Related
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 had some confusion about inner classes and lambda expression, and I tried to ask a question about that, but then another doubt arose, and It's probable better posting another question than commenting the previous one.
Straight to the point: I know (thank you Jon) that something like this won't compile
public class Main {
public static void main(String[] args) {
One one = new One();
F f = new F(){ //1
public void foo(){one.bar();} //compilation error
};
one = new One();
}
}
class One { void bar() {} }
interface F { void foo(); }
due to how Java manages closures, because one is not [effectively] final and so on.
But then, how come is this allowed?
public class Main {
public static void main(String[] args) {
One one = new One();
F f = one::bar; //2
one = new One();
}
}
class One { void bar() {} }
interface F { void foo(); }
Is not //2 equivalent to //1? Am I not, in the second case, facing the risks of "working with an out-of-date variable"?
I mean, in the latter case, after one = new One(); is executed f still have an out of date copy of one (i.e. references the old object). Isn't this the kind of ambiguity we're trying to avoid?
A method reference is not a lambda expression, although they can be used in the same way. I think that is what is causing the confusion. Below is a simplification of how Java works, it is not how it really works, but it is close enough.
Say we have a lambda expression:
Runnable f = () -> one.bar();
This is the equivalent of an anonymous class that implements Runnable:
Runnable f = new Runnable() {
public void run() {
one.bar();
}
}
Here the same rules apply as for an anonymous class (or method local class). This means that one needs to effectively final for it to work.
On the other hand the method handle:
Runnable f = one::bar;
Is more like:
Runnable f = new MethodHandle(one, one.getClass().getMethod("bar"));
With MethodHandle being:
public class MethodHandle implements Runnable {
private final Object object;
private final Method method;
public MethodHandle(Object object, java.lang.reflect.Method method) {
this.object = Object;
this.method = method;
}
#Override
public void run() {
method.invoke(object);
}
}
In this case, the object assigned to one is assigned as part of the method handle created, so one itself doesn't need to be effectively final for this to work.
Your second example is simply not a lambda expression. It's a method reference. In this particular case, it chooses a method from a particular object, which is currently referenced by the variable one. But the reference is to the object, not to the variable one.
This is the same as the classical Java case:
One one = new One();
One two = one;
one = new One();
two.bar();
So what if one changed? two references the object that one used to be, and can access its method.
Your first example, on the other hand, is an anonymous class, which is a classical Java structure that can refer to local variables around it. The code refers to the actual variable one, not the object to which it refers. This is restricted for the reasons that Jon mentioned in the answer you referred to. Note that the change in Java 8 is merely that the variable has to be effectively final. That is, it still can't be changed after initialization. The compiler simply became sophisticated enough to determine which cases will not be confusing even when the final modifier is not explicitly used.
The consensus appears to be that this is because when you do it using an anonymous class, one refers to a variable, whereas when you do it using a method reference, the value of one is captured when the method handle is created. In fact, I think that in both cases one is a value rather than a variable. Let's consider anonymous classes, lambda expressions and method references in a bit more detail.
Anonymous classes
Consider the following example:
static Supplier<String> getStringSupplier() {
final Object o = new Object();
return new Supplier<String>() {
#Override
public String get() {
return o.toString();
}
};
}
public static void main(String[] args) {
Supplier<String> supplier = getStringSupplier();
System.out.println(supplier.get()); // Use o after the getStringSupplier method returned.
}
In this example, we are calling toString on o after the method getStringSupplier has returned, so when it appears in the get method, o cannot refer to a local variable of the getStringSupplier method. In fact it is essentially equivalent to this:
static Supplier<String> getStringSupplier() {
final Object o = new Object();
return new StringSupplier(o);
}
private static class StringSupplier implements Supplier<String> {
private final Object o;
StringSupplier(Object o) {
this.o = o;
}
#Override
public String get() {
return o.toString();
}
}
Anonymous classes make it look as if you are using local variables, when in fact the values of these variables are captured.
