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I am programming a simple platformer game, and I have several types of platforms. I created a class for the most simple type and made the rest subclasses, with the only difference between each class being the value of their variables (so they all share the same variable and method names).
In my collision detection, I loop through a HashMap. Within that HashMap are ArrayLists of the instances of each class. But when I use a nested loop to loop through try to call their methods implicitly, I have found that I cannot access these methods without explicitly declaring which class I want to call the method from.
I have done research, although the only way I can see of doing this is to loop through the instances of each class separately, meaning one loop per class; I would rather not do this, since it would be a lot more code than I feel is necessary.
In order to be able to call a common method on classes of different types you need to give your objects a common supertype declaring the common method - i.e. they should have a common superclass, or implement a common interface.
Interfaces provide an easier way of declaring common functionality, because a class can implement multiple interfaces, but it can extend only one class.
Provide an interface with the common method, then declare the map to use objects of that interface, i.e.
interface CommonInterface {
void commonMethod(int arg);
}
class One implements CommonInterface {
public void commonMethod(int arg) {
...
}
}
class Two implements CommonInterface {
public void commonMethod(int arg) {
...
}
}
Here is what you can do now:
Map<String,CommonInterface> myMap = new HashMap<>();
myMap.put("one", new One());
myMap.put("two", new Two());
for (Map.Entry<String,CommonInterface> e : myMap.entrySet()) {
System.out.println(e.getKey());
CommonInterface c = e.getValue();
c.commonMethod(123);
}
Simple, make each platform class implement an IPlatform interface or extand a base class. Look up java polymorphism and interfaces.
Are your subclasses overriding the common methods from the super class?
In other words, are your subclass' common methods declared in your simpler class?
If it is the case, you can simply call the method as if it is a simple class:
public abstract class Fruit {
public abstract void method();
}
public class Apple extends Fruit {
#Override
public void method() {
System.out.println("I'm an apple");
}
}
public class Orange extends Fruit {
#Override
public void method()
System.out.println("I'm an orange");
}
}
Using this you can simply call your method from any fruit, since it has your method declared. No need to know which fruit it is. The following code:
Fruit fruit = new Orange();
fruit.method();
will output: "I'm an orange".
I am trying to understand better oop ,but I don't understand how to use abstract classes.
I have a car class, and a car can be mercedes , audi and volvo. Mercedes cars can have 5 passengers ,can open door and block windows; audi car can have 6 passengers, and just can open door. Volvo car can only have 3 passengers.
So, in order to do this, I created an interface:
public interface Car{
void openDoor();
void blockWindows();
int passengers=0;
}
then for each car I created an abstract class:
public abstract class Mercedes implements Car{
public void openDoor(){
System.out.println("Mercedes opendoor");
}
public void blockWindow(){
System.out.println("Mercedes blockwindow");
}
public Mercedes()
{
int passengers=5;
}
public abstract class Audi implements Car{
public void openDoor(){
System.out.println("Audi opendoor");
}
public Audi()
{
int passengers=6;
}
}
public abstract class Volvo implements Car{
public Volvo()
{
int passengers=6;
}
Now, I need to create an object that can transport maximum 15 cars. So I wrote:
public class TransportCars{
Car[] transport=new Car[15];}
//now I need to put in transport array differents types of cars. But I can not instantiate abstract classes. Should I use anything else? I used abstract classes because I can implement an interface and use just o part of it
Basically your design is completely wrong, as you are yet new to java, you first need to understand basic.
Design should be like this :
Car is a Vehical, so is-a relationship.
So you can create a Class Vehicle.
class Vehicle {
// properties of Vehicle like type of Vehicle, numberOfWheels etc.
String vType;
int numberOfWheels;
int passengers;
}
// Car is a Vehicle so it should extend Vehicle
class Car extends Vehicle {
String type; // sedan or hatchback
String manufacturer; // Mercedes, BMW, Audi, Volvo etc.
}
If you want to restrict Vehicle not to be instantiated, you can declare it as an abstract class
Interfaces and abstract classes have some similarities, but are not the same. For your example, you should probably make Car an abstract and have Mercedes, Audi, and Volvo extend that abstract class. Then make sure to implement any abstract methods in Car in each of your classes which extend it. Doing so will make them concrete classes, which can be instantiated.
One thing you can do with abstract classes which you cannot with interfaces, is include data. I see you're already doing that with your interface for number of passengers, but in an interface, the value will be static and final.
I see the problem why it's hard for you to understant abstraction, it's because your example is wrong. The type of car must be a concrete class which inherits from an abstract class. The specificity of an abstract class is that you can't create one of it, you can only inherit it ,that benefits polymorphism. But the real benefits comes from abstract methods.
