Consider the following Scala code:
package scala_java
object MyScala {
def setFunc(func: Int => String) {
func(10)
}
}
Now in Java, I would have liked to use MyScala as:
package scala_java;
public class MyJava {
public static void main(String [] args) {
MyScala.setFunc(myFunc); // This line gives an error
}
public static String myFunc(int someInt) {
return String.valueOf(someInt);
}
}
However, the above does not work (as expected since Java does not allow functional programming). What is the easiest workaround to pass a function in Java? I would like a generic solution that works with functions having arbitrary number of parameters.
EDIT: Does Java 8 have any better syntax than the classic solutions discussed below?
In the scala.runtime package, there are abstract classes named AbstractFunction1 and so on for other arities. To use them from Java you only need to override apply, like this:
Function1<Integer, String> f = new AbstractFunction1<Integer, String>() {
public String apply(Integer someInt) {
return myFunc(someInt);
}
};
If you're on Java 8 and want to use Java 8 lambda syntax for this, check out https://github.com/scala/scala-java8-compat.
You have to manually instantiate a Function1 in Java. Something like:
final Function1<Integer, String> f = new Function1<Integer, String>() {
public int $tag() {
return Function1$class.$tag(this);
}
public <A> Function1<A, String> compose(Function1<A, Integer> f) {
return Function1$class.compose(this, f);
}
public String apply(Integer someInt) {
return myFunc(someInt);
}
};
MyScala.setFunc(f);
This is taken from Daniel Spiewak’s “Interop Between Java and Scala” article.
The easiest way for me is to defined a java interface like:
public interface JFunction<A,B> {
public B compute( A a );
}
Then modify your scala code, overloading setFunc to accept also JFunction objects such as:
object MyScala {
// API for scala
def setFunc(func: Int => String) {
func(10)
}
// API for java
def setFunc(jFunc: JFunction[Int,String]) {
setFunc( (i:Int) => jFunc.compute(i) )
}
}
You will naturally use the first definition from scala, but still be able to use the second one from java:
public class MyJava {
public static void main(String [] args) {
MyScala.setFunc(myFunc); // This line gives an error
}
public static final JFunction<Integer,String> myFunc =
new JFunction<Integer,String>() {
public String compute( Integer a ) {
return String.valueOf(a);
}
};
}
Here's my attempt at a solution, a little library: https://github.com/eirslett/sc8
You wrap your Java 8 lambda in F(...) and then it's converted to a Scala function.
Related
My program requires that I accept a user input and, based on this input, a method is to be carried out. My basic thoughts are described well by the following question/answer:
How to call a method stored in a HashMap? (Java)
To do this, I have created an array of lambda expressions:
public final Runnable[] userCommandMethods = {
() -> userCommand1(),
() -> userCommand2(),
};
And an array of keys:
public final String[] userCommandKeys = {
commandKey1,
commandKey2,
};
Which are joined to create a HashMap using the following method:
public Map<String, Runnable> mapArrays (String[] array1, Runnable[] array2) {
Map<String, Runnable> mappedArrays = new HashMap<String, Runnable>();
for (int i = 0; i < array1.length; i ++) {
mappedArrays.put(array1[i], array2[i]);
}
return mappedArrays;
}
When I attempt to run a method by using myHashMap.get(userInput).run(); it works perfectly, provided none of the methods in userCommandMethods require input parameters.
My question:
How would I pass an input parameter (specifically a Hash Map) into the methods contained within userCommandMethods?
When the userCommand1() method takes an input parameter, but the lambda expression does not, I get the following error:
The method userCommand1(Map<String, String>) in the type ProgramCommands is not applicable for the arguments ()
However, when I do pass a parameter to the method, it states that it cannot be resolved to a variable.
Edit: to elaborate:
When the userCommand1() method takes no arguments:
public void userCommand1 () {
// Do some stuff
}
It works perfectly fine. However, I am unsure how to use the lambda expressions if the method does take an input parameter:
public void userCommand1 (Map<String, String> myMap) {
// Do some stuff
}
You just need to choose another functional interface (not Runnable).
For example, if your methods all take a String parameter, you should use Consumer<String>. If they take a String and an int, then you should use BiConsumer<String, Integer>. If your methods need more than 2 parameters, you need to create your own functional interface. For an example, see my answer here.
