I need to build my own library to handle events coming from an event bus, the first solution I came up with was an abstract class made like this one:
public abstract class MyEventListener<T> extends RealEventListener{
private final Class<T> type; //the event class type
private final String stream; //the stream
public abstract onEvent(T type); //the method my subclasses have to implement
#Overrides
public void onEvent(byte[] evt){ //takes the original, clunky method and evolves it with typing
//convert and do stuff
onEvent(convertedEvent); //call method
}
}
so, the classes only do:
#Component
public class Child extends MyEventListener<AType>{
public Child(){
super(AType.class, "stream"); //registers
}
#Overrides
public void onMessage(AType evt){ //do stuff
}
I find this approach somewhat limiting and outdated (at least seeing the latest libraries). An example I can think of is that, this way, you are forced to handle separate events in separate classes.
So, I though of using annotations to have something like this:
#EventListener("stream") //1. you define the stream in this custom class annotation
public class Child { //so no need to extend
#ListenTo(type=AType.class) //2. you define the type on methods, this way a single class can handle more
public void onMessage(AType event, //any other param){
}
Now, for the magic behind, I though about using a startup annotation processing to retrieve the methods:
public void initialize(#EventListener List<Object> listeners) { //I inject all objects
listeners.stream().map(..).collect(..) //use reflection to get all methods annotated with #ListenTo and put them inside of map <String, Method>
eventBus.registerListener(new MyEventListener(target, methodMap)); //
}
Now, the listener will take somewhat the type from the original byte event and call the method:
String className = getClassFromEvent(evt);
listenerMap.get(className).invoke(target, evt);
According to you is this approach valid and, most importantly, efficient? Or, excluding the initialization phase, it could lead to performance problems at runtime? Of course I should make static checks at initialization to make sure that the annotated methods declare the event as parameter but it seems cleaner to me than the first one.
Related
In Java: What is the best way to pass a method from one object to another so that it can be called at a later time by the second object?
I come from an ActionScript background where it is as easy to pass around references to methods as it is to pass around references to variables but this seems to be much more difficult in Java. The first few links I found flat out say it is not possible (and it may have been at the time of their posting), but then I found http://www.javacamp.org/javavscsharp/delegate.html which details how this can be accomplished.
My issue with using Javacamp's example is the string based reference to the method. Methods get renamed all the time and a string reference will only complain once you actually run that function runtime as opposed to compile time for a proper explicit link.
Is there no way to do this with proper explicit links to the method you want the other class to execute?
Model of what I am hoping to accomplish:
Player clicks an upgrade button on Activity1 > Activity1 passes upgrade method to a new confirmation activity
Player clicks "Yes" > Confirmation activity calls upgrade method passed in from Activity1
OR: Player clicks "No" > Confirmation Activity closes
EDIT:
To be clear I am not looking for a static method solution as that would require my Confirmation activity to hold many lines of logic for which static method to call. The Confirmation activity will be used all over my application: a simple "Are you sure you want to X?" -Yes -No, if yes execute X
I am currently looking at implementing onActivityResult to avoid this issue but that will be more logic than I like for this kind of issue.
you can use interfaces like this:
public interface MyMethod {
public void method();
}
public class FirtObject{
private SecondObject ob;
public void tellSecondObjectExecuteLater(){
ob.executeLater( new MyMethod() {
public void method(){System.out.println("duh Method");} });
}
}
public class SecondObject {
private MyMethod myMth;
public void executeLater(MyMethod mth){
myMth = mth;
}
public void executeNow(){
myMth.method();
}
}
does this solve your problem?
The typical way to pass methods is to use an Interface and Anonymous Inner Classes. In order to maintain static typing an Interface is used to declare the method signature and typing information. The caller can use either a concrete implementation of that interface as a normal class or using Anonymous Inner Classes for quick class creation. I'll use standard Java SDK classes to illustrate:
interface Comparator<T> {
public int compare( T a, T b);
}
class SpecialCollection<T> {
public void sort( Comparator<T> comparator ) {...}
}
public class SomeClient {
public void doSomething( SpecialCollection<SpecialObj> collection ) {
collection.sort( new Comparator<SpecialObj>() {
public int compare( SpecialObject a, SpecialObject b ) {
// some implementation
}
} );
}
}
The above is an example of a strategy pattern. The thing about the strategy pattern (and passing callback methods like in Javascript). The author has to plan for those types of extensions. The author has to predict up front where he/she wants you to extend. And it just happens it's cleanest if you use Interfaces.
