I read that you can add extra methods for an object, or override the abstract methods in this way, yet I have an error. Can you tell me if this is even possible or what I have done wrong?
public abstract class Pesti {
public abstract void ud();
}
public class EX2 {
public static void main(String[] args) {
Pesti p = new Pesti() {
public void ud() {
System.out.println("Pestele este ud!");
}
public void inn() {
System.out.println("Innoata!");
}
};
p.ud();
p.inn();
}
}
It tells me that it cannot find method inn.
What you are doing is creating an anonymous sub-class of your Pesti. However, since your p variable is of type Pesti, you can only call methods declared in the Pesti class (or any of its super-classes). Therefore you can call ud(), but not inn().
If you could cast p to the type of the sub-class, you could have called inn(), but since this is an anonymous sub-class instance, you can't cast to that type.
You could declare inn() as an abstract (or concrete) method in Pesti. Then your code would pass compilation, and the implementation of this method in the anonymous sub-class instance will be executed.
public abstract class Pesti {
public abstract void ud();
public abstract void inn();
}
You declared p as an instance of Pesti, but tried to use a method not defined in Pesti. When you declare your object as the abstract class you can only use the methods defined in this class. To use your inn() method you have to declare this method in the Pesti class as well. Alternativly you can write a class extending Pesti which implements the inn() method and declare your object as the extending class.
Pesti is an abstract class with exactly one method (which is abstract): ud(). As such, a variable declared with the type Pesti can only be used to call ud() or any method from the class Object (Pesti's implicit superclass). p is declared with this type, so despite actually being an instance of an anonymous class with the method inn(), this method is not available in the context of having a variable of declared type Pesti.
Perhaps you intended to add the inn() method to Pesti so all instances must implement it:
public abstract class Pesti {
public abstract void ud();
public abstract void inn();
}
Or perhaps you don't really want to use anonymous classes at all and want to create a subclass of Pesti with the inn() method available:
public class PestiImpl extends Pesti {
#Override
public void ud() {
System.out.println("Pestele este ud!");
}
public void inn() {
System.out.println("Innoata!");
}
}
(or you could have a layer of inheritance between the two; some abstract class extending Pesti with inn() declared as an abstract method, if you have other classes like this with the inn() method but some Pestis without).
Side note: I strongly recommend using the #Override annotation where appropriate to make your code more readable and to avoid mistakes like typos when trying to override a method. You can get most IDEs to insert it automatically.
Think you need to look at this link, I don't know what do you want to do , but you can't instantiate abstract class, you can find detailed description in my link, hope it'll help you
Related
I am starting java programming and I came across abstract classes. I know that you cannot instantiate them without creating concrete classes which extend them to become the subclass. However, I got really confused when I tried this code and it runs ok.
abstract class Communication{
public void FirstMethod()
{
System.out.println("I am first method()\n");
}
}
public class Main{
public static void main(String[] args){
Communication communication = new Communication() {
#Override
public void FirstMethod(){
super.FirstMethod();
}
};
communication.FisrtMethod();
}
}
Output is: I am first method().
If I modify it to:
Communication communication = new Communication() {
#Override
public void FirstMethod(){
System.out.println("I've been called from Main");
}
};
The output is: I've been called from Main.
Could somebody please explain if this is a kind of instantiation or what concept is this?
This is termed as
Anonymous Class
Definition:
An inner class declared without a class name is known as an anonymous inner class.
In case of anonymous inner classes, we declare and instantiate them at the same time. Generally, they are used whenever you need to override the method of a class or an interface.
This is called anonymous inner class. This way you can implement an interface or abstract class without having to find a name for it and instantiate it at the same time. This concept is useful when you use a certain implementation just once.
The construct looks always like that:
new SomeClass() {
//implementation of methods
};
This is known as anonymous class. The anonymous class definition allows you to provide a class definition within code and it has no formal name. The anonymous class expression has the class definition and instance creation expression.This is not limited to abstract classes but also for interfaces and concrete classes.
For example
abstract class A { }
// so the anonymous class expression is
A a = new A() {// class definition };
// This will actually create an instance of a
// class that extends the abstract class A
// that java will create at run time
You can even use anonymous class expression in the method arguments.Example of this is a Comparator in Collections.sort() method;
Collections.sort(listOfValues,new Comparator<Value>(){
public int compare(Value v1, Value v2){
// method implemetation.
}
})
I came across a piece of code where there is a call to an abstract method ? I was just curious to know what it means ? Meanwhile the class is being called from a non-abstract method.
When you implement an abstract class you will also implement a concrete class. In the concrete class all abstract methods of the abstract class need to be implemented.
