I have a method that's about ten lines of code. I want to create more methods that do exactly the same thing, except for a small calculation that's going to change one line of code. This is a perfect application for passing in a function pointer to replace that one line, but Java doesn't have function pointers. What's my best alternative?
Anonymous inner class
Say you want to have a function passed in with a String param that returns an int.
First you have to define an interface with the function as its only member, if you can't reuse an existing one.
interface StringFunction {
int func(String param);
}
A method that takes the pointer would just accept StringFunction instance like so:
public void takingMethod(StringFunction sf) {
int i = sf.func("my string");
// do whatever ...
}
And would be called like so:
ref.takingMethod(new StringFunction() {
public int func(String param) {
// body
}
});
EDIT: In Java 8, you could call it with a lambda expression:
ref.takingMethod(param -> bodyExpression);
For each "function pointer", I'd create a small functor class that implements your calculation.
Define an interface that all the classes will implement, and pass instances of those objects into your larger function. This is a combination of the "command pattern", and "strategy pattern".
#sblundy's example is good.
When there is a predefined number of different calculations you can do in that one line, using an enum is a quick, yet clear way to implement a strategy pattern.
public enum Operation {
PLUS {
public double calc(double a, double b) {
return a + b;
}
},
TIMES {
public double calc(double a, double b) {
return a * b;
}
}
...
public abstract double calc(double a, double b);
}
Obviously, the strategy method declaration, as well as exactly one instance of each implementation are all defined in a single class/file.
You need to create an interface that provides the function(s) that you want to pass around. eg:
/**
* A simple interface to wrap up a function of one argument.
*
* #author rcreswick
*
*/
public interface Function1<S, T> {
/**
* Evaluates this function on it's arguments.
*
* #param a The first argument.
* #return The result.
*/
public S eval(T a);
}
Then, when you need to pass a function, you can implement that interface:
List<Integer> result = CollectionUtilities.map(list,
new Function1<Integer, Integer>() {
#Override
public Integer eval(Integer a) {
return a * a;
}
});
Finally, the map function uses the passed in Function1 as follows:
public static <K,R,S,T> Map<K, R> zipWith(Function2<R,S,T> fn,
Map<K, S> m1, Map<K, T> m2, Map<K, R> results){
Set<K> keySet = new HashSet<K>();
keySet.addAll(m1.keySet());
keySet.addAll(m2.keySet());
results.clear();
for (K key : keySet) {
results.put(key, fn.eval(m1.get(key), m2.get(key)));
}
return results;
}
You can often use Runnable instead of your own interface if you don't need to pass in parameters, or you can use various other techniques to make the param count less "fixed" but it's usually a trade-off with type safety. (Or you can override the constructor for your function object to pass in the params that way.. there are lots of approaches, and some work better in certain circumstances.)
Method references using the :: operator
You can use method references in method arguments where the method accepts a functional interface. A functional interface is any interface that contains only one abstract method. (A functional interface may contain one or more default methods or static methods.)
IntBinaryOperator is a functional interface. Its abstract method, applyAsInt, accepts two ints as its parameters and returns an int. Math.max also accepts two ints and returns an int. In this example, A.method(Math::max); makes parameter.applyAsInt send its two input values to Math.max and return the result of that Math.max.
import java.util.function.IntBinaryOperator;
class A {
static void method(IntBinaryOperator parameter) {
int i = parameter.applyAsInt(7315, 89163);
System.out.println(i);
}
}
import java.lang.Math;
class B {
public static void main(String[] args) {
A.method(Math::max);
}
}
In general, you can use:
method1(Class1::method2);
instead of:
method1((arg1, arg2) -> Class1.method2(arg1, arg2));
which is short for:
method1(new Interface1() {
int method1(int arg1, int arg2) {
return Class1.method2(arg1, agr2);
}
});
For more information, see :: (double colon) operator in Java 8 and Java Language Specification §15.13.
You can also do this (which in some RARE occasions makes sense). The issue (and it is a big issue) is that you lose all the typesafety of using a class/interface and you have to deal with the case where the method does not exist.
It does have the "benefit" that you can ignore access restrictions and call private methods (not shown in the example, but you can call methods that the compiler would normally not let you call).
