Abstract class in Java

Question

What is an "abstract class" in Java?

Answer 1

An abstract class is a class which cannot be instantiated. An abstract class is used by creating an inheriting subclass that can be instantiated. An abstract class does a few things for the inheriting subclass:

1. Define methods which can be used by the inheriting subclass.
2. Define abstract methods which the inheriting subclass must implement.
3. Provide a common interface which allows the subclass to be interchanged with all other subclasses.

Here's an example:

abstract public class AbstractClass
{
    abstract public void abstractMethod();
    public void implementedMethod() { System.out.print("implementedMethod()"); }
    final public void finalMethod() { System.out.print("finalMethod()"); }
}

Notice that "abstractMethod()" doesn't have any method body. Because of this, you can't do the following:

public class ImplementingClass extends AbstractClass
{
    // ERROR!
}

There's no method that implements abstractMethod()! So there's no way for the JVM to know what it's supposed to do when it gets something like new ImplementingClass().abstractMethod().

Here's a correct ImplementingClass.

public class ImplementingClass extends AbstractClass
{
    public void abstractMethod() { System.out.print("abstractMethod()"); }
}

Notice that you don't have to define implementedMethod() or finalMethod(). They were already defined by AbstractClass.

Here's another correct ImplementingClass.

public class ImplementingClass extends AbstractClass
{
    public void abstractMethod() { System.out.print("abstractMethod()"); }
    public void implementedMethod() { System.out.print("Overridden!"); }
}

In this case, you have overridden implementedMethod().

However, because of the final keyword, the following is not possible.

public class ImplementingClass extends AbstractClass
{
    public void abstractMethod() { System.out.print("abstractMethod()"); }
    public void implementedMethod() { System.out.print("Overridden!"); }
    public void finalMethod() { System.out.print("ERROR!"); }
}

You can't do this because the implementation of finalMethod() in AbstractClass is marked as the final implementation of finalMethod(): no other implementations will be allowed, ever.

Now you can also implement an abstract class twice:

public class ImplementingClass extends AbstractClass
{
    public void abstractMethod() { System.out.print("abstractMethod()"); }
    public void implementedMethod() { System.out.print("Overridden!"); }
}

// In a separate file.
public class SecondImplementingClass extends AbstractClass
{
    public void abstractMethod() { System.out.print("second abstractMethod()"); }
}

Now somewhere you could write another method.

public tryItOut()
{
    ImplementingClass a = new ImplementingClass();
    AbstractClass b = new ImplementingClass();

    a.abstractMethod();    // prints "abstractMethod()"
    a.implementedMethod(); // prints "Overridden!"     <-- same
    a.finalMethod();       // prints "finalMethod()"

    b.abstractMethod();    // prints "abstractMethod()"
    b.implementedMethod(); // prints "Overridden!"     <-- same
    b.finalMethod();       // prints "finalMethod()"

    SecondImplementingClass c = new SecondImplementingClass();
    AbstractClass d = new SecondImplementingClass();

    c.abstractMethod();    // prints "second abstractMethod()"
    c.implementedMethod(); // prints "implementedMethod()"
    c.finalMethod();       // prints "finalMethod()"

    d.abstractMethod();    // prints "second abstractMethod()"
    d.implementedMethod(); // prints "implementedMethod()"
    d.finalMethod();       // prints "finalMethod()"
}

Notice that even though we declared b an AbstractClass type, it displays "Overriden!". This is because the object we instantiated was actually an ImplementingClass, whose implementedMethod() is of course overridden. (You may have seen this referred to as polymorphism.)