In contrast to this, if a method of an anonymous class references the fields of the enclosing instance, the values of these fields are not captured, and the instance of the anonymous class does not hold references to them; instead the anonymous class holds a reference to the enclosing instance and can access its fields (either directly or via synthetic accessors, depending on the visibility). One advantage is that an extra reference to just one object, rather than several, is required.
Lambda expressions
Lambda expressions also close over values, not variables. The reason given by Brian Goetz here is that
idioms like this:
int sum = 0;
list.forEach(e -> { sum += e.size(); }); // ERROR
are fundamentally serial; it is quite difficult to write lambda bodies
like this that do not have race conditions. Unless we are willing to
enforce -- preferably at compile time -- that such a function cannot
escape its capturing thread, this feature may well cause more trouble
than it solves.
Method references
The fact that method references capture the value of the variable when the method handle is created is easy to check.
For example, the following code prints "a" twice:
String s = "a";
Supplier<String> supplier = s::toString;
System.out.println(supplier.get());
s = "b";
System.out.println(supplier.get());
Summary
So in summary, lambda expressions and method references close over values, not variables. Anonymous classes also close over values in the case of local variables. In the case of fields, the situation is more complicated, but the behaviour is essentially the same as capturing the values because the fields must be effectively final.
In view of this, the question is, why do the rules that apply to anonymous classes and lambda expressions not apply to method references, i.e. why are you allowed to write o::toString when o is not effectively final? I do not know the answer to that, but it does seem to me to be an inconsistency. I guess it's because you can't do as much harm with a method reference; examples like the one quoted above for lambda expressions do not apply.
No. In your first example you define the implementation of F inline and try to access the instance variable one.
In the second example you basically define your lambda expression to be the call of bar() on the object one.
Now this might be a bit confusing. The benefit of this notation is that you can define a method (most of the time it is a static method or in a static context) once and then reference the same method from various lambda expressions:
msg -> System.out::println(msg);
How can I make scope of a String variable(In Java) global.So that it is accessed from another function
Eg
//String b="null"; I don't want to do this... because if i do this, fun2 will print Null
public int func1(String s)
{
String b=s;
}
public int func2(String q)
{
System.out.println(b);//b should be accessed here and should print value of s
}
Any Help... Thanks
One of the fundamental concepts in OOP is the concept of scope: in almost all cases it is wise to reduce the scope of a variable (i.e. where it is visible from) to its minimum viable range.
I'm going to assume you absolutely require the use of that variable in both functions. Therefore, the minimum viable scope in this case would cover both functions.
public class YourClass
{
private String yourStringVar;
public int pleaseGiveYourFunctionProperNames(String s){
this.yourStringVar = s;
}
public void thisFunctionPrintsValueOfMyStringVar(){
System.out.println(yourStringVar);
}
}
Depending on the situation, you must assess the required scope of a variable, and you must understand the implications of increasing the scope (more access = potentially more dependencies = harder to keep track).
As an example, let's say you absolutely needed it to be a GLOBAL variable (as you call it in your question). A variable with Global scope can be accessed by anything within the application. This is exceptionally dangerous, which I will demonstrate.
To make a variable with global scope (there are no such things as global variables, exactly, in Java), you create a class with a static variable.
public class GlobalVariablesExample
{
public static string GlobalVariable;
}
If I were to alter the original code, it would now look like this.
public class YourClass
{
public int pleaseGiveYourFunctionProperNames(String s){
GlobalVariablesExample.GlobalVariable = s;
}
public void thisFunctionPrintsValueOfMyStringVar(){
System.out.println(GlobalVariablesExample.GlobalVariable);
}
}
This can be exceptionally powerful, and exceptionally dangerous as it can lead to weird behaviour that you do not expect, and you lose many of the abilities that object oriented programming gives you, so use it carefully.
public class YourApplication{
public static void main(String args[]){
YourClass instance1 = new YourClass();
YourClass instance2 = new YourClass();
instance1.pleaseGiveYourFunctionProperNames("Hello");
instance1.thisFunctionPrintsValueOfMyStringVar(); // This prints "Hello"
instance2.pleaseGiveYourFunctionProperNames("World");
instance2.thisFunctionPrintsValueOfMyStringVar(); // This prints "World"
instance1.thisFunctionPrintsValueOfMyStringVar(); // This prints "World, NOT Hello, as you'd expect"
}
}
Always assess the minimum viable scope for your variables. Do not make it more accessible than it needs to be.