Again, it is a rare case that this makes sense, but on those occasions it is a nice tool to have.
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
class Main
{
public static void main(final String[] argv)
throws NoSuchMethodException,
IllegalAccessException,
IllegalArgumentException,
InvocationTargetException
{
final String methodName;
final Method method;
final Main main;
main = new Main();
if(argv.length == 0)
{
methodName = "foo";
}
else
{
methodName = "bar";
}
method = Main.class.getDeclaredMethod(methodName, int.class);
main.car(method, 42);
}
private void foo(final int x)
{
System.out.println("foo: " + x);
}
private void bar(final int x)
{
System.out.println("bar: " + x);
}
private void car(final Method method,
final int val)
throws IllegalAccessException,
IllegalArgumentException,
InvocationTargetException
{
method.invoke(this, val);
}
}
If you have just one line which is different you could add a parameter such as a flag and a if(flag) statement which calls one line or the other.
You may also be interested to hear about work going on for Java 7 involving closures:
What’s the current state of closures in Java?
http://gafter.blogspot.com/2006/08/closures-for-java.html
http://tech.puredanger.com/java7/#closures
New Java 8 Functional Interfaces and Method References using the :: operator.
Java 8 is able to maintain method references ( MyClass::new ) with "# Functional Interface" pointers. There are no need for same method name, only same method signature required.
Example:
#FunctionalInterface
interface CallbackHandler{
public void onClick();
}
public class MyClass{
public void doClick1(){System.out.println("doClick1");;}
public void doClick2(){System.out.println("doClick2");}
public CallbackHandler mClickListener = this::doClick;
public static void main(String[] args) {
MyClass myObjectInstance = new MyClass();
CallbackHandler pointer = myObjectInstance::doClick1;
Runnable pointer2 = myObjectInstance::doClick2;
pointer.onClick();
pointer2.run();
}
}
So, what we have here?
Functional Interface - this is interface, annotated or not with #FunctionalInterface, which contains only one method declaration.
Method References - this is just special syntax, looks like this, objectInstance::methodName, nothing more nothing less.
Usage example - just an assignment operator and then interface method call.
YOU SHOULD USE FUNCTIONAL INTERFACES FOR LISTENERS ONLY AND ONLY FOR THAT!
Because all other such function pointers are really bad for code readability and for ability to understand. However, direct method references sometimes come handy, with foreach for example.
There are several predefined Functional Interfaces:
Runnable -> void run( );
Supplier<T> -> T get( );
Consumer<T> -> void accept(T);
Predicate<T> -> boolean test(T);
UnaryOperator<T> -> T apply(T);
BinaryOperator<T,U,R> -> R apply(T, U);
Function<T,R> -> R apply(T);
BiFunction<T,U,R> -> R apply(T, U);
//... and some more of it ...
Callable<V> -> V call() throws Exception;
Readable -> int read(CharBuffer) throws IOException;
AutoCloseable -> void close() throws Exception;
Iterable<T> -> Iterator<T> iterator();
Comparable<T> -> int compareTo(T);
Comparator<T> -> int compare(T,T);
For earlier Java versions you should try Guava Libraries, which has similar functionality, and syntax, as Adrian Petrescu has mentioned above.
For additional research look at Java 8 Cheatsheet
and thanks to The Guy with The Hat for the Java Language Specification §15.13 link.
#sblundy's answer is great, but anonymous inner classes have two small flaws, the primary being that they tend not to be reusable and the secondary is a bulky syntax.
The nice thing is that his pattern expands into full classes without any change in the main class (the one performing the calculations).
When you instantiate a new class you can pass parameters into that class which can act as constants in your equation--so if one of your inner classes look like this:
f(x,y)=x*y
but sometimes you need one that is:
f(x,y)=x*y*2
and maybe a third that is:
f(x,y)=x*y/2
rather than making two anonymous inner classes or adding a "passthrough" parameter, you can make a single ACTUAL class that you instantiate as:
InnerFunc f=new InnerFunc(1.0);// for the first
calculateUsing(f);
f=new InnerFunc(2.0);// for the second
calculateUsing(f);
f=new InnerFunc(0.5);// for the third
calculateUsing(f);
It would simply store the constant in the class and use it in the method specified in the interface.
