Consider a method
public void doSomething(String actionID){
switch (actionID){
case "dance":
System.out.print("I'm dancing");
break;
case "sleep":
System.out.print("I'm sleeping");
break;
default:
System.out.print("I've no idea what I'm doing");
}
The implementation of the method depends on the value of the parameter. Is there a more elegant way to do this, or a different design pattern to replicate the behaviour?
If the caller decides what logic is executed by passing different strings, then why not just have them call different methods:
public void doSomething(String actionID) {...}
...
doSomething("dance");
doSomething("sleep");
VS.:
public void dance() {...}
public void sleep() {...}
...
dance();
sleep();
It seems like you're unnecessarily funnelling all the calls into doSomething
But the strings might not always be literals. What if you're taking them from the console?
You could provide static mappings from the strings to the corresponding functions:
class MyClass {
private static final Map<String, Consumer<MyClass>> map = new HashMap<>();
static {
map.put("sleep", MyClass::sleep);
map.put("dance", MyClass::dance);
}
public void doSomething(String actionID) {
map.getOrDefault(actionID, MyClass::doNothing).accept(this);
}
public void dance() {
System.out.print("I'm dancing");
}
public void sleep() {
System.out.print("I'm sleeping");
}
private void doNothing() {
System.out.println("I've no idea what I'm doing");
}
}
This makes scenarios where you have a lot of switch cases a lot cleaner.
Introduce an interface, e.g.
public interface HumanState {
public void tellMeWhatYouAreDoing();
}
encapsulate the logic in different implementations
public class DancingState implements HumanState {
#Override
public void tellMeWhatYouAreDoing() {
System.out.println("I'm dancing");
}
}
public class SleepingState implements HumanState {
#Override
public void tellMeWhatYouAreDoing() {
System.out.println("I'm sleeping");
}
}
public class UnknownState implements HumanState {
#Override
public void tellMeWhatYouAreDoing() {
System.out.println("I've no idea what I'm doing");
}
}
and use a map. E.g.
public class HumanStateExample {
public static void main(String[] args) {
HumanStateExample humanStateExample = new HumanStateExample();
humanStateExample.doSomething("dance");
humanStateExample.doSomething("sleep");
humanStateExample.doSomething("unknown");
}
private final HashMap<String, HumanState> humanStateMap;
public HumanStateExample(){
humanStateMap = new HashMap<String, HumanState>();
humanStateMap.put("dance", new DancingState());
humanStateMap.put("sleep", new SleepingState());
}
public void doSomething(String action) {
HumanState humanState = humanStateMap.get(action);
if(humanState == null){
humanState = new UnknownState();
}
humanState.tellMeWhatYouAreDoing();
}
}
I'm not sure how the pattern is called, but it is very useful if you need to delegate the method call based on more than one parameter:
Create a lot of handlers where each one knows when it is responsible for handling a call. Then just loop through them and invoke the first one matching the parameter.
edit: I renamed the class from FancyParameterActionFactory to FancyParameterActionUtility: it is not a factory, the name was misleading
//Your method, but this time with a complex object, not with a simple string.
public void doSomething(FancyParameterObject fpo){
FancyParameterActionUtility.invokeOn(fpo);
}
//The utility which can handle the complex object and decides what to do.
public class FancyParameterActionUtility{
public Interface FPAHandler{
void invoke(FancyParameterObject fpo);
boolean handles(FancyParameterObject fpo);
}
//Omitted: Different implementations of FPAHandler
public static List<FPAHandler> handlers = new LinkedList<>();
static{
handlers.add(new DanceHandler());
handlers.add(new SleepHandler());
//Omitted: Different implementations of FPAHandler
}
public static void invokeOn(FancyParameterObject fpo){
for(FPAHandler handler:handlers){
if (handler.handles(fpo)){
handler.invoke(fpo);
return;
}
}
//Default-Behavior
}
}
Here is a simple implementation of the command pattern based your sample problem. I define a general AbstractCommand abstract class which contains two methods. The first method, createCommand(), instantiates a command class based on an input string name. This is how you can delegate your string input to create the right type of command. The second method is doAction(), and this is left undefined, to be implemented later on by specific concrete command classes.
