How many instances one can have of an anonymous class? - java

An anonymous class is something like this:
SenseOfLife _42 = new SenseOfLife() {
public int eval() {
return 42;
}
};
I didn't understand why such an anonymous class can have an unbounded number of instances (this is a proposition).
While I'm defining this anonymous class it creates automatically an instance and assigns it to the variable _42. So I have only one instance and cannot create a new instance.

You could create multiple instances by:
Executing the same block of code more than once.
Cloning an instance of an anonymous class.

If the interface extends Cloneable the class could technically be cloned.
Reflection could get a new instance.
If the same new SenseOfLife() constructor is called in a loop that constructor will get compiled down to one class that is instantiated multiple times.

I haven't tried it, but most likely you can create other instances using reflection, i.e., instance.getClass().newInstance().

Related

Using methods from other classes

The difference between (a) import somePackage.someClass; and (b)someClass object = new someClass(); is that (a) will allow call the methods from the imported class without creating new instances of it, while (b) will create an object using the template class and therefore the methods for the class someClass will belong to the object object. So if I want to use a method someMethod() from someClass in (b) I'd call it through the object object. Is it how it works?
Yes you can use static methods from a class directly
Yes you can use methods from a class by creating an object
But more important thing than just the above options available is when to use which. First type of call is to class methods whereas the second class is to instance methods.
Instance Methods vs Class Methods: Each class represents a set of attributes and behaviour. Instance methods usually represent the behaviour. example if Person is a class and Robb is an object, then robb.weight can be attribute, robb.write() would be an instance method and Person.type() (ans: species) or Person.population (ans: total number of instances) can be class methods.
Also you represent instance methods in textual writing as ClassName#instanceMethod and ClassName.classMethods
No, you are wrong
Simplistically if the class that you want to use is not in the same package then you need to import it, or fully path the class e.g. java.util.ArrayList.
If the methods are not static, then you will need to create a new Instance of the class you want to use.
You can use methods from other class directly only if it is a static method. You will also have to add static in your import statement if you want to use the method name directly without prefixing it will the class name.
For non-static methods you have to create instance of the class and then call that method.

Is the object of an abstract class an anonymous inner class? [duplicate]

This question already has answers here:
Can we instantiate an abstract class?
(16 answers)
Closed 9 years ago.
When I create the object of an Abstract class, I have to do like this just like an interface.
AbstractClass abstractClass = new AbstractClass() {
#Override
public void abstractMethod() {
}
};
Does this mean that the object of AbstractClass is an anonymous inner class object?
AbstractClass abstractClass = new AbstractClass() {
#Override
public void abstractMethod() {
}
};
This block of code means that you are creating an anonymous class which extends AbstractClass. You can also use the same notation for an interface also.
SomeInterface someImplementingClass = new SomeInterface(){/*some code here*/};
This means you are creating a class that implements SomeInterface.
Note that there are certain limitation when you are creating an anonymous class. As and anonymous class is already extending the parent type, you cannot make it extend another class as in java you can extend only on class.
This code will help understand concept of overriding methods in anonymous classes
class Anonymous {
public void someMethod(){
System.out.println("This is from Anonymous");
}
}
class TestAnonymous{
// this is the reference of superclass
Anonymous a = new Anonymous(){ // anonymous class definition starts here
public void someMethod(){
System.out.println("This is in the subclass of Anonymous");
}
public void anotherMethod(){
System.out.println("This is in the another method from subclass that is not in suprerclass");
}
}; // and class ends here
public static void main(String [] args){
TestAnonymous ta = new TestAnonymous();
ta.a.someMethod();
// ta.a.anotherMethod(); commented because this does not compile
// for the obvious reason that we are using the superclass reference and it
// cannot access the method in the subclass that is not in superclass
}
}
This outputs
This is in the subclass of Anonymous
Remember that anotherMethod is implemented in implemented in the subclass that is created as anonymous class. and a is the reference variable of type Anonymous i.e. superclass of the anonymous class. So the statement ta.a.anotherMethod(); gives compiler error as anotherMethod() is not available in Anonymous.
An object is not a class object (in this context). It is derived from a class. In Java there is a difference between classes and objects, as compared to e.g. prototype based languages (e.g. JavaScript) where this difference does not exist.
In your example you create an anonymous class, create an object of that anonymous class and assign it to a variable; all in one step.
Anonymous classes are always inner classes: http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.9.5
http://docs.oracle.com/javase/specs/jls/se7/html/jls-8.html#jls-8.1.3
One fundamental properties of attract classes is the fact that there could be no direct instance of this type. Only classes that implement the complete interface of an class can be instantiated.
In order to create an object you need a non abstract class first, by extending the abstract class.
Abstract classes does not have any instances (objects of it's type). I recommend Mavia to take a look at following link for clearness:
http://docs.oracle.com/javase/tutorial/java/IandI/abstract.html
You cannot create an object of an abstract class. They are non-instantiable. What you are doing when you do this is creating a sort of dynamic subclass object, and instantiating that (at the same time). More or less, yes, you can create it the same way as an interface. See this answer for more information.
In fact here you create both: an anonymous inner class, that extends AbstractClass as well as an instance of this anonyomous class, the object. You do not and cannot create an instance of AbstractClass.
Also you declare a variable named abstractClass that has the Type AbstractClass. Inside this variable you store a newly created instance of your newly defined subclass of AbstractClass.
EDIT: You can of course not reuse the anonymous inner class, since it is anonymous, and the only place where an instance of it can be created or rather is created is right here.
Here might be a loop or function in which case you would be able to create many instances of this anonymous inner class. But it would still be only this piece of code where instances are created.