Instead of creating a Car interface ,create a Vehicle interface.
Since you don't know how many passengers each type of car can carry make Car an abstract
class. Every vehicle have to start and stop. And you know that a car must load the passengers first in order to start. In the end you can start all your vehicles regardless what type car is it , οr what type of vehicle.
interface Vehicle {
public start();
public stop();
}
abstract class Car implements Vehicle {
protected wheels = 4;
public start() {
loadPassengers();
// do extra stuff like
//closeDoors();
}
abstract public loadPassengers();
}
public class Volvo extends Car {
int passengers = 6;
public loadPassengers() {
doSomething(this.passengers);
}
}
public static void main() {
List<Car> cars = new ArrayList<Car>();
cars.add(new Volvo());
cars.add(new Mercedes());
for(Car car : cars) {
car.start();
}
}
In terms of relationships between classes, there's notmuch differences between abstract classes and interfaces: You can't instantiate any of them and they would be used as a template for objects depending on them. You can implement partially the methods of an abstract class however, but even if all of them are implemented still you can't instantiate a class defined as abstract. To be concise:
Interfaces:
Define methods.
A class can implement several interfaces.
Public visibility (or package, by default).
Can't be instantiated.
Abstract classes:
Define methods and may implement them.
A class can inherit from only one class (abstract or not).
User defined visibility.
Can't be instantiated.
If your cars are meant to implement several interfaces, use interfaces, but their scope will have to be public or package. If you just want to have an inheritance relation with one class, use abstract.
It seems that you are going about this the wrong way. It would be better for you to seek a tutorial, but I will try to clear up as much as I can for you:
The car class should be either an interface or an abstract class (or neither).
If it's an interface, than Mercedes, Audi and Volvo should implement it.
If that is the case, any method in "car" must be implemented in the others, so all of them must have "Open door" and "Block windows". you must choose for each of the implementing classes how it will implement it.
If it's an abstract class, you can have some of the methods implemented in "car" and they will work "as is" in Mercedes, Audi and Volvo (which will "extend" car), unless you re-define them in their respective classes. if you want to enforce their implementation in each of inheriting classes, you can define those methods to be abstract in "car", and not implement them in car at all.
If you want to implement all of the methods in car, you don't need it to be abstract at all. You could still re-define them as mentioned above.
Suppose I have the following situation:
public abstract class Vehicle {
public void turnOn() { ... }
}
public interface Flier {
public void fly();
}
Is there a way that I can guarantee that any class that implements Flier must also extend Vehicle? I don't want to make Flier an abstract class because I want to be able to mix a few other interfaces in a similar manner.
For instance:
// I also want to guarantee any class that implements Car must also implement Vehicle
public interface Car {
public void honk();
}
// I want the compiler to either give me an error saying
// MySpecialMachine must extend Vehicle, or implicitly make
// it a subclass of Vehicle. Either way, I want it to be
// impossible to implement Car or Flier without also being
// a subclass of Vehicle.
public class MySpecialMachine implements Car, Flier {
public void honk() { ... }
public void fly() { ... }
}
Java interfaces cannot extend classes, which makes sense since classes contain implementation details that cannot be specified within an interface..
The proper way to deal with this problem is to separate interface from implementation completely by turning Vehicle into an interface as well. The Car e.t.c. can extend the Vehicle interface to force the programmer to implement the corresponding methods. If you want to share code among all Vehicle instances, then you can use a (possibly abstract) class as a parent for any classes that need to implement that interface.
You could rearrange your classes and interfaces like this:
public interface IVehicle {
public void turnOn();
}
public abstract class Vehicle implements IVehicle {
public void turnOn() { ... }
}
public interface Flier extends IVehicle {
public void fly();
}
This way all implementations of Flier are guaranteed to implement the protocol of a vehicle, namely IVehicle.
If you have control on the Vehicle classes just extract Vehicle as an interface and then provide a base implementation.
If you have no control over Vehicle class, for example because it is part of a framework you are using or a third party library, it's not possible to do in Java.
The closest thing you can do is using Generics multiple wildcards notation.
<T extends Vehicle & Car>
But you can't really apply it directly to Car unless you do something like this:
public interface Car<T extends Vehicle & Car>() {
T self();
}
Which is bot weird and do not enforce the self method to actually return self, it's just a strong hint/suggestion.
You would implement a Car like this:
public class CitroenC3 extends Vehicle implements Car<CitroenC3> {
#Override
public CitroenC3 self() {
return this;
}
}
one can use a Car<?> like this:
Car<?> car = obtainCarInSomeWay();
Vehicle v = car.self();
Car c = car.self();
they should be both valid syntax.