public abstract class AbstractCommand {
public static AbstractCommand createCommand(String name) {
try {
String clsName = name + "Command";
Class<?> cls = Class.forName(clsName);
AbstractCommand command = (AbstractCommand) cls.newInstance();
return command;
}
catch (Exception e) {
System.out.println("Something went wrong.");
}
}
public abstract void doAction();
}
public class DanceCommand extends AbstractCommand {
public void doAction() {
System.out.println("I'm dancing");
}
}
public class TestCommandPattern {
public void doSomething(String actionID) {
AbstractCommand cmd = AbstractCommand.createCommand(actionID);
cmd.doAction();
}
public static void main(String[] args) {
TestCommandPattern test = new TestCommandPattern();
test.doSomething("Dance"); // should print "I'm dancing"
}
}
Now that this framework has been setup, you could easily add other commands for the various types of actions in your original problem. For example, you could create a SleepCommand class which would output I'm sleeping, or do whatever action you wish.
Let's say I have a class called Server, and I would like to allow others to write Plugins for it. Say Plugin is an Interface that extends Runnable and adds a single method: void init(...). It is the job of a plugin to collect data and send it to the server. When the time comes to send data to the server, however, how does it do this? Coming from C and C++ I am looking for a thinking along the lines of a function pointer. It seems to be possible in Java though I have not found examples outside the Java Standard Class Library.
How do I pass a method reference to the init method such that it can be stored by the Plugin, and then how do I invoke the method whenever the Plugin wants to send data? For now say that the desired Server method is: void sendData(Integer data).
For example:
// Inside Server
Plugin p = new PluginImplementation();
p.init(this::sendData);
// Plugin init
public void init(?? sendMethod) {
storedSendMethod = sendMethod;
// ...
}
// Plugin run
public void run() {
// ...
storedSendMethod(x) // Sends data to server
// ...
}
Using java.util.function.Function we can pass a function as an argument to a method, and then use apply() to apply it to the relevant arguments. Here's an example:
import java.util.function.Function;
public class FunctionDemo {
// we will pass a reference to this method
public static Integer square(Integer x) {
return x * x;
}
// this method accepts the function as an argument and applies it to the input: 5
public static Integer doSomething(Function<Integer, Integer> func) {
return func.apply(5);
}
public static void main(String[] args) {
// and here's how to use it
System.out.println(doSomething(FunctionDemo::square)); // prints 25
}
}
Additional version with multiple parameters (passed as an array):
public static Integer sum(Integer[] x) {
Integer result = 0;
for(int i = 0; i < x.length; i++)
result += x[i];
return result;
}
public static void main(String[] args) {
Integer[] arr = {1,2,3,4,5};
System.out.println(doSomething(Play::sum, arr));
}
public static Integer doSomething(Function<Integer[], Integer> func,
Integer[] arr) {
return func.apply(arr);
}
If the method is void sendData(Integer data) that corresponds to a consumer that takes an Integer and returns a void which is covered by the built in Consumer<Integer> interface which has an accept(Integer) method that will invoke your function when called.
So your code will look like this:
public void init(Consumer<Integer> sendMethod) {
storedSendMethod = sendMethod;
// ...
}
// Plugin run
void run() {
// ...
storedSendMethod.accept(x) // Sends data to server
// ...