Java Factory patterns

Here is a factory "inner class?" from the Java Jung graph package:
Factory<Graph<String, Integer>> graphFactory = new Factory<Graph<String, Integer>>()
{
public Graph<String, Integer> create()
{
return new SparseMultigraph<String, Integer>();
}
};
What I want to know is what programming language concept is the above? Especially what is this concept in Java? Is the above an inner class? It is clearly not a method. It seems odd to me because it constructs a Factory object and then has braces with a semicolon at the end with a method to create a graph.
It is an anonyous inner class. The above code creates a subclass of the Factory class (or a class implementing the Factory interface), overrides its create() method, calls its constructor, and assigns the result to the graphFactory variable.
It is an anonymous inner class.
It's an anonymous inner class concept.In layman terms,it can also be called an unnamed class.There is always a debate on the use of such classes against inheritance.Usually,if there is a one time usage requirement of a child class,then anonymous classes are more handy than inherited classes.
As per the post it seems that you are aware that it is the concept of anonymous class in java that you are talking about.
Now since you are questioning y this :
The answer is that , as the name implies the class has no name, hence in a single step
class declaration
the creation of an instance of the class
is complete. Due to this reason use of anonymous class save time and effort for creating a .java file. :)
As a rule anonymous class must implement all the abstract methods in the super class or the interface and should use all the default constructor of the super class.

This Way of Writing?

