Related
What is the use of anonymous classes in Java? Can we say that usage of anonymous class is one of the advantages of Java?
By an "anonymous class", I take it you mean anonymous inner class.
An anonymous inner class can come useful when making an instance of an object with certain "extras" such as overriding methods, without having to actually subclass a class.
I tend to use it as a shortcut for attaching an event listener:
button.addActionListener(new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
// do something
}
});
Using this method makes coding a little bit quicker, as I don't need to make an extra class that implements ActionListener -- I can just instantiate an anonymous inner class without actually making a separate class.
I only use this technique for "quick and dirty" tasks where making an entire class feels unnecessary. Having multiple anonymous inner classes that do exactly the same thing should be refactored to an actual class, be it an inner class or a separate class.
Anonymous inner classes are effectively closures, so they can be used to emulate lambda expressions or "delegates". For example, take this interface:
public interface F<A, B> {
B f(A a);
}
You can use this anonymously to create a first-class function in Java. Let's say you have the following method that returns the first number larger than i in the given list, or i if no number is larger:
public static int larger(final List<Integer> ns, final int i) {
for (Integer n : ns)
if (n > i)
return n;
return i;
}
And then you have another method that returns the first number smaller than i in the given list, or i if no number is smaller:
public static int smaller(final List<Integer> ns, final int i) {
for (Integer n : ns)
if (n < i)
return n;
return i;
}
These methods are almost identical. Using the first-class function type F, we can rewrite these into one method as follows:
public static <T> T firstMatch(final List<T> ts, final F<T, Boolean> f, T z) {
for (T t : ts)
if (f.f(t))
return t;
return z;
}
You can use an anonymous class to use the firstMatch method:
F<Integer, Boolean> greaterThanTen = new F<Integer, Boolean> {
Boolean f(final Integer n) {
return n > 10;
}
};
int moreThanMyFingersCanCount = firstMatch(xs, greaterThanTen, x);
This is a really contrived example, but its easy to see that being able to pass functions around as if they were values is a pretty useful feature. See "Can Your Programming Language Do This" by Joel himself.
A nice library for programming Java in this style: Functional Java.
Anonymous inner class is used in following scenario:
1.) For Overriding(subclassing), when class definition is not usable except current case:
class A{
public void methodA() {
System.out.println("methodA");
}
}
class B{
A a = new A() {
public void methodA() {
System.out.println("anonymous methodA");
}
};
}
2.) For implementing an interface, when implementation of interface is required only for current case:
interface InterfaceA{
public void methodA();
}
class B{
InterfaceA a = new InterfaceA() {
public void methodA() {
System.out.println("anonymous methodA implementer");
}
};
}
3.) Argument Defined Anonymous inner class:
interface Foo {
void methodFoo();
}
class B{
void do(Foo f) { }
}
class A{
void methodA() {
B b = new B();
b.do(new Foo() {
public void methodFoo() {
System.out.println("methodFoo");
}
});
}
}
I use them sometimes as a syntax hack for Map instantiation:
Map map = new HashMap() {{
put("key", "value");
}};
vs
Map map = new HashMap();
map.put("key", "value");
It saves some redundancy when doing a lot of put statements. However, I have also run into problems doing this when the outer class needs to be serialized via remoting.
They're commonly used as a verbose form of callback.
I suppose you could say they're an advantage compared to not having them, and having to create a named class every time, but similar concepts are implemented much better in other languages (as closures or blocks)
Here's a swing example
myButton.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent e) {
// do stuff here...
}
});
Although it's still messily verbose, it's a lot better than forcing you to define a named class for every throw away listener like this (although depending on the situation and reuse, that may still be the better approach)
You use it in situations where you need to create a class for a specific purpose inside another function, e.g., as a listener, as a runnable (to spawn a thread), etc.
The idea is that you call them from inside the code of a function so you never refer to them elsewhere, so you don't need to name them. The compiler just enumerates them.
They are essentially syntactic sugar, and should generally be moved elsewhere as they grow bigger.
I'm not sure if it is one of the advantages of Java, though if you do use them (and we all frequently use them, unfortunately), then you could argue that they are one.
GuideLines for Anonymous Class.
Anonymous class is declared and initialized simultaneously.
Anonymous class must extend or implement to one and only one class or interface resp.
As anonymouse class has no name, it can be used only once.
eg:
button.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent arg0) {
// TODO Auto-generated method stub
}
});
Yes, anonymous inner classes is definitely one of the advantages of Java.
With an anonymous inner class you have access to final and member variables of the surrounding class, and that comes in handy in listeners etc.
But a major advantage is that the inner class code, which is (at least should be) tightly coupled to the surrounding class/method/block, has a specific context (the surrounding class, method, and block).
new Thread() {
public void run() {
try {
Thread.sleep(300);
} catch (InterruptedException e) {
System.out.println("Exception message: " + e.getMessage());
System.out.println("Exception cause: " + e.getCause());
}
}
}.start();
This is also one of the example for anonymous inner type using thread
An inner class is associated with an instance of the outer class and there are two special kinds: Local class and Anonymous class. An anonymous class enables us to declare and instantiate a class at same time, hence makes the code concise. We use them when we need a local class only once as they don't have a name.