In fact, if KNOW that your function won't be stored/reused, you could do this:
InnerFunc f=new InnerFunc(1.0);// for the first
calculateUsing(f);
f.setConstant(2.0);
calculateUsing(f);
f.setConstant(0.5);
calculateUsing(f);
But immutable classes are safer--I can't come up with a justification to make a class like this mutable.
I really only post this because I cringe whenever I hear anonymous inner class--I've seen a lot of redundant code that was "Required" because the first thing the programmer did was go anonymous when he should have used an actual class and never rethought his decision.
The Google Guava libraries, which are becoming very popular, have a generic Function and Predicate object that they have worked into many parts of their API.
One of the things I really miss when programming in Java is function callbacks. One situation where the need for these kept presenting itself was in recursively processing hierarchies where you want to perform some specific action for each item. Like walking a directory tree, or processing a data structure. The minimalist inside me hates having to define an interface and then an implementation for each specific case.
One day I found myself wondering why not? We have method pointers - the Method object. With optimizing JIT compilers, reflective invocation really doesn't carry a huge performance penalty anymore. And besides next to, say, copying a file from one location to another, the cost of the reflected method invocation pales into insignificance.
As I thought more about it, I realized that a callback in the OOP paradigm requires binding an object and a method together - enter the Callback object.
Check out my reflection based solution for Callbacks in Java. Free for any use.
Sounds like a strategy pattern to me. Check out fluffycat.com Java patterns.
oK, this thread is already old enough, so very probably my answer is not helpful for the question. But since this thread helped me to find my solution, I'll put it out here anyway.
I needed to use a variable static method with known input and known output (both double). So then, knowing the method package and name, I could work as follows:
java.lang.reflect.Method Function = Class.forName(String classPath).getMethod(String method, Class[] params);
for a function that accepts one double as a parameter.
So, in my concrete situation I initialized it with
java.lang.reflect.Method Function = Class.forName("be.qan.NN.ActivationFunctions").getMethod("sigmoid", double.class);
and invoked it later in a more complex situation with
return (java.lang.Double)this.Function.invoke(null, args);
java.lang.Object[] args = new java.lang.Object[] {activity};
someOtherFunction() + 234 + (java.lang.Double)Function.invoke(null, args);
where activity is an arbitrary double value. I am thinking of maybe doing this a bit more abstract and generalizing it, as SoftwareMonkey has done, but currently I am happy enough with the way it is. Three lines of code, no classes and interfaces necessary, that's not too bad.
To do the same thing without interfaces for an array of functions:
class NameFuncPair
{
public String name; // name each func
void f(String x) {} // stub gets overridden
public NameFuncPair(String myName) { this.name = myName; }
}
public class ArrayOfFunctions
{
public static void main(String[] args)
{
final A a = new A();
final B b = new B();
NameFuncPair[] fArray = new NameFuncPair[]
{
new NameFuncPair("A") { #Override void f(String x) { a.g(x); } },
new NameFuncPair("B") { #Override void f(String x) { b.h(x); } },
};
// Go through the whole func list and run the func named "B"
for (NameFuncPair fInstance : fArray)
{
if (fInstance.name.equals("B"))
{
fInstance.f(fInstance.name + "(some args)");
}
}
}
}
class A { void g(String args) { System.out.println(args); } }
class B { void h(String args) { System.out.println(args); } }
Check out lambdaj
http://code.google.com/p/lambdaj/
and in particular its new closure feature
http://code.google.com/p/lambdaj/wiki/Closures
and you will find a very readable way to define closure or function pointer without creating meaningless interface or use ugly inner classes
Wow, why not just create a Delegate class which is not all that hard given that I already did for java and use it to pass in parameter where T is return type. I am sorry but as a C++/C# programmer in general just learning java, I need function pointers because they are very handy. If you are familiar with any class which deals with Method Information you can do it. In java libraries that would be java.lang.reflect.method.
If you always use an interface, you always have to implement it. In eventhandling there really isn't a better way around registering/unregistering from the list of handlers but for delegates where you need to pass in functions and not the value type, making a delegate class to handle it for outclasses an interface.