If we wish to access a member specific to a particular subclass, we must cast down to that subclass first:

// Say ImplementingClass also contains uniqueMethod()
// To access it, we use a cast to tell the runtime which type the object is
AbstractClass b = new ImplementingClass();
((ImplementingClass)b).uniqueMethod();

Lastly, you cannot do the following:

public class ImplementingClass extends AbstractClass, SomeOtherAbstractClass
{
    ... // implementation
}

Only one class can be extended at a time. If you need to extend multiple classes, they have to be interfaces. You can do this:

public class ImplementingClass extends AbstractClass implements InterfaceA, InterfaceB
{
    ... // implementation
}

Here's an example interface:

interface InterfaceA
{
    void interfaceMethod();
}

This is basically the same as:

abstract public class InterfaceA
{
    abstract public void interfaceMethod();
}

The only difference is that the second way doesn't let the compiler know that it's actually an interface. This can be useful if you want people to only implement your interface and no others. However, as a general beginner rule of thumb, if your abstract class only has abstract methods, you should probably make it an interface.

The following is illegal:

interface InterfaceB
{
    void interfaceMethod() { System.out.print("ERROR!"); }
}

You cannot implement methods in an interface. This means that if you implement two different interfaces, the different methods in those interfaces can't collide. Since all the methods in an interface are abstract, you have to implement the method, and since your method is the only implementation in the inheritance tree, the compiler knows that it has to use your method.

Answer 2

A Java class becomes abstract under the following conditions:

1. At least one of the methods is marked as abstract:

public abstract void myMethod()

In that case the compiler forces you to mark the whole class as abstract.

2. The class is marked as abstract:

abstract class MyClass

As already said: If you have an abstract method the compiler forces you to mark the whole class as abstract. But even if you don't have any abstract method you can still mark the class as abstract.

Common use:

A common use of abstract classes is to provide an outline of a class similar like an interface does. But unlike an interface it can already provide functionality, i.e. some parts of the class are implemented and some parts are just outlined with a method declaration. ("abstract")

An abstract class cannot be instantiated, but you can create a concrete class based on an abstract class, which then can be instantiated. To do so you have to inherit from the abstract class and override the abstract methods, i.e. implement them.

Answer 3

A class that is declared using the abstract keyword is known as abstract class. Abstraction is a process of hiding the data implementation details, and showing only functionality to the user. Abstraction lets you focus on what the object does instead of how it does it.

Main things of abstract class

• An abstract class may or may not contain abstract methods.There can be non abstract methods.

  An abstract method is a method that is declared without an implementation (without braces, and followed by a semicolon), like this:

  ex : abstract void moveTo(double deltaX, double deltaY);

• If a class has at least one abstract method then that class must be abstract

• Abstract classes may not be instantiated (You are not allowed to create object of Abstract class)

• To use an abstract class, you have to inherit it from another class. Provide implementations to all the abstract methods in it.

• If you inherit an abstract class, you have to provide implementations to all the abstract methods in it.

Declare abstract class Specifying abstract keyword before the class during declaration makes it abstract. Have a look at the code below:

abstract class AbstractDemo{ }

Declare abstract method Specifying abstract keyword before the method during declaration makes it abstract. Have a look at the code below,

abstract void moveTo();//no body

Why we need to abstract classes

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 ex -: position, orientation, line color, fill color) and behaviors (for ex -: moveTo, rotate, resize, draw) in common. Some of these states and behaviors are the same for all graphic objects (for ex : fill color, position, and moveTo). Others require different implementation(for ex: resize or draw). All graphic objects 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 ex : GraphicObject) as shown in the following figure.

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 declared abstract methods, such as draw or resize, that need to be a implemented by all subclasses but must be implemented in different ways. The GraphicObject class can look something like this:

abstract class GraphicObject {

  void moveTo(int x, int y) {
    // Inside this method we have to change the position of the graphic 
    // object according to x,y     
    // This is the same in every GraphicObject. Then we can implement here. 
  }

  abstract void draw(); // But every GraphicObject drawing case is 
                        // unique, not common. Then we have to create that 
                        // case inside each class. Then create these    
                        // methods as abstract 
  abstract void resize();
}

Usage of abstract method in sub classes Each non abstract subclasses of GraphicObject, such as Circle and Rectangle, must provide implementations for the draw and resize methods.