Consider the example from doc where we have a Person class:
public class Person {
public enum Sex {
MALE, FEMALE
}
String name;
LocalDate birthday;
Sex gender;
String emailAddress;
public int getAge() {
// ...
}
public void printPerson() {
// ...
}
}
and we have a method to print members that match search criteria as:
public static void printPersons(
List<Person> roster, CheckPerson tester) {
for (Person p : roster) {
if (tester.test(p)) {
p.printPerson();
}
}
}
where CheckPerson is an interface like:
interface CheckPerson {
boolean test(Person p);
}
Now we can make use of anonymous class which implements this interface to specify search criteria as:
printPersons(
roster,
new CheckPerson() {
public boolean test(Person p) {
return p.getGender() == Person.Sex.MALE
&& p.getAge() >= 18
&& p.getAge() <= 25;
}
}
);
Here the interface is very simple and the syntax of anonymous class seems unwieldy and unclear.
Java 8 has introduced a term Functional Interface which is an interface with only one abstract method, hence we can say CheckPerson is a functional interface. We can make use of Lambda Expression which allows us to pass the function as method argument as:
printPersons(
roster,
(Person p) -> p.getGender() == Person.Sex.MALE
&& p.getAge() >= 18
&& p.getAge() <= 25
);
We can use a standard functional interface Predicate in place of the interface CheckPerson, which will further reduce the amount of code required.
i use anonymous objects for calling new Threads..
new Thread(new Runnable() {
public void run() {
// you code
}
}).start();
Anonymous inner class can be beneficial while giving different implementations for different objects. But should be used very sparingly as it creates problem for program readability.
One of the major usage of anonymous classes in class-finalization which called finalizer guardian. In Java world using the finalize methods should be avoided until you really need them. You have to remember, when you override the finalize method for sub-classes, you should always invoke super.finalize() as well, because the finalize method of super class won't invoke automatically and you can have trouble with memory leaks.
so considering the fact mentioned above, you can just use the anonymous classes like:
public class HeavyClass{
private final Object finalizerGuardian = new Object() {
#Override
protected void finalize() throws Throwable{
//Finalize outer HeavyClass object
}
};
}
Using this technique you relieved yourself and your other developers to call super.finalize() on each sub-class of the HeavyClass which needs finalize method.
You can use anonymous class this way
TreeSet treeSetObj = new TreeSet(new Comparator()
{
public int compare(String i1,String i2)
{
return i2.compareTo(i1);
}
});
Seems nobody mentioned here but you can also use anonymous class to hold generic type argument (which normally lost due to type erasure):
public abstract class TypeHolder<T> {
private final Type type;
public TypeReference() {
// you may do do additional sanity checks here
final Type superClass = getClass().getGenericSuperclass();
this.type = ((ParameterizedType) superClass).getActualTypeArguments()[0];
}
public final Type getType() {
return this.type;
}
}
If you'll instantiate this class in anonymous way
TypeHolder<List<String>, Map<Ineger, Long>> holder =
new TypeHolder<List<String>, Map<Ineger, Long>>() {};
then such holder instance will contain non-erasured definition of passed type.
Usage
This is very handy for building validators/deserializators. Also you can instantiate generic type with reflection (so if you ever wanted to do new T() in parametrized type - you are welcome!).
Drawbacks/Limitations
You should pass generic parameter explicitly. Failing to do so will lead to type parameter loss
Each instantiation will cost you additional class to be generated by compiler which leads to classpath pollution/jar bloating
An Anonymous Inner Class is used to create an object that will never be referenced again. It has no name and is declared and created in the same statement.
This is used where you would normally use an object's variable. You replace the variable with the new keyword, a call to a constructor and the class definition inside { and }.
When writing a Threaded Program in Java, it would usually look like this
ThreadClass task = new ThreadClass();
Thread runner = new Thread(task);
runner.start();
The ThreadClass used here would be user defined. This class will implement the Runnable interface which is required for creating threads. In the ThreadClass the run() method (only method in Runnable) needs to be implemented as well.
It is clear that getting rid of ThreadClass would be more efficient and that's exactly why Anonymous Inner Classes exist.
Look at the following code
Thread runner = new Thread(new Runnable() {
public void run() {
//Thread does it's work here
}
});
runner.start();
This code replaces the reference made to task in the top most example. Rather than having a separate class, the Anonymous Inner Class inside the Thread() constructor returns an unnamed object that implements the Runnable interface and overrides the run() method. The method run() would include statements inside that do the work required by the thread.
Answering the question on whether Anonymous Inner Classes is one of the advantages of Java, I would have to say that I'm not quite sure as I am not familiar with many programming languages at the moment. But what I can say is it is definitely a quicker and easier method of coding.