However, pure delegation doesn't have to have always involve Interfaces. You can pass concrete classes, since Java can always pass a subclass that overrides various methods of that class to change what method or code will be invoked. For example in Java InputStream/OutputStream are abstract classes and you typically pass subclass instances to the methods.
If you need the method to act differently depending on the context (AKA, it is different depending on how it is created), you'll want to pass along the instance of the class that the method is in.
If it is a static method, you can just referenced the method itself if you import that class at the top of your new class.
For example, lets say you have a method that will tell you stuff about a certain string. IF the class looks like this:
class stringChecker {
private String stringToCheck;
public class stringChecker(String s) {
stringToCheck = s;
}
public int getStringLength() {
return stringToCheck.length();
}
public boolean stringStartsWith(String startsWith) {
return (stringToCheck.indexOf(startsWith) == 0);
}
}
Then you'll want to pass along the instance, since it is non-static. Different instances have different strings that they were created with, so you will get a different return if you use a different instance.
However, if your class looks more like this:
class stringChecker {
public static int getStringLength(String s) {
return s.length();
}
public static boolean stringStartsWith(String s, String startsWith) {
return (s.indexOf(startsWith) == 0);
}
}
Then you can just reference those methods with stringChecker.getStringLength("test");, because the methods are static. It doesn't matter what instance they are in. The returned result depends ONLY on what is being passed in. You just have to make sure to add import stringChecker; at the top or whatever your class will be called. For you, it'll probably be something like com.example.blah.otherthing.stringChecker, since you're working with android.
Good luck! I hope this helps :)
EDIT: Looks like I may have read the problem too quickly...if this isn't what you were asking about, just let me know with a comment and I'll delete this answer so as to not confuse anybody else.
You said that you are using it in a project to open a Confirmation activity.
Activities should not contain references to each other to avoid memory leaks. To pass data between activities should be used Intent class. To receive a result, call StartActivityForResult() and get result in the onActivityResult() method.
But in general for your task is more suitable AlertDialog or PopupWindow.
This is a two part question. First, is it possible use a generic defined objects method such as:
public class MyClass<T>{
public MyClass(T t){
t.setText("Hello World"); // Assume class T is JMenuIten has the special method setText
}
}
This code doesn't work as is, but show the general idea for what I'm aiming for. I want to use the methods which are particular to that encapsulated object. If however I were to pass in another object such as which contains the encapsulated method .doSomething. I would like to do ...
public class MyClass<T>{
public MyClass(T t){
t.doSomething("Hello World"); // Assume class T is JMenuIten has the special method setText
}
}
I'm hoping that it is possible to do this, otherwise I would have to write multiple constructors to take care of all my special cases.
My second question is similar in that I would like to return a GUI component and execute a statement such as ...
myJPanel.getComponent(1).setText("Hello"); // Assuming index 1 is a JLabel and setText is a specific method defined in the JLabel class
This code does not work because the compiler cannot tell ahead of time what symbols will be needed at runtime, though I was hoping that there was a way of making things like this work. I would also like to know if there is a method that can tell me what class type .getComponent() is returning if that is possible. I'm trying to make code as dynamic as possible without having to hardcode everything.
Thanks
You have to use a bounded wildcard.
e.g.
public interface MyObject {
void myMethod();
}
public class GenericObj<T extends MyObject> {
private T t;
public void invokeMethod() {
t.myMethod(); //this way you can invoke methods (declcared in MyObject) on T
}
}
I like to realize the following scenario. (example)
I have 3 Classes:
Controller
Apple
Car
public class Apple implements IBonus {
public String name;
public String weight;
}
public class Car implements IBonus{
public String brand;
public String vmax;
public String power;
}
public class Controller {
public List<String> values;
public void doWork(IBonus bonusObject){
if(bonusObject instanceOf Car){
Car c = (Car)bonusObject;
values.add(c.brand);
values.add(c.vmax);
values.add(c.power);
}
if(bonusObject instanceOf Apple){
Apple a = (Apple)bonusObject;
values.add(a.name);
values.add(a.weight);
}
}
}
Now, I have a lot of Classes like apple and car. And there is also the possibility that some variables of each class will be changed, added or removed in the near future.