class Circle extends GraphicObject {
  void draw() {
    //Add to some implementation here
  }
  void resize() {
    //Add to some implementation here   
  }
}
class Rectangle extends GraphicObject {
  void draw() {
    //Add to some implementation here
  }
  void resize() {
    //Add to some implementation here
  }
}

Inside the main method you can call all methods like this:

public static void main(String args[]){
   GraphicObject c = new Circle();
   c.draw();
   c.resize();
   c.moveTo(4,5);   
}

Ways to achieve abstraction in Java

There are two ways to achieve abstraction in java

• Abstract class (0 to 100%)
• Interface (100%)

Abstract class with constructors, data members, methods, etc

abstract class GraphicObject {

  GraphicObject (){
    System.out.println("GraphicObject  is created");
  }
  void moveTo(int y, int x) {
       System.out.println("Change position according to "+ x+ " and " + y);
  }
  abstract void draw();
}

class Circle extends GraphicObject {
  void draw() {
    System.out.println("Draw the Circle");
  }
}

class TestAbstract {  
 public static void main(String args[]){

   GraphicObject  grObj = new Circle ();
   grObj.draw();
   grObj.moveTo(4,6);
 }
}

Output:

GraphicObject  is created
Draw the Circle
Change position according to 6 and 4

Remember two rules:

• If the class has few abstract methods and few concrete methods, declare it as an abstract class.

• If the class has only abstract methods, declare it as an interface.

References:

• TutorialsPoint - Java Abstraction
• BeginnersBook - Java Abstract Class Method
• Java Docs - Abstract Methods and Classes
• JavaPoint - Abstract Class in Java

Answer 4

It's a class that cannot be instantiated, and forces implementing classes to, possibly, implement abstract methods that it outlines.

Answer 5

Simply speaking, you can think of an abstract class as like an Interface with a bit more capabilities.

You cannot instantiate an Interface, which also holds for an abstract class.

On your interface you can just define the method headers and ALL of the implementers are forced to implement all of them. On an abstract class you can also define your method headers but here - to the difference of the interface - you can also define the body (usually a default implementation) of the method. Moreover when other classes extend (note, not implement and therefore you can also have just one abstract class per child class) your abstract class, they are not forced to implement all of your methods of your abstract class, unless you specified an abstract method (in such case it works like for interfaces, you cannot define the method body).

public abstract class MyAbstractClass{
  public abstract void DoSomething();
}

Otherwise for normal methods of an abstract class, the "inheriters" can either just use the default behavior or override it, as usual.

Example:

public abstract class MyAbstractClass{

  public int CalculateCost(int amount){
     //do some default calculations
     //this can be overriden by subclasses if needed
  }

  //this MUST be implemented by subclasses
  public abstract void DoSomething();
}

Answer 6

Get your answers here:

Abstract class vs Interface in Java

Can an abstract class have a final method?

BTW - those are question you asked recently. Think about a new question to build up reputation...

Edit:

Just realized, that the posters of this and the referenced questions have the same or at least similiar name but the user-id is always different. So either, there's a technical problem, that keyur has problems logging in again and finding the answers to his questions or this is a sort of game to entertain the SO community ;)

Answer 7

From oracle documentation

Abstract Methods and Classes:

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.

Since abstract classes and interfaces are related, have a look at below SE questions:

What is the difference between an interface and abstract class?

How should I have explained the difference between an Interface and an Abstract class?

Answer 8

Solution - base class (abstract)

public abstract class Place {

String Name;
String Postcode;
String County;
String Area;

Place () {

        }

public static Place make(String Incoming) {
        if (Incoming.length() < 61) return (null);

        String Name = (Incoming.substring(4,26)).trim();
        String County = (Incoming.substring(27,48)).trim();
        String Postcode = (Incoming.substring(48,61)).trim();
        String Area = (Incoming.substring(61)).trim();

        Place created;
        if (Name.equalsIgnoreCase(Area)) {
                created = new Area(Area,County,Postcode);
        } else {
                created = new District(Name,County,Postcode,Area);
        }
        return (created);
        }

public String getName() {
        return (Name);
        }

public String getPostcode() {
        return (Postcode);
        }

public String getCounty() {
        return (County);
        }

public abstract String getArea();

}

Answer 9

Little addition to all these posts.