References: Sams Teach Yourself Java in 21 Days Seventh Edition
The best way to optimize code. also, We can use for an overriding method of a class or interface.
import java.util.Scanner;
abstract class AnonymousInner {
abstract void sum();
}
class AnonymousInnerMain {
public static void main(String []k){
Scanner sn = new Scanner(System.in);
System.out.println("Enter two vlaues");
int a= Integer.parseInt(sn.nextLine());
int b= Integer.parseInt(sn.nextLine());
AnonymousInner ac = new AnonymousInner(){
void sum(){
int c= a+b;
System.out.println("Sum of two number is: "+c);
}
};
ac.sum();
}
}
One more advantage:
As you know that Java doesn't support multiple inheritance, so if you use "Thread" kinda class as anonymous class then the class still has one space left for any other class to extend.
What is the use of anonymous classes in Java? Can we say that usage of anonymous class is one of the advantages of Java?
By an "anonymous class", I take it you mean anonymous inner class.
An anonymous inner class can come useful when making an instance of an object with certain "extras" such as overriding methods, without having to actually subclass a class.
I tend to use it as a shortcut for attaching an event listener:
button.addActionListener(new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
// do something
}
});
Using this method makes coding a little bit quicker, as I don't need to make an extra class that implements ActionListener -- I can just instantiate an anonymous inner class without actually making a separate class.
I only use this technique for "quick and dirty" tasks where making an entire class feels unnecessary. Having multiple anonymous inner classes that do exactly the same thing should be refactored to an actual class, be it an inner class or a separate class.
Anonymous inner classes are effectively closures, so they can be used to emulate lambda expressions or "delegates". For example, take this interface:
public interface F<A, B> {
B f(A a);
}
You can use this anonymously to create a first-class function in Java. Let's say you have the following method that returns the first number larger than i in the given list, or i if no number is larger:
public static int larger(final List<Integer> ns, final int i) {
for (Integer n : ns)
if (n > i)
return n;
return i;
}
And then you have another method that returns the first number smaller than i in the given list, or i if no number is smaller:
public static int smaller(final List<Integer> ns, final int i) {
for (Integer n : ns)
if (n < i)
return n;
return i;
}
These methods are almost identical. Using the first-class function type F, we can rewrite these into one method as follows:
public static <T> T firstMatch(final List<T> ts, final F<T, Boolean> f, T z) {
for (T t : ts)
if (f.f(t))
return t;
return z;
}
You can use an anonymous class to use the firstMatch method:
F<Integer, Boolean> greaterThanTen = new F<Integer, Boolean> {
Boolean f(final Integer n) {
return n > 10;
}
};
int moreThanMyFingersCanCount = firstMatch(xs, greaterThanTen, x);
This is a really contrived example, but its easy to see that being able to pass functions around as if they were values is a pretty useful feature. See "Can Your Programming Language Do This" by Joel himself.
A nice library for programming Java in this style: Functional Java.
Anonymous inner class is used in following scenario:
1.) For Overriding(subclassing), when class definition is not usable except current case:
class A{
public void methodA() {
System.out.println("methodA");
}
}
class B{
A a = new A() {
public void methodA() {
System.out.println("anonymous methodA");
}
};
}
2.) For implementing an interface, when implementation of interface is required only for current case:
interface InterfaceA{
public void methodA();
}
class B{
InterfaceA a = new InterfaceA() {
public void methodA() {
System.out.println("anonymous methodA implementer");
}
};
}
3.) Argument Defined Anonymous inner class:
interface Foo {
void methodFoo();
}
class B{
void do(Foo f) { }
}
class A{
void methodA() {
B b = new B();
b.do(new Foo() {
public void methodFoo() {
System.out.println("methodFoo");
}
});
}
}
I use them sometimes as a syntax hack for Map instantiation:
Map map = new HashMap() {{
put("key", "value");
}};
vs
Map map = new HashMap();
map.put("key", "value");
It saves some redundancy when doing a lot of put statements. However, I have also run into problems doing this when the outer class needs to be serialized via remoting.
They're commonly used as a verbose form of callback.
I suppose you could say they're an advantage compared to not having them, and having to create a named class every time, but similar concepts are implemented much better in other languages (as closures or blocks)
Here's a swing example
myButton.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent e) {
// do stuff here...
}
});
Although it's still messily verbose, it's a lot better than forcing you to define a named class for every throw away listener like this (although depending on the situation and reuse, that may still be the better approach)
You use it in situations where you need to create a class for a specific purpose inside another function, e.g., as a listener, as a runnable (to spawn a thread), etc.
The idea is that you call them from inside the code of a function so you never refer to them elsewhere, so you don't need to name them. The compiler just enumerates them.
They are essentially syntactic sugar, and should generally be moved elsewhere as they grow bigger.
I'm not sure if it is one of the advantages of Java, though if you do use them (and we all frequently use them, unfortunately), then you could argue that they are one.
GuideLines for Anonymous Class.
Anonymous class is declared and initialized simultaneously.
Anonymous class must extend or implement to one and only one class or interface resp.
As anonymouse class has no name, it can be used only once.
eg:
button.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent arg0) {
// TODO Auto-generated method stub
}
});
Yes, anonymous inner classes is definitely one of the advantages of Java.
With an anonymous inner class you have access to final and member variables of the surrounding class, and that comes in handy in listeners etc.