Also, please don't name your variables a,b,c. And don't name your variables func1,func2. It doesn't make your application any slower, and it won't kill you to type in a few extra letters.
Hmm. You clearly need some lessons in object-oriented programming. In OO there is no "global" variable. But any variable defined as a member in a class (outside a method) is global within that class.
public class MyClass {
private String myVar; // this can be accessed everywhere in MyClass
public void func1(String s) {
myVar = s;
}
public void func2(String q) { // why is q needed here? It's not used
System.out.println(myVar);
}
}
So func2 will output the value of s ONLY IF you call func1 first.
final Myclass myClass = new MyClass();
myClass.func1("value");
myClass.func2("whatever"); // will output "value"
Also, why are the methods returning int in your example? They should be void.
The Java for C++ programmers tutorial says that (highlight is my own):
The keyword final is roughly
equivalent to const in C++
What does "roughly" mean in this context? Aren't they exactly the same?
What are the differences, if any?
In C++ marking a member function const means it may be called on const instances. Java does not have an equivalent to this. E.g.:
class Foo {
public:
void bar();
void foo() const;
};
void test(const Foo& i) {
i.foo(); //fine
i.bar(); //error
}
Values can be assigned, once, later in Java only e.g.:
public class Foo {
void bar() {
final int a;
a = 10;
}
}
is legal in Java, but not C++ whereas:
public class Foo {
void bar() {
final int a;
a = 10;
a = 11; // Not legal, even in Java: a has already been assigned a value.
}
}
In both Java and C++ member variables may be final/const respectively. These need to be given a value by the time an instance of the class is finished being constructed.
In Java they must be set before the constructor has finished, this can be achieved in one of two ways:
public class Foo {
private final int a;
private final int b = 11;
public Foo() {
a = 10;
}
}
In C++ you will need to use initialisation lists to give const members a value:
class Foo {
const int a;
public:
Foo() : a(10) {
// Assignment here with = would not be legal
}
};
In Java final can be used to mark things as non-overridable. C++ (pre-C++11) does not do this. E.g.:
public class Bar {
public final void foo() {
}
}
public class Error extends Bar {
// Error in java, can't override
public void foo() {
}
}
But in C++:
class Bar {
public:
virtual void foo() const {
}
};
class Error: public Bar {
public:
// Fine in C++
virtual void foo() const {
}
};
this is fine, because the semantics of marking a member function const are different. (You could also overload by only having the const on one of the member functions. (Note also that C++11 allows member functions to be marked final, see the C++11 update section)
C++11 update:
C++11 does in fact allow you to mark both classes and member functions as final, with identical semantics to the same feature in Java, for example in Java:
public class Bar {
public final void foo() {
}
}
public class Error extends Bar {
// Error in java, can't override
public void foo() {
}
}
Can now be exactly written in C++11 as:
class Bar {
public:
virtual void foo() final;
};
class Error : public Bar {
public:
virtual void foo() final;
};
I had to compile this example with a pre-release of G++ 4.7. Note that this does not replace const in this case, but rather augments it, providing the Java-like behaviour that wasn't seen with the closest equivalent C++ keyword. So if you wanted a member function to be both final and const you would do:
class Bar {
public:
virtual void foo() const final;
};
(The order of const and final here is required).
Previously there wasn't a direct equivalent of const member functions although making functions non-virtual would be a potential option albeit without causing an error at compile time.