None of the Java 8 answers have given a full, cohesive example, so here it comes.
Declare the method that accepts the "function pointer" as follows:
void doCalculation(Function<Integer, String> calculation, int parameter) {
final String result = calculation.apply(parameter);
}
Call it by providing the function with a lambda expression:
doCalculation((i) -> i.toString(), 2);
If anyone is struggling to pass a function that takes one set of parameters to define its behavior but another set of parameters on which to execute, like Scheme's:
(define (function scalar1 scalar2)
(lambda (x) (* x scalar1 scalar2)))
see Pass Function with Parameter-Defined Behavior in Java
Since Java8, you can use lambdas, which also have libraries in the official SE 8 API.
Usage:
You need to use a interface with only one abstract method.
Make an instance of it (you may want to use the one java SE 8 already provided) like this:
Function<InputType, OutputType> functionname = (inputvariablename) {
...
return outputinstance;
}
For more information checkout the documentation: https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html
Prior to Java 8, nearest substitute for function-pointer-like functionality was an anonymous class. For example:
Collections.sort(list, new Comparator<CustomClass>(){
public int compare(CustomClass a, CustomClass b)
{
// Logic to compare objects of class CustomClass which returns int as per contract.
}
});
But now in Java 8 we have a very neat alternative known as lambda expression, which can be used as:
list.sort((a, b) -> { a.isBiggerThan(b) } );
where isBiggerThan is a method in CustomClass. We can also use method references here:
list.sort(MyClass::isBiggerThan);
The open source safety-mirror project generalizes some of the above mentioned solutions into a library that adds functions, delegates and events to Java.
See the README, or this stackoverflow answer, for a cheat sheet of features.
As for functions, the library introduces a Fun interface, and some sub-interfaces that (together with generics) make up a fluent API for using methods as types.
Fun.With0Params<String> myFunctionField = " hello world "::trim;`
Fun.With2Params<Boolean, Object, Object> equals = Objects::equals;`
public void foo(Fun.With1ParamAndVoid<String> printer) throws Exception {
printer.invoke("hello world);
}
public void test(){
foo(System.out::println);
}
Notice:
that you must choose the sub-interface that matches the number of parameters in the signature you are targeting. Fx, if it has one parameter, choose Fun.With1Param.
that Generics are used to define A) the return type and B) the parameters of the signature.
Also, notice that the signature of the Method Reference passed to the call to the foo() method must match the the Fun defined by method Foo. If it do not, the compiler will emit an error.
Related
I've faced a strange(for me) behavior when I was trying to use function composition with two void methods. I've written a simple example to illustrate the problem :
public class Startup {
public static void main(String[] args) {
List<Foo> foos = new ArrayList<>();
// 1) Does not compile
foos.forEach(Startup::doSomething1.andThen(Startup::doSomething2));
Consumer<Foo> doSomething1 = Startup::doSomething1;
Consumer<Foo> doSomething2 = Startup::doSomething2;
// 2) Works as expected
foos.forEach(doSomething1.andThen(doSomething2));
}
public static void doSomething1(Foo foo) {
}
public static void doSomething2(Foo foo) {
}
public static class Foo {
}
}
When I try to compile the first solution it says "')' expected" before andThen call.
When I explicitly say this are Consumers the code is compiled and it works as expected.
Can anyone explain to my why this is happening and is there another way of doing function composition of void methods with Java 8?
Let's make this simpler:
private static boolean test(String input){
return input.equals("123");
}
Predicate<String> predicate = YourClass::test;
Function<String, Boolean> function = YourClass::test;
So a method reference is a poly expression (like generics for example), they depend on the context where they are used. So your Startup::doSomething method reference could be any #FunctionalInterface that would comply to the that method. It might look to you that it is a Consumer in this case, but it's a different story for the compiler.
This has to do with the way Java inferes, converts and detects types in lambdas. As mentioned in a comment above, the conversion to Consumer<Foo> has not taken place yet meaning that the compiler does not know that this is a Consumer so that you can chain an andThen() afterwards.