}
As a sidenote, having an init method is probably a bad Java design. you are better moving the initialization to the constructor if possbile
Plugin p = new PluginImplementation( this::sendData);
In java, you do it with a callback,
This is your callback interface,
public interface SendCallback {
public void doSend(Object toSend);
}
This is the plugin interface, All plugin must implement this interface
public interface Plugin extends Runnable {
public void init(SendCallback callback);
}
This is the Server's code.
public class Server {
Plugin plugin;
SendCallback callback = new SendCallback() {
public void doSend(Object toSend) {
// logic to send object 'toSend'
}
}
public Server() {
plugin = new MyPlugin();
plugin.init(callback);
}
}
This is your plugin implementation.
public class MyPlugin implements Plugin {
SendCallback callback = null;
Object x = null;
public void init(SendCallback callback) {
this.callback = callback;
}
public void run() {
x = "Somthing"; // initialize the x object
callback.doSend(x);
}
}
You will notice, the server define the callback implementation.
The plugin will invoke the callback's method doSend.
I hope, this helps
There is method reference in Java 8, however you can just pass a whole object and call its sendData() method. In a 'plug-in' situation, using interfaces for each helps the plugin and the server have 'looser' coupling.
public interface Server {
void setData(...);
}
public class MyPlugin implements plugin {
private Server server;
void init(Server s ) {
this.server = s;
}
void run() {
...
this.server.setData(...);
...
}
}
interface Server{
...
void sendData(String message);
}
Plugin doesn't need a function reference, you can use the Server interface for informing Plugin to know about that method.
class PluginX implements Plugin{
...
private Server server;
void init(Server server) {
this.server = server;
}
public void run() {
// ...
server.sendData(x) // Sends data to server
// ...
}
}
Is there a way to pass a call back function in a Java method?
The behavior I'm trying to mimic is a .Net Delegate being passed to a function.
I've seen people suggesting creating a separate object but that seems overkill, however I am aware that sometimes overkill is the only way to do things.
If you mean somthing like .NET anonymous delegate, I think Java's anonymous class can be used as well.
public class Main {
public interface Visitor{
int doJob(int a, int b);
}
public static void main(String[] args) {
Visitor adder = new Visitor(){
public int doJob(int a, int b) {
return a + b;
}
};
Visitor multiplier = new Visitor(){
public int doJob(int a, int b) {
return a*b;
}
};
System.out.println(adder.doJob(10, 20));
System.out.println(multiplier.doJob(10, 20));
}
}
Since Java 8, there are lambda and method references:
Oracle Docs: Lambda Expressions
Oracle Docs: Method References
For example, if you want a functional interface A -> B, you can use:
import java.util.function.Function;
public MyClass {
public static String applyFunction(String name, Function<String,String> function){
return function.apply(name);
}
}
And here is how you can call it:
MyClass.applyFunction("42", str -> "the answer is: " + str);
// returns "the answer is: 42"
Also you can pass class method. For example:
#Value // lombok
public class PrefixAppender {
private String prefix;
public String addPrefix(String suffix){
return prefix +":"+suffix;
}
}
Then you can do:
PrefixAppender prefixAppender= new PrefixAppender("prefix");
MyClass.applyFunction("some text", prefixAppender::addPrefix);
// returns "prefix:some text"
Note:
Here I used the functional interface Function<A,B>, but there are many others in the package java.util.function. Most notable ones are
Supplier: void -> A
Consumer: A -> void
BiConsumer: (A,B) -> void
Function: A -> B
BiFunction: (A,B) -> C
and many others that specialize on some of the input/output type. Then, if it doesn't provide the one you need, you can create your own FunctionalInterface:
#FunctionalInterface
interface Function3<In1, In2, In3, Out> { // (In1,In2,In3) -> Out
public Out apply(In1 in1, In2 in2, In3 in3);
}
Example of use:
String computeAnswer(Function3<String, Integer, Integer, String> f){
return f.apply("6x9=", 6, 9);
}
computeAnswer((question, a, b) -> question + "42");
// "6*9=42"
And you can also do that with thrown exception:
#FunctionalInterface
interface FallibleFunction<In, Out, Ex extends Exception> {
Out get(In input) throws Ex;
}
public <Ex extends IOException> String yo(FallibleFunction<Integer, String, Ex> f) throws Ex {
return f.get(42);
}
For simplicity, you can use a Runnable:
private void runCallback(Runnable callback)
{
// Run callback
callback.run();
}
Usage:
runCallback(new Runnable()
{
#Override
public void run()
{
// Running callback
}
});
or with Java8 lambdas
runCallback(() -> {
// Running callback
});
yet i see there is most preferred way which was what i was looking for.. it's basically derived from these answers but i had to manipulate it to more more redundant and efficient.. and i think everybody looking for what i come up with
To the point::
first make an Interface that simple
public interface myCallback {
void onSuccess();
void onError(String err);
}
now to make this callback run when ever you wish to do to handle the results - more likely after async call and you wanna run some stuff which depends on these reuslts
// import the Interface class here
public class App {
public static void main(String[] args) {
// call your method
doSomething("list your Params", new myCallback(){
#Override
public void onSuccess() {
// no errors
System.out.println("Done");
}
#Override
public void onError(String err) {
// error happen
System.out.println(err);
}
});
}
private void doSomething(String param, // some params..
