>. By reading this list of interfaces, you can infer that an instance of
HashMap (regardless of the developer or company who implemented the class) can be cloned, is serializable (which means that it can be converted into a byte stream; see the section
Serializable Objects), and has the functionality of a map. In addition, the
Map<K, V> interface has been enhanced with many default methods such as
merge and
forEach that older classes that have implemented this interface do not have to define.
Note that many software libraries use both abstract classes and interfaces; the
HashMap class implements several interfaces and also extends the abstract class
AbstractMap.
An Abstract Class Example
In an object-oriented drawing application, you can draw circles, rectangles, lines, Bezier curves, and many other graphic objects. These objects all have certain states (for example: position, orientation, line color, fill color) and behaviors (for example: moveTo, rotate, resize, draw) in common. Some of these states and behaviors are the same for all graphic objects (for example: position, fill color, and moveTo). Others require different implementations (for example, resize or draw). All
GraphicObjects must be able to draw or resize themselves; they just differ in how they do it. This is a perfect situation for an abstract superclass. You can take advantage of the similarities and declare all the graphic objects to inherit from the same abstract parent object (for example,
GraphicObject) as shown in
the following figure.
Classes Rectangle, Line, Bezier, and Circle Inherit from GraphicObject
First, you declare an abstract class,
GraphicObject, to provide member variables and methods that are wholly shared by all subclasses, such as the current position and the
moveTo method.
GraphicObject also declares abstract methods for methods, such as
draw or
resize, that need to be implemented by all subclasses but must be implemented in different ways. The
GraphicObject class can look something like this:
abstract class GraphicObject {
int x, y;
...
void moveTo(int newX, int newY) {
...
}
abstract void draw();
abstract void resize();
}
Each nonabstract subclass of
GraphicObject, such as
Circle and
Rectangle, must provide implementations for the
draw and
resize methods:
class Circle extends GraphicObject {
void draw() {
...
}
void resize() {
...
}
}
class Rectangle extends GraphicObject {
void draw() {
...
}
void resize() {
...
}
}
When an Abstract Class Implements an Interface
In the section on
Interfaces, it was noted that a class that implements an interface must implement
all of the interface's methods. It is possible, however, to define a class that does not implement all of the interface's methods, provided that the class is declared to be
abstract. For example,
abstract class X implements Y {
// implements all but one method of Y
}
class XX extends X {
// implements the remaining method in Y
}
In this case, class
X must be
abstract because it does not fully implement
Y, but class
XX does, in fact, implement
Y.
Class Members
An abstract class may have
static fields and
static methods. You can use these static members with a class reference (for example,
AbstractClass.staticMethod()) as you would with any other class.
Nice example on Stackoverflow.com, discussing what happens in background when you create
http://stackoverflow.com/questions/13670991/interview-can-we-instantiate-abstract-class