Explicitly casting this to a Consumer and using parentheses properly will let you achieve the desired effect:
List<Foo> foos = new ArrayList<>();
foos.forEach(((Consumer<Foo>) Null::doSomething).andThen(Null::doSomething2));
I guess if you fiddle around with it, you can achieve the same behavior using type witnesses but I am not 100% sure whether they can achieve the desired result.
First time I noticed this was using chained comparators which may exhibit the same behavior. Doing an online search about that will show you some more intricate details regarding how this works.
Just as the Consumer mentioned:
This is a functional interface and can therefore be used as the assignment target for a lambda expression or method reference.
And the functional interface gives us two methods:
void accept(T t)
default Consumer<T> andThen(Consumer<? super T> after)
As for andThen(...):
Returns a composed Consumer that performs, in sequence, this operation followed by the after operation.
The Functional Interface is the syntactic sugar that Java 8 provides that we can just pass in a lambda or method reference, and we can get more helpful/assistant features that we frequently need (default behaviors).
And here, we can combine several functions altogether easily using andThen
As for your case, you can just try something like this:
public class CastToFunctionalInterface {
public static void main(String... args) {
((Consumer<Integer>) CastToFunctionalInterface::consumeInteger)
.andThen(CastToFunctionalInterface::consumeAnotherInteger)
.accept(10);
}
private static void consumeInteger(Integer a) {
System.out.println("I'm an Integer: " + a);
}
private static void consumeAnotherInteger(Integer b) {
System.out.println("I'm another integer: " + b);
}
}
Output:
I'm an Integer: 10
I'm another integer: 10
I've faced a strange(for me) behavior when I was trying to use function composition with two void methods. I've written a simple example to illustrate the problem :
public class Startup {
public static void main(String[] args) {
List<Foo> foos = new ArrayList<>();
// 1) Does not compile
foos.forEach(Startup::doSomething1.andThen(Startup::doSomething2));
Consumer<Foo> doSomething1 = Startup::doSomething1;
Consumer<Foo> doSomething2 = Startup::doSomething2;
// 2) Works as expected
foos.forEach(doSomething1.andThen(doSomething2));
}
public static void doSomething1(Foo foo) {
}
public static void doSomething2(Foo foo) {
}
public static class Foo {
}
}
When I try to compile the first solution it says "')' expected" before andThen call.
When I explicitly say this are Consumers the code is compiled and it works as expected.
Can anyone explain to my why this is happening and is there another way of doing function composition of void methods with Java 8?
Let's make this simpler:
private static boolean test(String input){
return input.equals("123");
}
Predicate<String> predicate = YourClass::test;
Function<String, Boolean> function = YourClass::test;
So a method reference is a poly expression (like generics for example), they depend on the context where they are used. So your Startup::doSomething method reference could be any #FunctionalInterface that would comply to the that method. It might look to you that it is a Consumer in this case, but it's a different story for the compiler.
This has to do with the way Java inferes, converts and detects types in lambdas. As mentioned in a comment above, the conversion to Consumer<Foo> has not taken place yet meaning that the compiler does not know that this is a Consumer so that you can chain an andThen() afterwards.
Explicitly casting this to a Consumer and using parentheses properly will let you achieve the desired effect:
List<Foo> foos = new ArrayList<>();
foos.forEach(((Consumer<Foo>) Null::doSomething).andThen(Null::doSomething2));
I guess if you fiddle around with it, you can achieve the same behavior using type witnesses but I am not 100% sure whether they can achieve the desired result.
First time I noticed this was using chained comparators which may exhibit the same behavior. Doing an online search about that will show you some more intricate details regarding how this works.
Just as the Consumer mentioned:
This is a functional interface and can therefore be used as the assignment target for a lambda expression or method reference.
And the functional interface gives us two methods:
void accept(T t)
default Consumer<T> andThen(Consumer<? super T> after)
As for andThen(...):
Returns a composed Consumer that performs, in sequence, this operation followed by the after operation.
The Functional Interface is the syntactic sugar that Java 8 provides that we can just pass in a lambda or method reference, and we can get more helpful/assistant features that we frequently need (default behaviors).