But a major advantage is that the inner class code, which is (at least should be) tightly coupled to the surrounding class/method/block, has a specific context (the surrounding class, method, and block).
new Thread() {
public void run() {
try {
Thread.sleep(300);
} catch (InterruptedException e) {
System.out.println("Exception message: " + e.getMessage());
System.out.println("Exception cause: " + e.getCause());
}
}
}.start();
This is also one of the example for anonymous inner type using thread
An inner class is associated with an instance of the outer class and there are two special kinds: Local class and Anonymous class. An anonymous class enables us to declare and instantiate a class at same time, hence makes the code concise. We use them when we need a local class only once as they don't have a name.
Consider the example from doc where we have a Person class:
public class Person {
public enum Sex {
MALE, FEMALE
}
String name;
LocalDate birthday;
Sex gender;
String emailAddress;
public int getAge() {
// ...
}
public void printPerson() {
// ...
}
}
and we have a method to print members that match search criteria as:
public static void printPersons(
List<Person> roster, CheckPerson tester) {
for (Person p : roster) {
if (tester.test(p)) {
p.printPerson();
}
}
}
where CheckPerson is an interface like:
interface CheckPerson {
boolean test(Person p);
}
Now we can make use of anonymous class which implements this interface to specify search criteria as:
printPersons(
roster,
new CheckPerson() {
public boolean test(Person p) {
return p.getGender() == Person.Sex.MALE
&& p.getAge() >= 18
&& p.getAge() <= 25;
}
}
);
Here the interface is very simple and the syntax of anonymous class seems unwieldy and unclear.
Java 8 has introduced a term Functional Interface which is an interface with only one abstract method, hence we can say CheckPerson is a functional interface. We can make use of Lambda Expression which allows us to pass the function as method argument as:
printPersons(
roster,
(Person p) -> p.getGender() == Person.Sex.MALE
&& p.getAge() >= 18
&& p.getAge() <= 25
);
We can use a standard functional interface Predicate in place of the interface CheckPerson, which will further reduce the amount of code required.
i use anonymous objects for calling new Threads..
new Thread(new Runnable() {
public void run() {
// you code
}
}).start();
Anonymous inner class can be beneficial while giving different implementations for different objects. But should be used very sparingly as it creates problem for program readability.
One of the major usage of anonymous classes in class-finalization which called finalizer guardian. In Java world using the finalize methods should be avoided until you really need them. You have to remember, when you override the finalize method for sub-classes, you should always invoke super.finalize() as well, because the finalize method of super class won't invoke automatically and you can have trouble with memory leaks.
so considering the fact mentioned above, you can just use the anonymous classes like:
public class HeavyClass{
private final Object finalizerGuardian = new Object() {
#Override
protected void finalize() throws Throwable{
//Finalize outer HeavyClass object
}
};
}
Using this technique you relieved yourself and your other developers to call super.finalize() on each sub-class of the HeavyClass which needs finalize method.
You can use anonymous class this way
TreeSet treeSetObj = new TreeSet(new Comparator()
{
public int compare(String i1,String i2)
{
return i2.compareTo(i1);
}
});
Seems nobody mentioned here but you can also use anonymous class to hold generic type argument (which normally lost due to type erasure):
public abstract class TypeHolder<T> {
private final Type type;
public TypeReference() {
// you may do do additional sanity checks here
final Type superClass = getClass().getGenericSuperclass();
this.type = ((ParameterizedType) superClass).getActualTypeArguments()[0];
}
public final Type getType() {
return this.type;
}
}
If you'll instantiate this class in anonymous way
TypeHolder<List<String>, Map<Ineger, Long>> holder =
new TypeHolder<List<String>, Map<Ineger, Long>>() {};
then such holder instance will contain non-erasured definition of passed type.
Usage
This is very handy for building validators/deserializators. Also you can instantiate generic type with reflection (so if you ever wanted to do new T() in parametrized type - you are welcome!).
Drawbacks/Limitations
You should pass generic parameter explicitly. Failing to do so will lead to type parameter loss
Each instantiation will cost you additional class to be generated by compiler which leads to classpath pollution/jar bloating
An Anonymous Inner Class is used to create an object that will never be referenced again. It has no name and is declared and created in the same statement.
This is used where you would normally use an object's variable. You replace the variable with the new keyword, a call to a constructor and the class definition inside { and }.
When writing a Threaded Program in Java, it would usually look like this
ThreadClass task = new ThreadClass();
Thread runner = new Thread(task);
runner.start();
The ThreadClass used here would be user defined. This class will implement the Runnable interface which is required for creating threads. In the ThreadClass the run() method (only method in Runnable) needs to be implemented as well.
It is clear that getting rid of ThreadClass would be more efficient and that's exactly why Anonymous Inner Classes exist.
Look at the following code
Thread runner = new Thread(new Runnable() {
public void run() {
//Thread does it's work here
}
});
runner.start();
This code replaces the reference made to task in the top most example. Rather than having a separate class, the Anonymous Inner Class inside the Thread() constructor returns an unnamed object that implements the Runnable interface and overrides the run() method. The method run() would include statements inside that do the work required by the thread.