This will mean that I always have to adjust the code in the controller class.
Does anyone know a suitable pattern?
You don't need any pattern for this. Just plain old polymorphism:
public interface IBonus {
void fillStringList(List<String> values);
}
...
public void doWork(IBonus bonusObject){
bonusObject.fillStringList(values);
}
EDIT: Design pattern "Visitor".
Change your iBonus interface to add a method:
void add(List<String> values);
Have the Controller instead do
bonusObject.add(values);
This way you delegate the specific task (adding an object to 'values') to the specific implementation.
Basically whenever you find yourself writing code like the above (if instanceof or switch()) you should instead consider delegating the task to an abstract method.
You may want to consider "double dispatch" aka. "Visitor"
http://en.wikipedia.org/wiki/Visitor_pattern
The Wikipedia version is very generic, and splits out the "Visitor" logic (adding to the list) into a seperate class. In the below version the "Bonus" objects play that part, and the double dispatch structure is used instead.
In that scenario you have two interfaces: One for the bonus object, and one for the controller. The bonus object has a method called
void visit(Controller c);
The controller will then invoke the bonus object thus:
bonusObjecv.visit(this);
The purpose of "Visitor" is largely to let you vary the implementations independant of each other. It is a more generic version of the simple, polymorphic solution. Instead of using a generic class such as List you use the Controller interface. That way you make the interaction between controller and visited object explicit.
You could add a method:
public List<String> getValues();
to your IBonus interface and just use the following in doWork:
values.addAll(bonusObject.getValues());
Each IBonus type will then need to implement how to create the list.
I think Factory pattern will suit in this condition.
Factory pattern Example
There is a part in my java code where I am extending a class from a library which I haven't written.
#override
public Object getPropertyValue(Object id) {
if(id.equals(model.PROPERTY_RENAME))
model.setName((String)value);
else if(id.equals(model.PROPERTY_COLOUR))
model.setColor((Color)value);
}
Now in this case how should I modify this code to make it scalable. There would be many more properties like location, dimension, etc. Now this model is instance of an abstract class AbsModel.
So every class implementing the AbsModel would have different properties. So the class architecture should be there, so that this part of code remains unchanged, no matter how many more model classes I add.
It looks like you want to carry out some operation on the model when this method (getPropertyValue) is called. I would create a Map of id onto the interface ModelOperation defined as follows:
public interface ModelOperation {
void operate(Object value);
}
Then the map would be defines as follows:
map.put(model.PROPERTY_RENAME, new RenameOperation(model));
Your extension class would then look like this:
#Override
public Object getPropertyValue(Object id) {
map.get(id).operate(model);
// etc...
}
For example, RenameOperation would be defined like this:
public class RenameOperation implements ModelOperation {
public RenameOperation(Model model) {
// etc...
}
public void operate(Object value) {
model.setName((String)value);
}
}
This allows you to support as many model operations as you like and means you don't have to change the extension class you have to write. The above is just an outline. You could use generics on the ModelOperation implementations to avoid the cast of the value in each one.
I guess reflection is probably the answer here if you can rely on some naming to help direct you.