And here, we can combine several functions altogether easily using andThen
As for your case, you can just try something like this:
public class CastToFunctionalInterface {
public static void main(String... args) {
((Consumer<Integer>) CastToFunctionalInterface::consumeInteger)
.andThen(CastToFunctionalInterface::consumeAnotherInteger)
.accept(10);
}
private static void consumeInteger(Integer a) {
System.out.println("I'm an Integer: " + a);
}
private static void consumeAnotherInteger(Integer b) {
System.out.println("I'm another integer: " + b);
}
}
Output:
I'm an Integer: 10
I'm another integer: 10
Can I require classes implementing an interface to have a certain static field or method and access/invoke that field or method through a generic type argument?
I have an interface, Arithmetical<T>, which specifies several functions like T plus(T o) and T times(T o). I have as well a Vector<N extends Arithmetical<N>> class, which is intended for vectors (of variable dimension) with components of type N. I ran into an issue, however, when trying to implement the dot product.
I want to implement the method N dot(Vector<N> o). For this, I plan to start with whatever N's zero is and iterate through both Vector<N>s' List<N>s, adding the product of each pair of elements to my total. Is there a way to specify in Arithmetical<T> that all implementing classes must have a static (and preferably final) field ZERO and start dot(Vector<N> o)'s body with something along the lines of N sum = N.ZERO;?
If not, what other approaches might there be to this problem? I want to allow 0-dimensional vectors, so I can't just begin by multiplying the vectors' first components. Is there a way to instantiate an object of a generic type, so I can merely specify a T zero() method in Arithmetical<T>?
I have a reason for not using Java's numerical types—I want to have vectors with complex components.
Here's Arithmetical:
public interface Arithmetical<T> {
public T plus(T o);
public T minus(T o);
public T negate();
public T times(T o);
public T over(T o);
public T inverse();
// Can I put a line here that requires class Complex (below) to define ZERO?
}
Vector:
public class Vector<N extends Arithmetical<N>> {
private List<N> components;
public Vector<N>(List<N> cs) {
this.components = new ArrayList<N>(cs);
}
public N dot(Vector<N> o) {
// Here's where I need help.
}
}
And Complex:
public class Complex implements Arithmetical<Complex> {
public static final Complex ZERO = new Complex(0, 0); // Can I access this value through N if <N extends Arithmetical<N>>?
private double real;
private double imag;
public Complex(double r, double i) {
this.real = r;
this.imag = i;
}
/* Implementation of Arithmetical<Complex> (and some more stuff) not shown... */
}
I'm quite new to Java (and programming in general); I will likely not understand complex (ha) explanations and workarounds.
Thanks!
(Python is a suggested tag... Huh.)
You need a "zero" for every possible implementation type. A constant in the interface won't do, because a constant cannot be overridden and must remain the same.
The solution is to add a new method to your Arithmetical interface:
public T zero();
Each implementation is forced to implement this and return its own version of zero. In this case, you're using it as a starting point for adding; it's the additive identity.
The Complex class implementation would look like this.
#Override
public Complex zero() {
return ZERO;
}
If your instances are mutable, then don't use a constant; just return new Complex(0, 0).
Another idea is to borrow from what Streams do when reduce-ing items and combining them to one single item -- take an identity value that represents the initial state, i.e. no items collected yet -- zero.
public N dot(Vector<N> o, N identity) {
N dotProduct = identity;
// Perform operations on each item in your collection
// to accumulate and return a dot product.
}
The caller will have to supply the identity value.
Complex dotProduct = vectorOfComplex.dotProduct(otherVector, new Complex(0, 0));
Can I put a line here that requires class Complex (below) to define ZERO?