myCallback callback) {
// now call onSuccess whenever you want if results are ready
if(results_success)
callback.onSuccess();
else
callback.onError(someError);
}
}
doSomething is the function that takes some time you wanna add a callback to it to notify you when the results came, add the call back interface as a parameter to this method
hope my point is clear, enjoy ;)
A little nitpicking:
I've seem people suggesting creating a
separate object but that seems
overkill
Passing a callback includes creating a separate object in pretty much any OO language, so it can hardly be considered overkill. What you probably mean is that in Java, it requires you to create a separate class, which is more verbose (and more resource-intensive) than in languages with explicit first-class functions or closures. However, anonymous classes at least reduce the verbosity and can be used inline.
This is very easy in Java 8 with lambdas.
public interface Callback {
void callback();
}
public class Main {
public static void main(String[] args) {
methodThatExpectsACallback(() -> System.out.println("I am the callback."));
}
private static void methodThatExpectsACallback(Callback callback){
System.out.println("I am the method.");
callback.callback();
}
}
I found the idea of implementing using the reflect library interesting and came up with this which I think works quite well. The only down side is losing the compile time check that you are passing valid parameters.
public class CallBack {
private String methodName;
private Object scope;
public CallBack(Object scope, String methodName) {
this.methodName = methodName;
this.scope = scope;
}
public Object invoke(Object... parameters) throws InvocationTargetException, IllegalAccessException, NoSuchMethodException {
Method method = scope.getClass().getMethod(methodName, getParameterClasses(parameters));
return method.invoke(scope, parameters);
}
private Class[] getParameterClasses(Object... parameters) {
Class[] classes = new Class[parameters.length];
for (int i=0; i < classes.length; i++) {
classes[i] = parameters[i].getClass();
}
return classes;
}
}
You use it like this
public class CallBackTest {
#Test
public void testCallBack() throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
TestClass testClass = new TestClass();
CallBack callBack = new CallBack(testClass, "hello");
callBack.invoke();
callBack.invoke("Fred");
}
public class TestClass {
public void hello() {
System.out.println("Hello World");
}
public void hello(String name) {
System.out.println("Hello " + name);
}
}
}
A method is not (yet) a first-class object in Java; you can't pass a function pointer as a callback. Instead, create an object (which usually implements an interface) that contains the method you need and pass that.
Proposals for closures in Java—which would provide the behavior you are looking for—have been made, but none will be included in the upcoming Java 7 release.
When I need this kind of functionality in Java, I usually use the Observer pattern. It does imply an extra object, but I think it's a clean way to go, and is a widely understood pattern, which helps with code readability.