Though i have often come across,i dont understand this way of writing code:
Runnable r=new Runnable() {//<----- (braces start here?)
public void run() {
System.out.println("Hello");
}
}; // ?
What is this?
Please explain very clearly.
That's an anonymous inner class. It's creating an implementation of the Runnable interface using code within the braces. As well as implementing interfaces, you can extend other classes. The nice aspects are that you can do this without explicitly creating a separate class, and you can also refer to final local variables (including parameters) within the body of the anonymous inner class.
See the Java tutorial for more details, or just search for "anonymous inner class" for loads of related pages.
As mentioned by others, what is being created here is an anonymous inner class. Specifically, the person who wrote the code is saying:
Instead of an instance of the Runnable class, I want to create a subclass that overrides the "run()" method and create an instance of that. Its not worth my time to create a named subclass, since I'm only going to create this one instance. Instead, just override the method and return the subclass instance I need.
That's an anonymous class declaration - basically, a class that implements the Runnable interface, declared and instantiated inline as an anonymous nested class.
Note that you can also declare anonymous subclasses the same way:
Object o = new Object(){
public String toString(){ return "boo!" };
}
Also note that you can use variables of the enclosing method inside the anonymous class code, but only if the variables are final (because the anonymous class actually gets a copy of the variable).
I would start with these http://www.google.co.uk/search?q=anonymous+classes+tutorial 23 million results.
Basically, it allows you to define an implementation or a subclass without having to create a fully formed class defintiions.
EDIT: For your own interest, see if you can figure out what this does.
Map<String, String> map = new LinkedHashMap<String, String>() {{ // two brackets
put("a", "aye");
put("b", "bee");
put("c", "see");
put("d", "dee");
put("e", "ee");
put("f", "eff");
}};
It's so called 'anonymous class'.
You can do this:
class MyRunnable implements Runnable{
public void run() {
System.out.println("Hello");
}
}
Then:
Runnable r = new MyRunnable();
And achieve the same thing. However, the MyRunnable class that you created is never needed in any other part of your code. So creating a named class is not necessary. The code that you wrote on the other hand is creating an anonymous inner class such that the implementation of the class is precisely where it is needed. It will not be accessible rom anywhere else in the code but that is the idea, you do not need it anywhere else.

Anonymous class definition based on interface... maybe?

I saw this Java snippet in the book Spring in Action, but I'm not familiar with the language construct.
new RowMapper() {
public Object mapRow() throws SQLException, DataAccessException {
Motorist motorist = new Motorist();
motorist.setId(rs.getInt(1));
motorist.setEmail(rs.getString(2));
motorist.setPassword(rs.getString(3));
motorist.setFirstName(rs.getString(4));
motorist.setLastName(rs.getString(5));
return motorist;
}
}
According the Spring documentation, RowMapper is an interface. It looks to me like an anonymous class definition based on the RowMapper interface. The new keyword is a little confusing, making me wonder if this also creates one instance of the anonymous class. I would guess yes, because if the class has no name, how will you ever create an instance after the line that defines it?
Can anyone confirm my guesses that:
this is an anonymous class definition based on the RowMapper interface, and
it creates a single instance of that class?
This is an anonymous class definition based on the RowMapper interface
That's precisely what it is.
It creates a single instance of that class?
Yep. That's correct.
That code is implementing the interface in an anonymous way.
The syntax would be similar to:
Runnable runnable = new Runnable() {
public void run() {
}
};
Note the semicolon at the end of the declaration. Here the runnable object, though holds the reference to the Runnable interface actually contains the implemented object. That's runtime polymorphism for you!
Your guesses are entirely correct. An anonymous class definition may be based on either a non-final class or on an interface, and you must implement all abstract (or interface) methods. The only available syntax for declaring anonymous classes is new, which also has the effect of instantiating exactly one instance of the anonymous class (in the course of the program, though, many instances of the same anonymous class could be created, if this code is executed several times).
Interface tells what methods the built class instance should have or if thy are label interfaces, then what kind of behavior to associate with it.
Anonymous classes are classes that basically while instantiating a class instance thy are also extending it with custom code. So if you are instantiating a interface, then you must write all the methods described with that interface, and as long as you do at least that much, then compiler will be happy. This is what is done here.
IS this is an anonymous class definition based on the RowMapper interface?
Yes. As you can see mapRow() function has been written. And if you debug the code you can see, that is not a class of an instance of interface, but class that extends interface. In case of abstract class or just class, it would be same - extended. So if class is final you cant write anonymous class for it.
Does it create a single instance of that class?
Well, it extends it and makes an instance of it. It will be single instance and any sequent call to it would result in a different class. If you debug the code, then you can even see different class names dynamically associated with it.
Solely from the code above and without knowing about RowMapper, all you can assume is that a new anonymous class based on RowMapper (which may be an interface or a class) is instantiated.
Declaring Anonymous class and in below example it creates two instances .
public class Multithread {
void test(){
new Runnable() {
#Override
public void run() {
System.out.println("1");
}
}.run();
new Runnable() {
#Override
public void run() {
System.out.println("11");
}
}.run();}
public static void main(String[] args) {
new Multithread().test();
}
}

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