No. The best you can do is to define an interface, for example:
interface ZeroProvider<A extends Arithmetical<A>> {
A zero();
}
and then supply a compatible instance of that where you need to provide a zero, for example:
class ComplexZeroProvider implements ZeroProvider<Complex> {
public Complex zero() { return new Complex(0, 0); }
}
There's something you can do sometimes using reflection in situations like this. If you put the following method in the Vector class, it will invoke a static method N.zero() (with caveats, below):
protected N zero() {
try {
Type s = getClass().getGenericSuperclass();
#SuppressWarnings("unchecked")
Class<N> n = (Class<N>) ((ParameterizedType) s).getActualTypeArguments()[0];
Method zero = n.getMethod("zero");
return n.cast(zero.invoke(null));
} catch (RuntimeException | ReflectiveOperationException x) {
// probably better to make a custom exception type
throw new IllegalArgumentException("illegal type argument", x);
}
}
However, it's important to understand what this is actually doing. This is getting the type argument from the class file of the direct superclass of this. In other words, there must actually be a superclass of this with an actual type argument (which is a class).
The usual idiom then is that you'd create all of your vectors like this:
new Vector<Complex>() {}
instead of this:
new Vector<Complex>()
Or you'd declare subclasses like this:
public class Vector<N> {
// ...
public static class OfComplex extends Vector<Complex> {
}
}
Since you need an actual superclass with a type argument which is a class, instantiations like in the following examples will fail:
new Vector<Complex>()
new Vector() // never use this anyway
new Vector() {} // never use this anyway
// also, you can't do stuff like this:
public Vector<T> copy() {
return new Vector<T>(this) {};
}
In your case I think the suggestions in the other answers are better, but I wanted to post this answer along with the proper explanation and caveats which are sometimes not included. There are cases where this technique is actually good, mainly when you have pretty tight restrictions on how the class in question is extended. Guava TypeToken will also do some of the reflection for you.
Also, this is the best Java can do at doing exactly what you're asking for (at the moment), so it's worthwhile to point out just as a comparison.
I had a question about reusability of lambda expression without code duplication. For example if I have a helper method I can easily code it as a static method and can refer to it from other classes without code duplication. How would this work in lambda expression ?
Example: I have the following static method written
public class MyUtil {
public static int doubleMe(int x) {
return x * 2;
}
}
I can reuse the same method without code duplication in multiple places across the project
public class A {
public void someOtherCalculation() {
MyUtil.doubleMe(5);
}
}
public class B {
public void myCalculation() {
MyUtil.doubleMe(3);
}
}
How would it work when it comes to a lambda function, write the function once and use the same at multiple class.
Function<Integer, Integer> doubleFunction = x -> x * 2;
In my example, where would I write the above lambda function and how would I reuse the same in class A and B ?
Where would I write the above lambda function
Since your function does not reference any fields, it is appropriate to put it in a static final field:
class Utility {
public static final Function<Integer,Integer> doubleFunction = x -> x * 2;
}
how would I reuse the same in class A and B?
You would refer to it as Utility.doubleFunction, and pass it in the context where it is required:
callMethodWithLambda(Utility.doubleFunction);
Note that method references let you define a function, and use it as if it were lambda:
class Utility {
public static Integer doubleFunction(Integer x) {
return x*2;
}
}
...
callMethodWithLambda(Utility::doubleFunction);
This approach is very flexible, because it lets you reuse the same code in multiple contexts as you find appropriate.
Really, anonymous functions are for cases where code reuse isn't necessary.
Dumb example, but say you're using map to add two to every number in a list. If this is a common action that you may need all over the place, a static function that adds two to a number makes more sense than writing the same lambda everywhere.
If, however you have a single function that adds two to a list, it makes more sense to define the "add two" function locally as a lambda so you dont plug up your class with code that isn't needed anywhere else.
When writing Clojure, which makes extensive use of higher-order functions, it's pretty common for me to create local anonymous functions that tidy up the code in the "full" function that I'm writing. The vast majority of these anonymous functions would be non-sensical in the "global" scope (or class-scope); especially since they usually have closures over local variables, so they couldn't be global anyways.
With lambda expressions, you don't need to worry about reusability (in fact, most of the lambdas are not being re-used at all). If you want a Function pointer to point to this method the you can declare one like below:
Function<Integer, Integer> doubleFunction = MyUtil::doubleMe;
And pass it to any method or stream to apply/map, e.g.:
public static void consume(Function<Integer, Integer> consumer, int value){
System.out.println(consumer.apply(value));
}
public static void main(String[] args) throws Exception{
Function<Integer, Integer> doubleFunction = MyUtil::doubleMe;
consume(doubleFunction, 5);
}
Different from other answers. I'd like to answer your question in TDD way.