What the compiler enforce here is that what you specify in Car<WHICH> as WHICH must both extend Vehicle and implement Car. And by adding self() you are saying to the programmer that the T object is supposed to be the object itself, thus forcing the wildcard instance to match the class if he want to be compliant with the specification.
in Java 8 you can even define a default implementation for the self method.
I also wish there was a better way to handle something like this.
It's a strange requirement, but you can accomplish something of the sort with Generics:
<T extends MyInterface & MyAbstractClass>
This question shows that you haven't grasped the essence of interface and class. Forgetting the concrete Java syntax right now, all you need to understand first is that: interface is a set of protocol, which should be implementation-agnostic. It makes no sense to let an interface extend a class(which is implementation-oriented).
Back to your concrete question, if you want to guarantee that a Flier is always a kind of Vehicle, just change the latter to an interface and let former extends it(It does make sense to extend one protocol from the other protocol). After that, you may create any class(abstract or concrete) that implements Vehicle or Flier.
Define a new Package
Create a new interface (ie. HiddenOne) with scope "default" with a method "implementMe(HiddenOne)"
Move Vehicle and Flier to the new Package.
Inherit Vehicle and Flier from HiddenOne
Implement the method implementMe in Vehicle.
Now: Whenever you like to implement from "Flier" you must extends from Vehicle !
(because only Vehicle can implement implementMe).
This is tricky but works great.
Can you have a class which implements an interface, and choose whether to use the methods in the interface during instantiation of this class? Therefore having object A which uses the interface and object B which does not use it.
Thanks
Updated:
Assuming you have a Professor class and this class implements an interface called Employer, which has employ(rAssist x) abstract method.
Now I want to instantiated 2 objects from the Professor class implementing this interface Object A - Professor can employ a research assistant and Object B - Professor cannot employ research assistants.
Can you have a class which implements an interface, and choose whether to use the methods in the interface during instantiation of this class?
No, if class C implements the interface, then all instances of C will provide the methods declared in the interface.
What you can do is something like
class MyClass implements MyInterface {
#Override
void interfaceMethod() {
System.out.println("Interface method");
}
}
and then do
MyClass x = new MyClass();
MyClass y = new MyClass() {
#Override
void interfaceMethod() {
throw new UnsupportedOperationException();
}
};
In effect, x supports the use of interfaceMethod while y does not. Note however that...
The usage of y.interfaceMethod is not prevented at compile-time, i.e. it will not be enforced by the type system.
With this solution, you are in fact creating an (anonymous) subclass of MyClass and assigning an instance of it to y.
Do you mean you want class A and Class B to implement a common Interface but you dont want to implement all methods in Class B?
An Interface in simple terms means it is sort of a contract and all the classes which implement it should follow that contract.So if you want Class B to implement the interface , Class B should also follow the same contract. But if you dont want to implement any methos you can always do this.
class ISampleInterface {
void sampleMethod();
void optionalMethod();
}
Class A implements ISampleInterface {
void sampleMethod() {
//Your Implementation
}
void optionalMethod() {
//Your Implementation
}
}
class B implements ISampleInterface {
void sampleMethod() {
//Your Implementation
}
void optionalMethod() {
throw new UnsupportedMethodException();
}
}
No, that's not the point of an Interface.
An Interface is contract that guarantees that implementations WILL implement it's signature
The idea of interface is to establish a obligation for the class that implements the interface.
If your's is a requirement, you can use the java.lang.reflect.Method reflection class to change the visibility of the method at runtime. However, this is not a clean way.
1. Interfaces were introduced in Java because Multiple Inheritance was not allowed in Java.
2. But as far as Design Pattern are concerned, following are the uses..
- To implement certain Roles.
Consider Dog a Super class, but then Pet dog and Wild dog can be interfaces, which
can be implemented by the Sub Classes of Dog class.
- Used when Behaviors keeps changing.
Consider you have a Class Drawing, and paint method() in it, now paint can be stroking, shading, etc...
You must Encapsulate such behaviors in an Interface or an Abstract class.
What is an "abstract class" in Java?
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:
Define methods which can be used by the inheriting subclass.
Define abstract methods which the inheriting subclass must implement.
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.
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.
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
It's a class that cannot be instantiated, and forces implementing classes to, possibly, implement abstract methods that it outlines.
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();
}
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?
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 ;)
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.
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
}
}
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();
}
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
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();
}
}
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.
Class which can have both concrete and non-concrete methods i.e. with and without body.
Methods without implementation must contain 'abstract' keyword.
Abstract class can't be instantiated.
It do nothing, just provide a common template that will be shared for it's subclass