Check the closures how they have been implemented in the lambdaj library. They actually have a behavior very similar to C# delegates:
http://code.google.com/p/lambdaj/wiki/Closures
You also can do theCallback using the Delegate pattern:
Callback.java
public interface Callback {
void onItemSelected(int position);
}
PagerActivity.java
public class PagerActivity implements Callback {
CustomPagerAdapter mPagerAdapter;
public PagerActivity() {
mPagerAdapter = new CustomPagerAdapter(this);
}
#Override
public void onItemSelected(int position) {
// Do something
System.out.println("Item " + postion + " selected")
}
}
CustomPagerAdapter.java
public class CustomPagerAdapter {
private static final int DEFAULT_POSITION = 1;
public CustomPagerAdapter(Callback callback) {
callback.onItemSelected(DEFAULT_POSITION);
}
}
I tried using java.lang.reflect to implement 'callback', here's a sample:
package StackOverflowQ443708_JavaCallBackTest;
import java.lang.reflect.*;
import java.util.concurrent.*;
class MyTimer
{
ExecutorService EXE =
//Executors.newCachedThreadPool ();
Executors.newSingleThreadExecutor ();
public static void PrintLine ()
{
System.out.println ("--------------------------------------------------------------------------------");
}
public void SetTimer (final int timeout, final Object obj, final String methodName, final Object... args)
{
SetTimer (timeout, obj, false, methodName, args);
}
public void SetTimer (final int timeout, final Object obj, final boolean isStatic, final String methodName, final Object... args)
{
Class<?>[] argTypes = null;
if (args != null)
{
argTypes = new Class<?> [args.length];
for (int i=0; i<args.length; i++)
{
argTypes[i] = args[i].getClass ();
}
}
SetTimer (timeout, obj, isStatic, methodName, argTypes, args);
}
public void SetTimer (final int timeout, final Object obj, final String methodName, final Class<?>[] argTypes, final Object... args)
{
SetTimer (timeout, obj, false, methodName, argTypes, args);
}
public void SetTimer (final int timeout, final Object obj, final boolean isStatic, final String methodName, final Class<?>[] argTypes, final Object... args)
{
EXE.execute (
new Runnable()
{
public void run ()
{
Class<?> c;
Method method;
try
{
if (isStatic) c = (Class<?>)obj;
else c = obj.getClass ();
System.out.println ("Wait for " + timeout + " seconds to invoke " + c.getSimpleName () + "::[" + methodName + "]");
TimeUnit.SECONDS.sleep (timeout);
System.out.println ();
System.out.println ("invoking " + c.getSimpleName () + "::[" + methodName + "]...");
PrintLine ();
method = c.getDeclaredMethod (methodName, argTypes);
method.invoke (obj, args);
}
catch (Exception e)
{
e.printStackTrace();
}
finally
{
PrintLine ();
}
}
}
);
}
public void ShutdownTimer ()
{
EXE.shutdown ();
}
}
public class CallBackTest
{
public void onUserTimeout ()
{
System.out.println ("onUserTimeout");
}
public void onTestEnd ()
{
System.out.println ("onTestEnd");
}
public void NullParameterTest (String sParam, int iParam)
{
System.out.println ("NullParameterTest: String parameter=" + sParam + ", int parameter=" + iParam);
}
public static void main (String[] args)
{
CallBackTest test = new CallBackTest ();
MyTimer timer = new MyTimer ();
timer.SetTimer ((int)(Math.random ()*10), test, "onUserTimeout");
timer.SetTimer ((int)(Math.random ()*10), test, "onTestEnd");
timer.SetTimer ((int)(Math.random ()*10), test, "A-Method-Which-Is-Not-Exists"); // java.lang.NoSuchMethodException
timer.SetTimer ((int)(Math.random ()*10), System.out, "println", "this is an argument of System.out.println() which is called by timer");
timer.SetTimer ((int)(Math.random ()*10), System.class, true, "currentTimeMillis");
timer.SetTimer ((int)(Math.random ()*10), System.class, true, "currentTimeMillis", "Should-Not-Pass-Arguments"); // java.lang.NoSuchMethodException
timer.SetTimer ((int)(Math.random ()*10), String.class, true, "format", "%d %X", 100, 200); // java.lang.NoSuchMethodException
timer.SetTimer ((int)(Math.random ()*10), String.class, true, "format", "%d %X", new Object[]{100, 200});
timer.SetTimer ((int)(Math.random ()*10), test, "NullParameterTest", new Class<?>[]{String.class, int.class}, null, 888);
timer.ShutdownTimer ();
}
}
I've recently started doing something like this:
public class Main {
#FunctionalInterface
public interface NotDotNetDelegate {
int doSomething(int a, int b);
}
public static void main(String[] args) {
// in java 8 (lambdas):
System.out.println(functionThatTakesDelegate((a, b) -> {return a*b;} , 10, 20));
}
public static int functionThatTakesDelegate(NotDotNetDelegate del, int a, int b) {
// ...