It's not going to be nice, but the idea would be that you'd have a method that would reflect on the type and look up the appropriate method. The code belwo
public Object setPropertyValue(Object id) {
String className = id.getClass().getSimpleName();
// Hope that the method is called set<CLASS> and takes a single parameter that is the class
Method method = model.class.getMethod("set" + className, id.getClass());
// Invoke the method (TODO deal with all of the exceptions)
method.invoke(model, id);
}
There are multiple ways of doing this -- though it depends on what do you mean by "scalable" (being able to cope with lots of requests per second or being able to cope with lots of properties?):
one way -- if you're going to go down the path you have outlined in your code is to have those properties that are used very often at the top of your if/then/else block -- so their execution path is very short. this would "scale up" well for lots of requests as not too much time is being spent in actually executing the method (in most cases at least!)
another way -- and this scales up well for lots of properties and easiness of maintaining the code but you will take a hit on execution time: have a Map that maps property names to setxxx() method names, then you can use reflection to invoke these methods on the target object (id in your case) on each call. Classes extended your class will only have to provide a getMap() method which will return the mapping name-to-setter method, which can be a static member and initialized on class load.
Store your properties in a Map -- in which case setName() is the same as map.put( PROPERTY_RENAME, value)
Since in Java functions are not first class citizens, the "nice" route would be very awkward: define an enum with one value per each constant above (i.e. for each property), and a virtual method e.g. update(Object value, then override the method in each enum to update the corresponding property. If you can, redefine the constants PROPERTY_RENAME etc. themselves as enums. This still results in code bloat.
The other way is to use reflection. If you can use the same ids as the property names you want to update, you only need to invoke the setter for the property (as illustrated in other answers). Otherwise you may need to introduce a mapping from ids to property names.
A version not using reflection, call the base class's implementation:
public Object getValue(Object id) {
Object ret = super.getValue(id);
if (ret == null) {
// Subclass specific properties
}
return ret;
}
A common way around this is to use reflection like
public Object getValue(IdType id) {
Method getter = model.getClass().getMethod("get" + id);
return getter.invoke(model); // throws Exceptions.
}
OR
public void setValue(IdType id, Object value) {
Method setter = model.getClass().getMethod("set" + id, value.getClass());
setter.invoke(model, value); // throws Exceptions.
}
I solved this issue by creating an interface. So the code is.
public interface IModel
{
public void setProperty(String propertyName);
}
Rest of the classes were
public class HelloModel implements IModel
{
public void setProperty(String propertyName)
{ code for handling the properties goes here ... }
}
So in this case every class has to handle it's own property setters.
Is this the best way to handle abstraction ? I think this model is very scalable ...
Introduction
As a disclaimer, I'v read Why can't static methods be abstract in Java and, even if I respectfully disagree with the accepted answer about a "logical contradiction", I don't want any answer about the usefulness of static abstract just an answer to my question ;)
I have a class hierarchy representing some tables from a database. Each class inherits the Entity class which contains a lot of utility methods for accessing the database, creating queries, escaping characters, etc.
Each instance of a class is a row from the database.
The problem
Now, in order to factorize as much code as possible, I want to add information about related columns and table name for each class. These informations must be accessible without a class instance and will be used in Entity to build queries among other things.
The obvious way to store these data are static fields returned by static methods in each class. Problem is you can't force the class to implement these static methods and you can't do dynamic linking on static methods call in Java.
My Solutions
Use a HashMap, or any similar data structure, to hold the informations. Problem : if informations are missing error will be at runtime not compile time.
Use a parallel class hierarchy for the utility function where each corresponding class can be instantiated and dynamic linking used. Problem : code heavy, runtime error if the class don't exist
The question
How will you cope with the absence of abstract static and dynamic linking on abstract method ?
In a perfect world, the given solution should generate a compile error if the informations for a class are missing and data should be easily accessible from withing the Entity class.
The answer doesn't need to be in Java, C# is also ok and any insight on how to do this without some specific code in any language will be welcomed.
Just to be clear, I don't have any requirement at all besides simplicity. Nothing have to be static. I only want to retrieve table and columns name from Entity to build a query.
Some code
class Entity {
public static function afunction(Class clazz) { // this parameter is an option
// here I need to have access to table name of any children of Entity
}
}
class A extends Entity {
static String table = "a";
}
class B extends Entity {
static String table = "b";
}
You should use the Java annotation coupled with the javac annotation processor, as it's the most efficient solution. It's however a bit more complicated than the usual annotation paradigm.
This link shows you how you can implement an annotation processor that will be used at the compile time.