Answering the question on whether Anonymous Inner Classes is one of the advantages of Java, I would have to say that I'm not quite sure as I am not familiar with many programming languages at the moment. But what I can say is it is definitely a quicker and easier method of coding.
References: Sams Teach Yourself Java in 21 Days Seventh Edition
The best way to optimize code. also, We can use for an overriding method of a class or interface.
import java.util.Scanner;
abstract class AnonymousInner {
abstract void sum();
}
class AnonymousInnerMain {
public static void main(String []k){
Scanner sn = new Scanner(System.in);
System.out.println("Enter two vlaues");
int a= Integer.parseInt(sn.nextLine());
int b= Integer.parseInt(sn.nextLine());
AnonymousInner ac = new AnonymousInner(){
void sum(){
int c= a+b;
System.out.println("Sum of two number is: "+c);
}
};
ac.sum();
}
}
One more advantage:
As you know that Java doesn't support multiple inheritance, so if you use "Thread" kinda class as anonymous class then the class still has one space left for any other class to extend.
See the following example:
interface I {}
class A implements I {}
class B implements I {}
class Foo{
void f(A a) {}
void f(B b) {}
static public void main(String[]args ) {
I[] elements = new I[] {new A(), new B(), new B(), new A()};
Foo o = new Foo();
for (I element:elements)
o.f(element);//won't compile
}
}
Why doesn't overloading methods support upcasting?
If overloading was implemented at run time, it would provide much more flexibility. E.g, the Visitor Pattern would be simpler. Is there any technical reason that prevents Java from doing this?
Overload resolution involves some non-trivial rules to determine which overload is the best fit, and it'd be hard to do these efficiently at runtime. In contrast, override resolution is easier -- in the hard case you have to just look up the foo function for the object's class, and in the easy case (e.g. when there's only one implementation, or only one implementation in this code path), you can turn the virtual method into a statically-compiled, non-virtual, non-dynamically-dispatching call (if you're doing it based on the code path, you have to do a quick check to verify that the object is actually the one you expect).
As it turns out, it's a good thing Java 1.4 and lower didn't have runtime override resolution, because that would make generics much harder to retrofit. Generics play a role in override resolution, but this information wouldn't be available at runtime due to erasure.
There is no theoretical reason why it cannot be done. The Common Lisp Object System supports this type of construction — called multiple dispatch — although it does so in a somewhat different paradigm (methods, rather than being attached to objects, are instances of generics (or generic functions), which can do virtual dispatch at run-time on the values of multiple parameters). I believe there have also been extensions to Java to enable it (Multi-Java comes to mind, although that may have been multiple inheritance rather than multiple dispatch).
There may, however, be Java language reasons why it cannot be done, besides the language designers just thinking it shouldn't be done, that I'll leave others to reason about. It does introduce complications for inheritance, though. Consider:
interface A {}
interface B {}
class C implements A {}
class Foo {
public void invoke(A a) {}
public void invoke(B b) {}
}
class Bar extends Foo {
public void invoke(C c) {}
}
class Baz extends Bar {
public void invoke(A a) {}
}
Baz obj = new Baz();
obj.invoke(new C);
Which invoke is invoked? Baz? Bar? What is super.invoke? It is possible to come up with deterministic semantics, but they will likely involve confusion and surprise in at least some cases. Given that Java aims to be a simple language, I don't think features introducing such confusion are likely to be seen as according with its goals.
Is there any technical reason that prevents Java from doing this?
Code correctness: your current example provides two implementations of I and two corresponding methods f. However nothing prevents the existence of other classes implementing I - moving the resolution to runtime would also replace compile errors to possibly hidden runtime errors.
Performance: as others have mentioned method overloading involves rather complex rules, doing so once at compile time is certainly faster than doing it for every method invocation at runtime.
Backwards compatibility: currently overloaded methods are resolved using the compile time type of passed arguments rather than their runtime type, changing the behavior to use runtime information would break a lot of existing applications.
How to work around it
Use the visitor pattern, I do not understand how someone would think that it is hard.
interface I{
void accept(IVisitor v);
}
interface IVisitor{
void f(A a);
void f(B b);
}
class A implements I{
void accept(IVisitor v){v.f(this);}
}
class B implements I{
void accept(IVisitor v){v.f(this);}
}
class Foo implements IVisitor{
void f(A a) {}
void f(B b) {}
static public void main(String[]args ) {
I[] elements = new I[] {new A(), new B(), new B(), new A()};
Foo o = new Foo();
for (I element:elements)
element.accept(o);
}
}
I don't think anyone but the designers of the language could possible answer this question. I am not nearly an expert on the subject, but I will provide just my opinion.
By reading the JLS 15.12 about Method Invocation Expressions, it is pretty clear that choosing the right method to execute is an already complicated compile-time process; above all after the introduction of generics.
Now imagine moving all this to the runtime just to support the single feature of mutimethods. To me it sounds like a small feature that adds too much complexity to the language, and probably a feature with certain amount of performance implications now that all these decisions would need to be made, over and over, at runtime, and not just once, as today it is, at compile time.