IF your doubleMe is so simple as you have wrote, that is clrealy you should stop abusing method expression reference and just call it directly as a common method invocation.
IF your doubleMe is so complicated that you want to test doubleMe independent , you need to make implicit dependencies explicity by dependency injection to testing whether they can working together by their cummunication protocols. But java can't refer a method dierctly except you using reflection api Method/using a anonymous class that implements SAM interface which delegates request to a method before in jdk-8. What the happy thing is you can refer a method expression reference to a functional interface in jdk-8. so you can make implicit dependencies explicit by using functional interface, then I would like write some communication protocol test as below:
#Test
void applyingMultiplicationWhenCalculating???(){
IntUnaryOperator multiplication = mock(IntUnaryOperator.class);
B it = new B(multiplication);
it.myCalculation();
verify(multiplication).applyAsInt(3);
}
AND then your classes like as B applied dependency injection is more like as below:
public class B {
IntUnaryOperator multiplication;
public B(IntUnaryOperator multiplication){
this.multiplication = multiplication;
}
public void myCalculation() {
multiplication.applyAsInt(3);
}
}
THEN you can reuse a method by refer a method expression reference to a functional interface as below:
A a = new A(MyUtil::doubleMe);
B b = new B(MyUtil::doubleMe);
You can do something like below.
class Fn {
public static final Function<Integer, Integer> X2TIMES = x -> x *2;
}
class Test {
public static void main (String[] args) {
System.out.println(Fn.X2TIMES.apply(5));
}
}
I have recently started reading about java 8 features and i am confused with what seems like a very basic thing. How to organize code in 'Functional style' ?
Whatever i do, it looks very object oriented to me.
Best to explain what i ask with an example.
#FunctionalInterface
public interface SubstringOperator {
String splitAtLastOccurence(String plainText, String delimiter);
}
Let's say that in certain class i always need exactly one specific implementation of the SubstringOperator interface. I could provide implementation in the constructor like below:
public class SomeClass {
private SubstringOperator substringOperator;
public SomeClass() {
substringOperator = (s, d) -> { return s.substring(s.lastIndexOf(d)+1);};
}
}
I could now use this implementation in any method within SomeClass like this:
//...
String valueAfterSplit = substringOperator.splitAtLastOccurence(plainText, "=");
If i now wish to add another class which reuses that specific SubstringOperator implementation, should i create another class which exposes the implementation via getters?
Am i missing something obvious, or:
functions must be contained in classes in order to reuse them ?
How is that any different than object oriented paradigm ?
Put aside Stream API and other thingies, i would like to get basic understanding about code organization in java 8 for Functional style programming.
Usually it's better to reuse existing functional interfaces instead of creating new ones. In your case the BinaryOperator<String> is what you need. And it's better to name the variables by their meaning, not by their type. Thus you may have:
public class SomeClass {
private BinaryOperator<String> splitAtLastOccurence =
(s, d) -> s.substring(s.lastIndexOf(d)+1);
}
Note that you can simplify single-statement lambda removing the return keyword and curly brackets. It can be applied like this:
String valueAfterSplit = splitAtLastOccurence.apply(plainText, "=");
Usually if your class uses the same function always, you don't need to store it in the variable. Use plain old method instead:
protected static String splitAtLastOccurence(String s, String d) {
return s.substring(s.lastIndexOf(d)+1);
}
And just call it:
String valueAfterSplit = splitAtLastOccurence(plainText, "=");
Functions are good when another class or method is parameterized by function, so it can be used with different functions. For example, you are writing some generic code which can process list of strings with additional other string:
void processList(List<String> list, String other, BinaryOperator<String> op) {
for(int i=0; i<list.size(); i++) {
list.set(i, op.apply(list.get(i), other));
}
}
Or more in java-8 style:
void processList(List<String> list, String other, BinaryOperator<String> op) {
list.replaceAll(s -> op.apply(s, other));
}
In this way you can use this method with different functions. If you already have splitAtLastOccurence static method defined as above, you can reuse it using a method reference:
processList(myList, "=", MyClass::splitAtLastOccurence);