Array of function pointers in Java [duplicate]

This question already has answers here: How to call a method stored in a HashMap? (Java) [duplicate] 开发者_JAVA百科 (3 answers) Closed 8 years ago.

I have read this question and I'm still not sure whether it is possible to keep pointers to methods in an array in Java. If anyone knows if this is possible (or not), it would be a real help. I'm trying to find an elegant solution of keeping a list of Strings and associated functions without writing a mess of hundreds of if statements.

Cheers

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Java doesn't have a function pointer per se (or "delegate" in C# parlance). This sort of thing tends to be done with anonymous subclasses.

public interface Worker {
  void work();
}

class A {
  void foo() { System.out.println("A"); }
}

class B {
  void bar() { System.out.println("B"); }
}

A a = new A();
B b = new B();

Worker[] workers = new Worker[] {
  new Worker() { public void work() { a.foo(); } },
  new Worker() { public void work() { b.bar(); } }
};

for (Worker worker : workers) {
  worker.work();
}

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You can achieve the same result with the functor pattern. For instance, having an abstract class:

abstract class Functor
{
  public abstract void execute();
}

Your "functions" would be in fact the execute method in the derived classes. Then you create an array of functors and populate it with the apropriated derived classes:

class DoSomething extends Functor
{
  public void execute()
  {
    System.out.println("blah blah blah");
  }
}

Functor [] myArray = new Functor[10];
myArray[5] = new DoSomething();

And then you can invoke:

myArray[5].execute();

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It is possible, you can use an array of Method. Grab them using the Reflection API (edit: they're not functions since they're not standalone and have to be associated with a class instance, but they'd do the job -- just don't expect something like closures)

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Java does not have pointers (only references), nor does it have functions (only methods), so it's doubly impossible for it to have pointers to functions. What you can do is define an interface with a single method in it, have your classes that offer such a method declare they implement said interface, and make a vector with references to such an interface, to be populated with references to the specific objects on which you want to call that method. The only constraint, of course, is that all the methods must have the same signature (number and type of arguments and returned values).

Otherwise, you can use reflection/introspection (e.g. the Method class), but that's not normally the simplest, most natural approach.

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I found the reflection approach the cleanest -- I added a twist to this solution since most production classes have nested classes and I didn't see any examples that demonstrates this (but I didn't look for very long either). My reason for using reflection is that my "updateUser()" method below had a bunch of redundant code and just one line that changed (for every field in the user object) in the middle that updated the user object:

NameDTO.java

public class NameDTO {

    String first, last;

    public String getFirst() {
        return first;
    }

    public void setFirst(String first) {
        this.first = first;
    }

    public String getLast() {
        return last;
    }

    public void setLast(String last) {
        this.last = last;
    }   
}

UserDTO.java

public class UserDTO {

    private NameDTO name;
    private Boolean honest;

    public UserDTO() {
        name = new NameDTO();
        honest = new Boolean(false);
    }

    public NameDTO getName() {
        return name;
    }

    public void setName(NameDTO name) {
        this.name = name;
    }

    public Boolean getHonest() {
        return honest;
    }

    public void setHonest(Boolean honest) {
        this.honest = honest;
    }
}

Example.java

import java.lang.reflect.Method;

public class Example {

    public Example ()  {
        UserDTO dto = new UserDTO();

        try {
            Method m1 = dto.getClass().getMethod("getName", null);
            NameDTO nameDTO = (NameDTO) m1.invoke(dto, null);

            Method m2 = nameDTO.getClass().getMethod("setFirst", String.class);
            updateUser(m2, nameDTO, "Abe");

            m2 = nameDTO.getClass().getMethod("setLast", String.class);
            updateUser(m2, nameDTO, "Lincoln");

            m1 = dto.getClass().getMethod("setHonest", Boolean.class);
            updateUser(m1, dto, Boolean.TRUE);
            System.out.println (dto.getName().getFirst() + " " + dto.getName().getLast() + ": honest=" + dto.getHonest().toString());

        } catch (Exception e) {
            e.printStackTrace();
        } 
    }

    public  void updateUser(Method m,  Object o, Object v) {
        //  lots of code here
        try {
            m.invoke(o, v);
        } catch (Exception e) {
            e.printStackTrace();
        } 
        // lots of code here -- including a retry loop to make sure the
        // record hadn't been written since my last read
    }

    public static void main(String[] args) {
        Example mp = new Example();
    }
}

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You are right that there are no pointers in java because a reference variables are the same as the & syntax in C/C++ holding the reference to the object but no * because the JVM can reallocate the heap when necessary causing the pointer to be lost from the address which would cause a crash. But a method is just a function inside a class object and no more than that so you are wrong saying there are no functions, because a method is just a function encapsulated inside an object. As far as function pointers, the java team endorses the use of interfaces and nested classes which all fine and dandy, but being a C++/C# programmer who uses java from time to time, I use my Delegate class I made for java because I find it more convenient when I need to pass a function only having to declare the return type of the method delegate. It all depends on the programmer. I read the white pages on why delegates are not support but I disagree and prefer to think outside the box on that topic.