// use a list instead of an array, because arrays don't work well with generic types
public final List<Consumer<String>> userCommandMethods = List.of(
x -> userCommand1(x),
x -> userCommand2() // it's fine if the method takes fewer parameters
);
Instead of run, you would call accept, which is what Consumer and BiConsumer's single abstraction method is called.
Note that you can also use the method reference syntax. If userCommand1 is static, x -> userCommand1(x) can be rewritten as SomeClass::userCommand1 where SomeClass is the enclosing class of userCommand1. If userCommand1 is non static, it can be rewritten as this::userCommand1.
You don't need to build the map from two arrays. You can use ofEntries and entry to write the entries inline.
private final Map<String, Consumer<String>> someMap = Map.ofEntries(
Map.entry("foo", SomeClass::userCommand1),
Map.entry("bar", SomeClass::userCommand2),
Map.entry("baz", SomeClass::userCommand3),
// and so on
)
You are using Runnable interface that takes no argument on input:
#FunctionalInterface
public interface Runnable {
public abstract void run();
}
Instead, you can define your custom interface and consume it.
As a simple example:
#FunctionalInterface
public interface RunnableWithArg {
void apply(String t) throws RuntimeException;
}
And implementation may look like:
public class RunnableTest {
//also fine:
//public final RunnableWithArg[] userCommandMethods = { t -> this.userCommand1(t), t -> this.userCommand2(t) };
public final RunnableWithArg[] userCommandMethods = { this::userCommand1, this::userCommand2 };
public String commandKey1 = "commandKey1";
public String commandKey2 = "commandKey2";
public final String[] userCommandKeys = { commandKey1, commandKey2, };
public Map<String, RunnableWithArg> mapArrays(String[] array1, RunnableWithArg[] array2) {
Map<String, RunnableWithArg> mappedArrays = new HashMap<>();
for (int i = 0; i < array1.length; i++) {
mappedArrays.put(array1[i], array2[i]);
}
return mappedArrays;
}
public void userCommand1(String data) {
System.out.println("userCommand1 called with " + data);
}
public void userCommand2(String data) {
System.out.println("userCommand2 called with " + data);
}
public void test()
{
var fncMap = mapArrays(userCommandKeys, userCommandMethods);
for(String key: fncMap.keySet())
{
var fnc = fncMap.get(key);
fnc.apply(key);
}
}
}
And of course you can also define some generic types of "#FunctionalInterface" like this, so you can use it for both taking input and returning some output of generic types:
#FunctionalInterface
public interface AbcFunction<T, R> {
R apply(T t) throws AbcException;
static <T> Function<T, T> identity() {
return t -> t;
}
}
Is this something you are thinking of?
interface Command<T> {
public void run(T arg);
}
class SayHelloCommand implements Command<String>{
public void run(String name){
System.out.println("hello " + name);
}
}
class CountCommand implements Command<Integer>{
public void run(Integer limit){
for(int i=0; i<=limit; i++)
System.out.println(i);
}
}
public class Main{
public static void main(String[] args) {
Command[] commands = new Command[3];
commands[0] = new SayHelloCommand();
commands[1] = new CountCommand();
commands[0].run("Joe");
commands[1].run(5);
}
}
I have Java enum:
public enum ConflictResolutionStrategy {
softResolve,
hardResolve,
}
I want to call it like ConflictResolutionStrategy.hardResolve.apply(case1, case2).
Both case1 and case2 objects of the same type. apply in my case should return nothing.
The basic idea behind this design. Create Strategy design pattern and resolve conflicts based on the set enum value.
I cannot find any similar questions on StackOveflow even simple search gives me tons of similar cases which don't resolve my case directly.
I tried The following:
public enum ConflictResolutionStrategy {
softResolve ((CaseType case1, CaseType case2) -> case1.update(case2)),
hardResolve,
}
This version above doesn't compile.
I tried another solution:
public enum ConflictResolutionStrategy {
softResolve,
hardResolve {
public void apply(CaseType case1, CaseType case2) {
case1.update(case2);
}
},
}
The second solution, works okay but requires too much code.
A function accepting two parameters and returning nothing is a BiConsumer.