When a method in the abstract class call an abstract method, it refers to the implementation of this method in the concrete class.
public abstract class A {
public String method() {
// do something
int i = abstractMethod(); // abstract method call
}
public abstract int abstractMethod();
}
public class B extends A {
public int abstractMethod() { // <-- this implementation of the method is called.
return 1;
}
}
public class C extends A {
public int abstractMethod() { // <-- this implementation of the method is called.
return 100;
}
public static void main(String[] args) {
A a = new B();
a.method(); // -> abstractMethod of class B is called
a = new C();
c.method(); // -> abstractMethod of class C is called
}
}
An abstract method is a method that belongs to an abstract class but has no implementation which means that it has to be implemented (or overridden) by any children of the abstract class.
That is the whole idea of an abstract method / class: It allows the developer to define the behavior of the class later.
Assume, your class requires to "load some resource", but you want to provide different implementations for that task. Then you can override the abstract method with either loading a resource over the network or from the file system etc.
See: http://docs.oracle.com/javase/tutorial/java/IandI/abstract.html
An abstract method is a method inside an abstract class or an interface. It has no body, and one must be defined by a subclass before it can be instantiated. This allows interface classes that define some common methods, but leave some to the implementations.
The abstract methods can be called from non-abstract ones, because:
They are non-static, so they need an instance of that class to be called.
An instance of that class may not exist if the method hasn't been implemented, as the resulting class would also be abstract.
Therefore a caller will always have an instance of a class implementing that method.
I came to situation :
public interface Intr {
public void m1();
}
public abstract class Abs {
public void m1() {
System.out.println("Abs.m1()");
}
// public abstract void m1();
}
public class A extends Abs implements Intr {
#Override
public void m1() {
// which method am I overriding, well it is Abs.m1() but why?
// if method implemented is Abs.m1(), then why I am not getting error for Intr.m1() not implemented.
}
}
You are satisfying both conditions at once; ie. the one implementation is at the same time fulfilling the abstract class requirements and the interface requirements.
As a note, unless you are using Intr in another inheritance chain, you don't need it. Also, it might make sense to move the implements Intr up to the abstract class definition.
You can only override methods defined in another class.
Methods declared in an interface are merely implemented. This distinction exists in Java to tackle the problem of multiple inheritance. A class can only extend one parent class, therefore any calls to super will be resolved without ambiguity. Classes however can implement several interfaces, which can all declare the same method. It's best to think of interfaces as a list of "must have"s: to qualify as a Comparable your cluss must have a compareTo() method but it doesn't matter where it came from or what other interfaces require that same method.
So technically you override Abs.m1() and implement Intr.m1() in one fell swoop.
Note that this would be fine too:
public class B extends Abs implements Intr {
//m1() is inherited from Abs, so there's no need to override it to satisfy the interface
}
Here, both the interface and abstract class have the same method.
You have one class with named Derived which extends an abstract class and implement an interface. It's true and you override print method on Derived class it's fine and it compiles correctly and does not give any error but here you can't identify which class method is overridden like abstract class or interface.
This is runtime polymorphism you can't create an instance of an abstract class or interface but you can create a reference variable of that type. Here the solution is you can't identify that on compile-time it's actually overridden at run time.
interface AnInterface
{
public void print();
}
abstract class Base
{
public abstract void print();
}
public class Derived extends Base implements AnInterface
{
public void print(){
System.out.println("hello");
}
AnInterface iRef = new Derived();
iRef.print(); // It means interface method is overridden and it's decided when we call the method.
Base base = new Derived();
base.print(); // It means abstract class method is overridden.
}
#Override ensures you override the method with no difference Interface or abstract superclass. So no error with override.
On the other hand Interface method is also implemented in the superclass which is enough for Interface contracts.
Let's consider the following simple code in Java.
package temppkg;
interface Interface
{
#Override
public abstract boolean equals(java.lang.Object arg);
#Override
public abstract String toString();
public void show();
}
final class Demo implements Interface
{
public void show()
{
System.out.println("Method invoked.");
}
}
final public class Main
{
public static void main(String...args)
{
new Demo().show();
}
}
In the above code snippet, the interface named Interface has some Object class methods from JDK and they are with the #Override annotation even though they are abstract. Now, the class Demo has implemented Interface and has not implemented the equals() and the toString(); methods. Still the compiler doesn't complain and the program is running successfully. Why?
What is the relation between interfaces and the object class in Java?