Likewise the Java:
public final class Bar {
}
public class Error extends Bar {
}
becomes in C++11:
class Bar final {
};
class Error : public Bar {
};
(Previously private constructors was probably the closest you could get to this in C++)
Interestingly, in order to maintain backwards compatibility with pre-C++11 code final isn't a keyword in the usual way. (Take the trivial, legal C++98 example struct final; to see why making it a keyword would break code)
A const object can only call const methods, and is generally considered immutable.
const Person* person = myself;
person = otherPerson; //Valid... unless we declared it const Person* const!
person->setAge(20); //Invalid, assuming setAge isn't a const method (it shouldn't be)
A final object cannot be set to a new object, but it is not immutable - there is nothing stopping someone from calling any set methods.
final Person person = myself;
person = otherPerson; //Invalid
person.setAge(20); //Valid!
Java has no inherent way of declaring objects immutable; you need to design the class as immutable yourself.
When the variable is a primitive type, final/const work the same.
const int a = 10; //C++
final int a = 10; //Java
a = 11; //Invalid in both languages
In Java the final keyword can be used for four things:
on a class or method to seal it (no subclasses / overriding allowed)
on a member variable to declare that is it can be set exactly once (I think this is what you are talking about)
on a variable declared in a method, to make sure that it can be set exactly once
on a method parameter, to declare that it cannot be modified within the method
One important thing is:
A Java final member variable must be set exactly once! For example, in a constructor, field declaration, or intializer. (But you cannot set a final member variable in a method).
Another consequence of making a member variable final relates to the memory model, which is important if you work in a threaded environment.
Java final is equivalent to C++ const on primitive value types.
With Java reference types, the final keyword is equivalent to a const pointer... i.e.
//java
final int finalInt = 5;
final MyObject finalReference = new MyObject();
//C++
const int constInt = 5;
MyObject * const constPointer = new MyObject();
Java's final works only on primitive types and references, never on object instances themselves where the const keyword works on anything.
Compare const list<int> melist; with final List<Integer> melist; the first makes it impossible to modify the list, while the latter only stops you from assigning a new list to melist.
You have some great answers here already, but one point that seemed worth adding: const in C++ is commonly used to prevent other parts of the program changing the state of objects. As has been pointed out, final in java can't do this (except for primitives) - it just prevents the reference from being changed to a different object. But if you are using a Collection, you can prevent changes to your objects by using the static method
Collection.unmodifiableCollection( myCollection )
This returns a Collection reference that gives read-access to the elements, but throws an exception if modifications are attempted, making it a bit like const in C++
Aside from having certain and subtle multi-threading properties, variables declared final don't need to be initialized on declaration!
i.e. This is valid in Java:
// declare the variable
final int foo;
{
// do something...
// and then initialize the variable
foo = ...;
}
This would not be valid if written with C++'s const.
According to wikipedia:
In C++, a const field is not only protected from being reassigned, but there is the additional limitation that only const methods can be called on it and it can only be passed as the const argument of other methods.
Non-static inner classes can freely access any field of the enclosing class, final or not.
I am guessing it says "roughly" because the meaning of const in C++ gets complicated when you talk about pointers, i.e. constant pointers vs. pointers to constant objects. Since there are no "explicit" pointers in Java, final does not have these issues.
Let me explain what I understood with an example of switch/case statement.
The values in each case statement must be compile-time constant values of the same data type as the switch value.
declare something like below (either in your method as local instances, or in your class as static variable(add static to it then), or an instance variable.
final String color1 = "Red";
and
static final String color2 = "Green";
switch (myColor) { // myColor is of data type String
case color1:
//do something here with Red
break;
case color2:
//do something with Green
break;
}
This code will not compile, if color1 is a class/instance variable and not a local variable.
This will compile if color1 is defined as static final (then it becomes static final variable).
When it does not compile, you will get the following error
error: constant string expression required
keyword "const" mean that your variable is saved in ROM (with Microprocessor). in computer, your variable is saved in RAM area for Assembly code (read only RAM). it means that your variable is not in the writeable RAM include: static memory, stack memory and heap memory.
keyword "final" mean that your variable is saved in writeable RAM, but you notice to compiler that your variable is only change only one time.
//in java language you can use:
static final int i =10;
i =11; //error is showed here by compiler
//the same in C++ the same as follows
int i =10;
const int &iFinal = i;
iFinal = 11; //error is showed here by compiler the same as above
I think, "const" is bad in performance, so Java does not use it.
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.