If I reuse your example, I'd go this way:
#Target(ElementType.TYPE)
#Retention(RetentionType.SOURCE)
#interface MetaData {
String table();
}
abstract class Entity {}
#MetaData(table="a")
class A extends Entity {}
#MetaData(table="b")
class B extends Entity {}
class EntityGetter {
public <E extends Entity> E getEntity(Class<E> type) {
MetaData metaData = type.getAnnotation(MetaData.class);
if (metaData == null) {
throw new Error("Should have been compiled with the preprocessor.");
// Yes, do throw an Error. It's a compile-time error, not a simple exceptional condition.
}
String table = metaData.table();
// do whatever you need.
}
}
In your annotation processing, you then should check whether the annotation is set, whether the values are correct, and make the compilation fail.
The complete documentation is available in the documentation for the package javax.annotation.processing.
Also, a few tutorials are available on the Internet if you search for "java annotation processing".
I will not go deeper in the subject as I never used the technology myself before.
I have run into the same problems as you, and am using the following approach now. Store Metadata about columns as annotations and parse them at runtime. Store this information in a map. If you really want compile time errors to appear, most IDEs (Eclipse e.g.) support custom builder types, that can validate the classes during build time.
You could also use the compile time annotation processing tool which comes with java, which can also be integrated into the IDE builds. Read into it and give it a try.
In Java the most similar approach to "static classes" are the static enums.
The enum elements are handed as static constants, so they can be accesed from any static context.
The enum can define one or more private constructors, accepting some intialization parameters (as it could be a table name, a set of columns, etc).
The enum class can define abstract methods, which must be implemented by the concrete elements, in order to compile.
public enum EntityMetadata {
TABLE_A("TableA", new String[]{"ID", "DESC"}) {
#Override
public void doSomethingWeirdAndExclusive() {
Logger.getLogger(getTableName()).info("I'm positively TableA Metadata");
}
},
TABLE_B("TableB", new String[]{"ID", "AMOUNT", "CURRENCY"}) {
#Override
public void doSomethingWeirdAndExclusive() {
Logger.getLogger(getTableName()).info("FOO BAR message, or whatever");
}
};
private String tableName;
private String[] columnNames;
private EntityMetadata(String aTableName, String[] someColumnNames) {
tableName=aTableName;
columnNames=someColumnNames;
}
public String getTableName() {
return tableName;
}
public String[] getColumnNames() {
return columnNames;
}
public abstract void doSomethingWeirdAndExclusive();
}
Then to access a concrete entity metadata this would be enough:
EntityMetadata.TABLE_B.doSomethingWeirdAndExclusive();
You could also reference them from an Entity implemetation, forcing each to refer an EntityMetadata element:
abstract class Entity {
public abstract EntityMetadata getMetadata();
}
class A extends Entity {
public EntityMetadata getMetadata() {
return EntityMetadata.TABLE_A;
}
}
class B extends Entity {
public EntityMetadata getMetadata() {
return EntityMetadata.TABLE_B;
}
}
IMO, this approach will be fast and light-weight.
The dark side of it is that if your enum type needs to be really complex, with lot of different params, or a few different complex overriden methods, the source code for the enum can become a little messy.
Mi idea, is to skip the tables stuff, and relate to the "There are not abstract static methods". Use "pseudo-abstract-static" methods.
First define an exception that will ocurr when an abstract static method is executed:
public class StaticAbstractCallException extends Exception {
StaticAbstractCallException (String strMessage){
super(strMessage);
}
public String toString(){
return "StaticAbstractCallException";
}
} // class
An "abstract" method means it will be overriden in subclasses, so you may want to define a base class, with static methods that are suppouse to be "abstract".
abstract class MyDynamicDevice {
public static void start() {
throw new StaticAbstractCallException("MyDynamicDevice.start()");
}
public static void doSomething() {
throw new StaticAbstractCallException("MyDynamicDevice.doSomething()");
}
public static void finish() {
throw new StaticAbstractCallException("MyDynamicDevice.finish()");
}
// other "abstract" static methods
} // class
...
And finally, define the subclasses that override the "pseudo-abstract" methods.
class myPrinterBrandDevice extends MyDynamicDevice {
public static void start() {
// override MyStaticLibrary.start()
}
/*
// ops, we forgot to override this method !!!
public static void doSomething() {
// ...