To all these we could add the fact that due to type erasure it would be impossible to determine the actual type of certain generic types. It appears to me that abandoning the safety of the static type checking is not in the best interest of Java.
At any rate, there are valid alternatives to deal with the multiple dispatch problem, and perhaps these alternatives pretty much justify why it has not been implemented in the language. So, you can use the classical visitor pattern or you can use certain amount of reflection.
There is an outdated MultiJava Project that implemented mutiple dispatch support in Java and there are a couple of other projects out there using reflection to support multimethods in Java: Java Multimethods, Java Multimethods Framework. Perhaps there are even more.
You could also consider an alternative Java-based language which does support multimethods, like Clojure or Groovy.
Also, since C# is a language pretty similar to Java in its general phillosopy, it might be interesting to investigate more on how it supports multimethods and meditate on what would be the implications of offering a similar feature in Java. If you think it's a feature worth having in Java you can even submit a JEP and it may be taken into account for future releases of the Java language.
Not the answer to Java. This functionality exists in C# 4 though:
using System;
public class MainClass {
public static void Main() {
IAsset[] xx = {
new Asset(), new House(), new Asset(), new House(), new Car()
};
foreach(IAsset x in xx) {
Foo((dynamic)x);
}
}
public static void Foo(Asset a) {
Console.WriteLine("Asset");
}
public static void Foo(House h) {
Console.WriteLine("House");
}
public static void Foo(Car c) {
Console.WriteLine("Car");
}
}
public interface IAsset { }
public class Asset : IAsset { }
public class House : Asset { }
public class Car : Asset { }
Output:
Asset
House
Asset
House
Car
If you are using C# 3 and below, you have to use reflection, I made a post about it on my blog Multiple Dispatch in C# : http://www.ienablemuch.com/2012/04/multiple-dispatch-in-c.html
If you want to do multiple dispatch in Java, you might go the reflection route.
Here's another solution for Java: http://blog.efftinge.de/2010/03/multiple-dispatch-and-poor-mens-patter.html
Guess you just have to settle with reflection:
import java.lang.reflect.*;
interface I {}
class A implements I {}
class B implements I {}
public class Foo {
public void f(A a) { System.out.println("from A"); }
public void f(B b) { System.out.println("from B"); }
static public void main(String[]args ) throws InvocationTargetException
, NoSuchMethodException, IllegalAccessException
{
I[] elements = new I[] {new A(), new B(), new B(), new A()};
Foo o = new Foo();
for (I element : elements) {
o.multiDispatch(element);
}
}
void multiDispatch(I x) throws NoSuchMethodException
, InvocationTargetException, IllegalAccessException
{
Class cls = this.getClass();
Class[] parameterTypes = { x.getClass() };
Object[] arguments = { x };
Method fMethod = cls.getMethod("f", parameterTypes);
fMethod.invoke(this,arguments);
}
}
Output:
from A
from B
from B
from A
Your method says it will accept A or B which are derived classes of I, they can contain more details then I
void f(A a) {}
When you try to send super class of A in your case interface I, compiler wants a confirmation that you are actually sending A as details available in A may not be available in I, also only during runtime I will actually refer to an instance of A no such information available at compile time, so you will have to explicitly tell the compiler that I is actually A or B and you do a cast to say so.
I have a method in my static state machine that is only used once when my application is first fired up. The method needs to be public, but I still want it hidden. Is there a way to use an annotation or something that will hide the method from the rest of the project?
You cannot make a public method hidden (unless you can declare it private). You can however put in a subclass and only let the users of the object know the type of the superclass, that is:
class A {
//Externally visible members
}
class B extends A {
//Secret public members
}
Then you instantiate the class B, but only let the type A be known to others...
Once you declare public method it becomes part of your class's contract. You can't hide it because all class users will expect this method to be available.
You could use package level instead of public. That way it can only be called by your application.
If a method is public, it can't be hidden. What you may really be looking for is just a way to restrict access to calling a method. There are other ways to achieve a similar effect.
If there are some things that your state machine does that are "only used once when my application is first fired up" it sounds a lot like those are things that could happen in the constructor. Although it depends on how complex those tasks are, you may not want to do that at construction time.
Since you said your state machine is static, is it also a Singleton? You could maybe use the Singleton Pattern.
public class SimpleStateMachine {
private static SimpleStateMachine instance = new SimpleStateMachine();
private SimpleStateMachine() {
super();
System.out.println("Welcome to the machine"); // prints 1st
}
public static SimpleStateMachine getInstance() {
return instance;
}
public void doUsefulThings() {
System.out.println("Doing useful things"); // prints 3rd
}
}
Here's some code for a client of this Singleton:
public class MachineCaller {
static SimpleStateMachine machine = SimpleStateMachine.getInstance();
public static void main(String... args) {
System.out.println("Start at the very beginning"); // prints 2nd
machine.doUsefulThings();
}
}
Note that the SimpleStateMachine instance isn't built until the first time your class is accessed. Because it's declared as static in the MachineCaller client, that counts as a "first access" and creates the instance. Keep this tidbit in mind if you definitely want your state machine to perform some of those initialization tasks at the time your application starts up.