return del.doSomething(a, b);
}
}
it's a bit old, but nevertheless... I found the answer of Peter Wilkinson nice except for the fact that it does not work for primitive types like int/Integer.
The problem is the .getClass() for the parameters[i], which returns for instance java.lang.Integer, which on the other hand will not be correctly interpreted by getMethod(methodName,parameters[]) (Java's fault) ...
I combined it with the suggestion of Daniel Spiewak (in his answer to this); steps to success included: catching NoSuchMethodException -> getMethods() -> looking for the matching one by method.getName() -> and then explicitly looping through the list of parameters and applying Daniels solution, such identifying the type matches and the signature matches.
with java 8 this task is kinda easy, if you want to use callback in multi-thread scenario you can do something similar like the following:
public void methodA (int n, IntConsumer consumer) {
// create a thread
Thread t = new Thread(() -> {
// some time consuming operation
int result = IntStream.range(0, n).sum();
// after the result is ready do something with it.
consumer.accept(result);
});
t.start();
}
and to use this method do:
methodA(1000000, System.out::println);
public class HelloWorldAnonymousClasses {
//this is an interface with only one method
interface HelloWorld {
public void printSomething(String something);
}
//this is a simple function called from main()
public void sayHello() {
//this is an object with interface reference followed by the definition of the interface itself
new HelloWorld() {
public void printSomething(String something) {
System.out.println("Hello " + something);
}
}.printSomething("Abhi");
//imagine this as an object which is calling the function'printSomething()"
}
public static void main(String... args) {
HelloWorldAnonymousClasses myApp =
new HelloWorldAnonymousClasses();
myApp.sayHello();
}
}
//Output is "Hello Abhi"
Basically if you want to make the object of an interface it is
not possible, because interface cannot have objects.
The option is to let some class implement the interface and then call that function using the object of that class.
But this approach is really verbose.
Alternatively, write new HelloWorld() (*oberserve this is an interface not a class) and then follow it up with the defination of the interface methods itself. (*This defination is in reality the anonymous class).
Then you get the object reference through which you can call the method itself.
Create an Interface, and Create the Same Interface Property in Callback Class.
interface dataFetchDelegate {
void didFetchdata(String data);
}
//callback class
public class BackendManager{
public dataFetchDelegate Delegate;
public void getData() {
//Do something, Http calls/ Any other work
Delegate.didFetchdata("this is callbackdata");
}
}
Now in the class where you want to get called back implement the above Created Interface.
and Also Pass "this" Object/Reference of your class to be called back.
public class Main implements dataFetchDelegate
{
public static void main( String[] args )
{
new Main().getDatafromBackend();
}
public void getDatafromBackend() {
BackendManager inc = new BackendManager();
//Pass this object as reference.in this Scenario this is Main Object
inc.Delegate = this;
//make call
inc.getData();
}
//This method is called after task/Code Completion
public void didFetchdata(String callbackData) {
// TODO Auto-generated method stub
System.out.println(callbackData);
}
}
Simpliest and easiest way is by creating a reusable model and trigger.... https://onecompiler.com/java/3wejrcby2?fbclid=IwAR0dHbGDChRUJoCZ3CIDW-JQu7Dz3iYGNGYjxYVCPCWfEqQDogFGTwuOuO8