}
*/
public static void finish() {
// override MyStaticLibrary.finish()
}
// other abstract static methods
} // class
When the static myStringLibrary doSomething is called, an exception will be generated.
I do know of a solution providing all you want, but it's a huge hack I wouldn't want in my own code nowadays:
If Entity may be abstract, simply add your methods providing the meta data to that base class and declare them abstract.
Otherwise create an interface, with methods providing all your data like this
public interface EntityMetaData{
public String getTableName();
...
}
All subclasses of Entity would have to implement this interface though.
Now your problem is to call these methods from your static utility method, since you don't have an instance there. So you need to create an instance. Using Class.newInstance() is not feasable, since you'd need a nullary constructor, and there might be expensive initialization or initialization with side-effects happening in the constructor, you don't want to trigger.
The hack I propose is to use Objenesis to instantiate your Class. This library allows instatiating any class, without calling the constructor. There's no need for a nullary constructor either. They do this with some huge hacks internally, which are adapted for all major JVMs.
So your code would look like this:
public static function afunction(Class clazz) {
Objenesis objenesis = new ObjenesisStd();
ObjectInstantiator instantiator = objenesis.getInstantiatorOf(clazz);
Entity entity = (Entity)instantiator.newInstance();
// use it
String tableName = entity.getTableName();
...
}
Obviously you should cache your instances using a Map<Class,Entity>, which reduces the runtime cost to practically nothing (a single lookup in your caching map).
I am using Objenesis in one project of my own, where it enabled me to create a beautiful, fluent API. That was such a big win for me, that I put up with this hack. So I can tell you, that it really works. I used my library in many environments with many different JVM versions.
But this is not good design! I advise against using such a hack, even if it works for now, it might stop in the next JVM. And then you'll have to pray for an update of Objenesis...
If I were you, I'd rethink my design leading to the whole requirement. Or give up compile time checking and use annotations.
Your requirement to have static method doesn't leave much space for clean solution. One of the possible ways is to mix static and dynamic, and lose some CPU for a price of saving on RAM:
class Entity {
private static final ConcurrentMap<Class, EntityMetadata> metadataMap = new ...;
Entity(EntityMetadata entityMetadata) {
metadataMap.putIfAbsent(getClass(), entityMetadata);
}
public static EntityMetadata getMetadata(Class clazz) {
return metadataMap.get(clazz);
}
}
The way I would like more would be to waste a reference but have it dynamic:
class Entity {
protected final EntityMetadata entityMetadata;
public Entity(EntityMetadata entityMetadata) {
this.entityMetadata=entityMetadata;
}
}
class A extends Entity {
static {
MetadataFactory.setMetadataFor(A.class, ...);
}
public A() {
super(MetadataFactory.getMetadataFor(A.class));
}
}
class MetadataFactory {
public static EntityMetadata getMetadataFor(Class clazz) {
return ...;
}
public static void setMetadataFor(Class clazz, EntityMetadata metadata) {
...;
}
}
You could get even get rid of EntityMetadata in Entity completely and leave it factory only. Yes, it would not force to provide it for each class in compile-time, but you can easily enforce that in the runtime. Compile-time errors are great but they aren't holy cows after all as you'd always get an error immediately if a class hasn't provided a relevant metadata part.
I would have abstracted away all meta data for the entities (table names, column names) to a service not known by the entities them selfs. Would be much cleaner than having that information inside the entities
MetaData md = metadataProvider.GetMetaData<T>();
String tableName = md.getTableName();
First, let me tell you I agree with you I would like to have a way to enforce static method to be present in classes.
As a solution you can "extend" compile time by using a custom ANT task that checks for the presence of such methods, and get error in compilation time. Of course it won't help you inside you IDE, but you can use a customizable static code analyzer like PMD and create a custom rule to check for the same thing.
And there you java compile (well, almost compile) and edit time error checking.
The dynamic linking emulation...well, this is harder. I'm not sure I understand what you mean. Can you write an example of what you expect to happen?