So, if you don't want to turn your state machine class into a true singleton... you can use a static initialization block do your one-time tasks the first time the class is accessed. That would look something like this:
public class SimpleStateMachine {
static {
System.out.println("First time tasks #1");
System.out.println("First time tasks #2");
}
public SimpleStateMachine() {
super();
System.out.println("Welcome to the machine");
}
public void doUsefulThings() {
System.out.println("Doing useful things");
}
}
While we're at it, since you mentioned that it's a state machine... the Head First Design Patterns book does a nice, easily understandable treatment of the State Pattern. I recommend reading it if you haven't already.
The idiomatic approach to doing this is to use interfaces to limit the visibility of your methods.
For example, say you have the following class:
public class MyClass {
public void method1() {
// ...
}
public void method2() {
// ...
}
}
If you want to limit some parts of the project to only see method1(), then what you do is describe it in an interface, and have the class implement that interface:
public interface Method1Interface {
public void method1();
}
...
public class MyClass implements Method1Interface {
public void method1() {
// ...
}
public void method2() {
// ...
}
}
Then, you can limit the visibility of the methods by choosing to pass the class around either as a MyClass reference, or as a Method1Interface reference:
public class OtherClass {
public void otherMethod1(MyClass obj) {
// can access both obj.method1() and obj.method2()
}
public void otherMethod2(Method1Interface obj) {
// can only access obj.method1(), obj.method2() is hidden.
}
}
A bonus of this approach is that it can also be easily extended. Say, for example, you now also want to independently control access to method2(). All you need do is create a new Method2Interface along the same lines as Method1Interface, and have MyClass implement it. Then, you can control access to method2() in exactly the same manner as method1().
This is a similar approach to that advocated in #MathiasSchwarz's answer, but is much more flexible:
The independent access control described in the preceding paragraph isn't possible with Mathias' technique, due to Java not supporting multiple inheritance.
Not requiring an inheritance relationship also allows more flexibility in designing the class hierarchy.
The only change required to the original class is to add implements Method1Interface, which means that it is a very low-impact refactor since existing users of MyClass don't have to be changed at all (at least, until the choice is made to change them to use Method1Interface).
An alternative solution: You can make it private and create a invokeHiddenMethod(String methodName, Object ... args) method using reflection.
You said that your public method is used only once when the application is started up.
Perhaps you could leave the method public, but make it do nothing after the first call?
There is a (non-)keyword level package level visibility. Instead of public, protected, or private, you use nothing.
This would make the method or class visible to the class and others in the package, but would give you a certain modicum of privacy. You may want to look at What is the use of package level protection in java?.
Hmm... You want a private method, but want to access it outside?
Try do this with reflection.
http://download.oracle.com/javase/tutorial/reflect/index.html
I have seen many Java programmers do something like this:
public static void main(String args[]) {
new MyClass();
}
So basically they create just one object of the class. If there is a method which should run only once, I guess this approach can achieve that. Your method will be called from inside the constructor. But since I don't know how your app works, what are the constraints, so it is just a thought.
I know that an interface must be public. However, I don't want that.
I want my implemented methods to only be accessible from their own package, so I want my implemented methods to be protected.
The problem is I can't make the interface or the implemented methods protected.
What is a work around? Is there a design pattern that pertains to this problem?
From the Java guide, an abstract class wouldn't do the job either.
read this.
"The public access specifier indicates that the interface can be used by any class in any package. If you do not specify that the interface is public, your interface will be accessible only to classes defined in the same package as the interface."
Is that what you want?
You class can use package protection and still implement an interface:
class Foo implements Runnable
{
public void run()
{
}
}
If you want some methods to be protected / package and others not, it sounds like your classes have more than one responsibility, and should be split into multiple.
Edit after reading comments to this and other responses:
If your are somehow thinking that the visibility of a method affects the ability to invoke that method, think again. Without going to extremes, you cannot prevent someone from using reflection to identify your class' methods and invoke them. However, this is a non-issue: unless someone is trying to crack your code, they're not going to invoke random methods.
Instead, think of private / protected methods as defining a contract for subclasses, and use interfaces to define the contract with the outside world.
Oh, and to the person who decided my example should use K&R bracing: if it's specified in the Terms of Service, sure. Otherwise, can't you find anything better to do with your time?
When I have butted up against this I use a package accessible inner or nested class to implement the interface, pushing the implemented method out of the public class.
Usually it's because I have a class with a specific public API which must implement something else to get it's job done (quite often because the something else was a callback disguised as an interface <grin>) - this happens a lot with things like Comparable. I don't want the public API polluted with the (forced public) interface implementation.
Hope this helps.
Also, if you truly want the methods accessed only by the package, you don't want the protected scope specifier, you want the default (omitted) scope specifier. Using protected will, of course, allow subclasses to see the methods.
BTW, I think that the reason interface methods are inferred to be public is because it is very much the exception to have an interface which is only implemented by classes in the same package; they are very much most often invoked by something in another package, which means they need to be public.