Your enum may implement BiConsumer:
public enum ConflictResolutionStrategy implements BiConsumer<CaseType, CaseType> {
softResolve ((case1, case2) -> case1.update(case2)),
hardResolve((case1, case2) -> {/* do something else */});
private final BiConsumer<CaseType, CaseType> consumer;
ConflictResolutionStrategy(BiConsumer<CaseType, CaseType> consumer){
this.consumer = consumer;
}
#Override
public void accept(CaseType case1, CaseType case2) {
consumer.accept(case1, case2);
}
}
Suppose you have a method for processing your cases:
public void processCases(Collection<CaseType> cases, BiConsumer<CaseType, CaseType> conflictResolutionStrategy){
// ...
}
Now you can pass in either one of your existing strategies:
processCases(cases, ConflictResolutionStrategy.softResolve);
or an inline strategy:
processCases(cases, (c1, c2) -> { /* do smth here */ });
Maybe you're looking for something like a BiConsumer within the enum as :
public enum ConflictResolutionStrategy {
softResolve(CaseType::update), // just for example
hardResolve((caseType, caseType2) -> {
caseType.update(caseType2);
}); // lambda representation
BiConsumer<CaseType, CaseType> caseTypeBiConsumer;
ConflictResolutionStrategy(BiConsumer<CaseType, CaseType> caseTypeBiConsumer) {
this.caseTypeBiConsumer = caseTypeBiConsumer;
}
}
class CaseType {
void update(CaseType caseType){}
}
I am trying to wrap my head around generic and functions... what I am trying to achieve: Passing function name as a string to get it executed:
I want to do Wrapper.useFunction("eleven") or Wrapper.useFunction("ten")
public class Wrapper<T> {
public F useFunction(Function<F, F> function) {
return function.apply(F);
}
Function<F, String> ten = s -> "10";
Function<F, String> eleven = s -> "11";
}
But this code not even close to compiling. Maybe it doesn't make any sense. Any suggestions?
If you have a finite set of functions which you would like to be able to call I would recommend building a Map which maps Strings to instances of Runnable (or similar functional interfaces). Your useFunction method may then look up the function implementation in the Map and call it if it exists.
Example:
public class SomeClass {
private final Map<String, Runnable> methods = new HashMap<>();
{
methods.put("helloworld", () -> {
System.out.println("Hello World!");
});
methods.put("test", () -> {
System.out.println("test!");
});
methods.put("doStuff", () -> {
System.out.println("doStuff!");
});
}
public void perform(String code) {
methods.getOrDefault(code,
() -> {
System.err.println("No such Method: "+code);
})
.run();
}
}
If you want to call arbitrary methods you should probably use Reflection as stated by others.
I've just started working with Java 8 and I'm struggling with this code snippet:
paramsValues[idx++] = new ReplyMessage<JsonObject>() {
#Override
public void reply(JsonObject reply) {
message.reply(reply);
}
};
How to convert this to Lambda style?
If ReplyMessage is a functional interface, you could do
paramsValues[idx++] = reply -> message.reply(reply);
Here's a full example with stub implementations of the other classes in your question:
// Stub classes
class JsonObject { }
#FunctionalInterface
interface ReplyMessage<T> {
void reply(T jo);
}
class LambdaDemo {
public static void main(String args[]) {
// Dummy variables
ReplyMessage<JsonObject> message = new ReplyMessage<JsonObject>() {
public void reply(JsonObject jo) {}
};
ReplyMessage[] paramsValues = new ReplyMessage[5];
int idx = 0;
// Your code, lambdafied
paramsValues[idx++] = reply -> message.reply(reply);
// Or,
// paramsValues[idx++] = message::reply;
// But then you could perhaps do with just ...
// paramsValues[idx++] = message;
}
}
Lambda expressions are only possible with Functional Interfaces (Interfaces with only one method, such as Runnable or ActionEvent)
If ReplyMessage is a functional interface, you can do:
paramsValues[idx++] = reply -> message.reply(reply);
Lambda expressions are formed in this pattern: parameters that the method should take, then -> then the method body
Here is the code of how ReplyMessage interface should look like:
#FunctionalInterface
interface ReplyMessage<T> {
void reply(T jo);
}
For more information, consider reading this.
paramValues[idx++] = reply -> message.reply(reply);
Or
paramValues[idx++] = reply -> {
return message.reply(reply);
}
It will work as long as ReplyMessage<JsonObject> is functional interface and paramValues is of type ReplyMessage<JsonObject>.