Sometimes you may want to declare a class and yet not know how to define all of the methods that belong to that class. For example, you may want to declare a class called Writer and include in it a member method called write(). However, you don't know how to code write() because it is different for each type of Writer devices. Of course, you plan to handle this by deriving subclass of Writer, such as Printer, Disk, Network and Console.

Answer 10

An abstract class can not be directly instantiated, but must be derived from to be usable. A class MUST be abstract if it contains abstract methods: either directly

abstract class Foo {
    abstract void someMethod();
}

or indirectly

interface IFoo {
    void someMethod();
}

abstract class Foo2 implements IFoo {
}

However, a class can be abstract without containing abstract methods. Its a way to prevent direct instantation, e.g.

abstract class Foo3 {
}

class Bar extends Foo3 {

}

Foo3 myVar = new Foo3(); // illegal! class is abstract
Foo3 myVar = new Bar(); // allowed!

The latter style of abstract classes may be used to create "interface-like" classes. Unlike interfaces an abstract class is allowed to contain non-abstract methods and instance variables. You can use this to provide some base functionality to extending classes.

Another frequent pattern is to implement the main functionality in the abstract class and define part of the algorithm in an abstract method to be implemented by an extending class. Stupid example:

abstract class Processor {
    protected abstract int[] filterInput(int[] unfiltered);

    public int process(int[] values) {
        int[] filtered = filterInput(values);
        // do something with filtered input
    }
}

class EvenValues extends Processor {
    protected int[] filterInput(int[] unfiltered) {
        // remove odd numbers
    }
}

class OddValues extends Processor {
    protected int[] filterInput(int[] unfiltered) {
        // remove even numbers
    }
}

Answer 11

What is Abstract class? Ok! lets take an example you known little bit about chemistry we have an element carbon(symbol C).Carbon has some basic atomic structure which you can't change but using carbon you can make so many compounds like (CO2),Methane(CH4),Butane(C4H10).

So Here carbon is abstract class and you do not want to change its basic structure however you want their childrens(CO2,CH4 etc) to use it.But in their own way

Answer 12

Class which can have both concrete and non-concrete methods i.e. with and without body.

1. Methods without implementation must contain 'abstract' keyword.
2. Abstract class can't be instantiated.

Answer 13

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.

In other words, a class that is declared with abstract keyword, is known as abstract class in java. It can have abstract(method without body) and non-abstract methods (method with body).

Important Note:- Abstract classes cannot be used to instantiate objects, they can be used to create object references, because Java's approach to run-time Polymorphism is implemented through the use of superclass references. Thus, it must be possible to create a reference to an abstract class so that it can be used to point to a subclass object. You will see this feature in the below example

abstract class Bike{  
  abstract void run();  
}  

class Honda4 extends Bike{  
    void run(){
        System.out.println("running safely..");
    }  

    public static void main(String args[]){  
       Bike obj = new Honda4();  
       obj.run();  
    }  
} 

Answer 14

An abstract class is one that isn't fully implemented but provides something of a blueprint for subclasses. It may be partially implemented in that it contains fully-defined concrete methods, but it can also hold abstract methods. These are methods with a signature but no method body. Any subclass must define a body for each abstract method, otherwise it too must be declared abstract. Because abstract classes cannot be instantiated, they must be extended by at least one subclass in order to be utilized. Think of the abstract class as the generic class, and the subclasses are there to fill in the missing information.

Answer 15

It do nothing, just provide a common template that will be shared for it's subclass