This question is based on a wrong statement:
I know that an interface must be public
Not really, you can have interfaces with default access modifier.
The problem is I can't make the interface or the implemented methods protected
Here it is:
C:\oreyes\cosas\java\interfaces>type a\*.java
a\Inter.java
package a;
interface Inter {
public void face();
}
a\Face.java
package a;
class Face implements Inter {
public void face() {
System.out.println( "face" );
}
}
C:\oreyes\cosas\java\interfaces>type b\*.java
b\Test.java
package b;
import a.Inter;
import a.Face;
public class Test {
public static void main( String [] args ) {
Inter inter = new Face();
inter.face();
}
}
C:\oreyes\cosas\java\interfaces>javac -d . a\*.java b\Test.java
b\Test.java:2: a.Inter is not public in a; cannot be accessed from outside package
import a.Inter;
^
b\Test.java:3: a.Face is not public in a; cannot be accessed from outside package
import a.Face;
^
b\Test.java:7: cannot find symbol
symbol : class Inter
location: class b.Test
Inter inter = new Face();
^
b\Test.java:7: cannot find symbol
symbol : class Face
location: class b.Test
Inter inter = new Face();
^
4 errors
C:\oreyes\cosas\java\interfaces>
Hence, achieving what you wanted, prevent interface and class usage outside of the package.
Here's how it could be done using abstract classes.
The only inconvenient is that it makes you "subclass".
As per the java guide, you should follow that advice "most" of the times, but I think in this situation it will be ok.
public abstract class Ab {
protected abstract void method();
abstract void otherMethod();
public static void main( String [] args ) {
Ab a = new AbImpl();
a.method();
a.otherMethod();
}
}
class AbImpl extends Ab {
protected void method(){
System.out.println( "method invoked from: " + this.getClass().getName() );
}
void otherMethod(){
System.out.println("This time \"default\" access from: " + this.getClass().getName() );
}
}
Here's another solution, inspired by the C++ Pimpl idiom.
If you want to implement an interface, but don't want that implementation to be public, you can create a composed object of an anonymous inner class that implements the interface.
Here's an example. Let's say you have this interface:
public interface Iface {
public void doSomething();
}
You create an object of the Iface type, and put your implementation in there:
public class IfaceUser {
private int someValue;
// Here's our implementor
private Iface impl = new Iface() {
public void doSomething() {
someValue++;
}
};
}
Whenever you need to invoke doSomething(), you invoke it on your composed impl object.
I just came across this trying to build a protected method with the intention of it only being used in a test case. I wanted to delete test data that I had stuffed into a DB table. In any case I was inspired by #Karl Giesing's post. Unfortunately it did not work. I did figure a way to make it work using a protected inner class.
The interface:
package foo;
interface SomeProtectedFoo {
int doSomeFoo();
}
Then the inner class defined as protected in public class:
package foo;
public class MyFoo implements SomePublicFoo {
// public stuff
protected class ProtectedFoo implements SomeProtectedFoo {
public int doSomeFoo() { ... }
}
protected ProtectedFoo pFoo;
protected ProtectedFoo gimmeFoo() {
return new ProtectedFoo();
}
}
You can then access the protected method only from other classes in the same package, as my test code was as show:
package foo;
public class FooTest {
MyFoo myFoo = new MyFoo();
void doProtectedFoo() {
myFoo.pFoo = myFoo.gimmeFoo();
myFoo.pFoo.doSomeFoo();
}
}
A little late for the original poster, but hey, I just found it. :D
You can go with encapsulation instead of inheritance.
That is, create your class (which won't inherit anything) and in it, have an instance of the object you want to extend.
Then you can expose only what you want.
The obvious disadvantage of this is that you must explicitly pass-through methods for everything you want exposed. And it won't be a subclass...
I would just create an abstract class. There is no harm in it.
With an interface you want to define methods that can be exposed by a variety of implementing classes.
Having an interface with protected methods just wouldn't serve that purpose.
I am guessing your problem can be solved by redesigning your class hierarchy.
One way to get around this is (depending on the situation) to just make an anonymous inner class that implements the interface that has protected or private scope. For example:
public class Foo {
interface Callback {
void hiddenMethod();
}
public Foo(Callback callback) {
}
}
Then in the user of Foo:
public class Bar {
private Foo.Callback callback = new Foo.Callback() {
#Override public void hiddenMethod() { ... }
};
private Foo foo = new Foo(callback);
}
This saves you from having the following:
public class Bar implements Foo.Callback {
private Foo foo = new Foo(this);
// uh-oh! the method is public!
#Override public void hiddenMethod() { ... }
}
I think u can use it now with Java 9 release. From the openJdk notes for Java 9,
Support for private methods in interfaces was briefly in consideration
for inclusion in Java SE 8 as part of the effort to add support for
Lambda Expressions, but was withdrawn to enable better focus on higher
priority tasks for Java SE 8. It is now proposed that support for
private interface methods be undertaken thereby enabling non abstract
methods of an interface to share code between them.
refer https://bugs.openjdk.java.net/browse/JDK-8071453