Our code has several processors, each one having several api methods, where each method is overloaded also with same method that can accept collection.
For example:
public class Foo {
public X foo(Y y){...}
public Collection<X> foo(Collection<Y> y){... // iterate and execute foo(y) ... }
public Z bar(W w){...}
public Collection<Z> bar(Collection<W> w){... // iterate and execute bar(w) ... }
}
public class Other{
// also method and method on collection
}
Naturally, those methods on collections are actually duplication code of iteration.
What we are looking for, is kind of way to make some pattern or use generics, so the iteration over collection will be implemented once, also for that need a way to somehow pass the method name.
I'd suggest Startegy pattern. And do something like:
public interface Transformer<X, Y> {
Y transform( X input );
}
class Processor {
public <X,Y> Collection<Y> process( Collection<X> input, Transformer<X, Y> transformer) {
Collection<Y> ret = new LinkedList<Y>();
// generic loop, delegating transformation to specific transformer
for( X x : input) {
ret.add( transformer.transform( x ) );
}
return ret;
}
}
Example:
public static void main( String[] args ) {
List<String> strings = new LinkedList<String>();
strings.add( "1" );
strings.add( "2" );
strings.add( "3" );
Processor p = new Processor();
Collection<Integer> numbers = p.process( strings, new Transformer<String, Integer>() {
#Override
public Integer transform( String input ) {
return Integer.parseInt( input );
}
} );
}
I can't see how reflection could help here. You're trying to replace something as trivial as
public Collection<X> foo(Collection<Y> y) {
List<X> result = Lists.newArrayList();
for (Y e : y) result.add(foo(e));
return result;
}
by something probably much slower. I don't think that saving those 3 lines (several times) is worth it, but you might want to try either annotation processing (possibly without using annotations) or dynamic code generation. In both cases you'd write the original class as is without the collection methods and use a different one containing both the scalar and the collection methods.
Or you might want to make it more functionally styled:
public class Foo {
public final RichFunction<Y, X> foo = new RichFunction<Y, X>() {
X apply(Y y) {
return foo(y);
}
}
// after some refactoring the original method can be made private
// or inlined into the RichFunction
public X foo(Y y){...}
// instead of calling the original method like
// foo.foo(y)
// you'd use
// foo.foo.apply(y)
// which would work for both the scalar and collection methods
}
public abstract class RichFunction<K, V> extends com.google.common.base.Function<K, V> {
Collection<V> apply(Collection<K> keys) {
List<V> result = Lists.newArrayList();
for (K k : keys) result.add(apply(k));
return result;
}
}
RUAKH - I chosed to implement your suggestion for reflection (although, admit, I don't like reflection). So, I did something like the code below THANKS :)
public class Resource {
private static final int CLIENT_CODE_STACK_INDEX;
static {
// Finds out the index of "this code" in the returned stack trace - funny but it differs in JDK 1.5 and 1.6
int i = 0;
for (StackTraceElement ste : Thread.currentThread().getStackTrace()) {
i++;
if (ste.getClassName().equals(Resource.class.getName())) {
break;
}
}
CLIENT_CODE_STACK_INDEX = i;
}
public static String getCurrentMethodName() {
return Thread.currentThread().getStackTrace()[CLIENT_CODE_STACK_INDEX].getMethodName();
}
protected <IN,OUT> Collection<OUT> doMultiple(String methodName, Collection<IN> inCol, Class<?>... parameterTypes){
Collection<OUT> result = new ArrayList<OUT>();
try {
Method m = this.getClass().getDeclaredMethod(methodName, parameterTypes);
if (inCol==null || inCol.size()==0){
return result;
}
for (IN in : inCol){
Object o = m.invoke(this, in);
result.add((OUT) o);
}
}catch (Exception e){
e.printStackTrace();
}
return result;
}
}
public class FirstResource extends Resource{
public String doSomeThing(Integer i){
// LOTS OF LOGIC
return i.toString();
}
public Collection<String> doSomeThing(Collection<Integer> ints){
return doMultiple(getCurrentMethodName(), ints, Integer.class);
}
}
You should use Strategy pattern. By using Strategy pattern you can omit the usage if/else which makes the code more complex. Where strategy pattern creates less coupled code which is much simpler. By using Strategy pattern you can achieve more ways to configure code dynamically. So I would like to suggest you to use Strategy pattern.