The Java Language Specification clearly says that the members of an
interface are those which are declared in the interface and those
which are inherited from direct super interfaces. If an interface has
no direct superinterface then the interface implicitly declares a
public abstract member method corresponding to each public instance
method declared in the Object class, unless a method with the same
signature, same return type, and a compatible throws clause is
explicitly declared by that interface. This is what makes the
signatures of the Object methods available to the compiler and the
code compiles without any error. Remember if the interface tries to
declare a public instance method declared 'final' in the Object class
then it'll result into a compile-time error. For example, 'public
final Class getClass()' is a public instance method declared 'final'
in the Object class and therefore if an interface tries to declare a
method with this signature then the compilation will fail.
http://geekexplains.blogspot.com/2008/06/do-interfaces-really-inherit-from-class.html
Check out JLS 9.2:
If an interface has no direct superinterfaces, then the interface implicitly declares a public abstract member method m with signature s, return type r, and throws clause t corresponding to each public instance method m with signature s, return type r, and throws clause t declared in Object, unless a method with the same signature, same return type, and a compatible throws clause is explicitly declared by the interface. It is a compile-time error if the interface explicitly declares such a method m in the case where m is declared to be final in Object.
In other words, every interface implicitly defines each of Object's methods, and you can therefore #Override those methods. The other methods aren't defined in Object, so you can't override them.
In Interface you're not actually overriding anything - an interface by definition can not provide implementations for any of its methods. The class Demo just inherits the equals and toString implementation from Object.
In essence an interface in Java contains a set of zero or more method signatures (all of them are implicitly abstract, in your code you made it explicit by adding the keyword abstract), and the concrete classes that implement the interface must provide an implementation for those methods. In the case of your code, that implementation comes from Object, since all the classes implicitly extend Object, which provides default implementations for equals and toString (among other methods).
You really shouldn't mark the methods in an interface with #Override, as you have seen, it's confusing and serves for no practical purpose. Instead, use #Override in the methods in the concrete class that implement the methods of the interface, like this:
class Demo implements Interface {
#Override
public void show() {
System.out.println("Method invoked.");
}
}
Also, it's completely unnecessary to declare equals and toString in an interface, so you're better off with this definition:
interface Interface {
public void show();
}
#Override can only be used for functions defined in the base class, Object. Object defines equals and toString, so you can use #Override with them, but not with, say, the function show. It indicates a relationship between the #Override functions in a class and its base classes, not it's derived classes.
In Java, I can't create instances of abstract classes. So why doesn't eclipse scream about the following code?
public abstract class FooType {
private final int myvar;
public FooType() {
myvar = 1;
}
}
The code is fine, the final variable is initialized in the constructor of FooType.
You cannot instantiate FooType because of it being abstract. But if you create a non abstract subclass of FooType, the constructor will be called.
If you do not have an explicit call to super(...) in a constructor, the Java Compiler will add it automatically. Therefore it is ensured that a constructor of every class in the inheritance chain is called.
You can have constructors, methods, properties, everything in abstract classes that you can have in non-abstract classes as well. You just can't instantiate the class. So there is nothing wrong with this code.
In a deriving class you can call the constructor and set the final property:
public class Foo extends FooType
{
public Foo()
{
super(); // <-- Call constructor of FooType
}
}
if you don't specify a call to super(), it will be inserted anyway by the compiler.
You can create concrete sub-classes of FooType and they will all have a final field called myvar.
BTW: A public constructor in an abstract class is the same as a protected one as it can only be called from a sub-class.
What is your doubt?
Ok. See, an abstract class can have a constructor. It's always there-implicit or explicit. In fact when you create an object of a subclass of an abstract class, the first thing that the constructor of the subclass does is call the constructor of its abstract superclass by using super(). It is just understood, that's why you don't have to write super() explicitly unless you use parameterized constructors. Every class even if it is abstract, has an implicit constructor which you cannot see. It is called unless you create some constructor of your own. so long you created abstract classes without creating any custom constructor in it, so you didn't know about the existence of the implicit constructor.
You definitely can declare final variable in abstract class as long as you assign value to it either in the constructor or in declaration. The example that guy gave makes no sense.
No you can't declare final variables inside an Abstract class.
Check Below example.
public abstract class AbstractEx {
final int x=10;
public abstract void AbstractEx();
}
public class newClass extends AbstractEx{
public void AbstractEx(){
System.out.println("abc");
}
}
public class declareClass{
public static void main(String[] args) {
AbstractEx obj = new newClass ();
obj.AbstractEx();
// System.out.println(x);
}
}
This code runs correct and produce output as
abc
But if we remove comment symbol of
System.out.println(x);
it will produce error.