Solved

C++ function pointers

Posted on 1999-01-22
6
16,411 Views
Last Modified: 2011-08-18
In C++ i have a pointer to a function. I can use this pointer to point to a number of functions. How do I implement function pointers in java?
0
Comment
Question by:orchi
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
6 Comments
 
LVL 1

Expert Comment

by:velev
ID: 1234495
there is no way to implement this in Java
0
 

Author Comment

by:orchi
ID: 1234496
how then, do i port c++ apps that use function pointers?
0
 
LVL 1

Expert Comment

by:velev
ID: 1234497
see classes & interfaces in java.lang.reflect
/Method, Member, Field/ and invoke() method of Method class
they may help you

0
PeopleSoft Has Never Been Easier

PeopleSoft Adoption Made Smooth & Simple!

On-The-Job Training Is made Intuitive & Easy With WalkMe's On-Screen Guidance Tool.  Claim Your Free WalkMe Account Now

 
LVL 16

Accepted Solution

by:
heyhey_ earned 50 total points
ID: 1234498
there is a standard way to implement such behaviour in Java.
because Java is strictly OO language, you'll need to define Interface that describes some methods
(analog to defining a function type).

I'm posting an small example
// C++
void qsort( void *base, size_t num, size_t width, int (__cdecl *compare )(const void *elem1, const void *elem2 ) );

// Java
interface SorterFunction
{
      public int sort(Object o1, Object o2);
}

public class qsort
{
      public static sort (Object[] objects, SorterFunction sorter)
      {
            // .... implementation
      }
}



now you can implement a StringSorter
class StringSorter implements SorterFunction
{
      public int sort(Object o1, Object o2)
      {
            /// you'll have to add a lot of more cheking here
            String st1 = (String) o1;
            String st2 = (String) o2;
            return st1.compare(st2);
      }
}


and now you can use it

String strings[] = {"zz", "aa", "bb", "cc"};
SorterFunction f = new StringSorter();
qsort.sort(srings, f);

I think that you'll get the idea.
if you have more questions - just ask.


best regards
  heyhey
0
 
LVL 8

Expert Comment

by:diakov
ID: 1234499
In general, look at velev's sggestion it is the right approach.

In Java you do not have pointers at all, but you have the introspection scheme with which you can find out about an object.

Let's suppose you have a reference to an object - r.

Class c = r.getClass() retrieves an instance to a object of type class that describes the class of the r reference.

Then you can enumerate all methods of this class by calls to c.getMethods() or just by a call to c.getMethods() if you know what you're looking for.

The Method m returned by these calls can be then called by m.invoke() with the appropriate params specified in an array.

This is the basic scenario.

Cheers,
  Nik
0
 
LVL 1

Expert Comment

by:haho
ID: 1234500
I agree with Nik.  A concrete example would be:
class A {
...
   public boolean aFunc() {
       ...
   }
}
class B {
...
   //pretendedFuncPointer is object of A that you will be using
   //to call its function.  Effectively, this works as
   //"function pointer"
   public void myFunc(A pretendedFuncPointer) {
       ...
       pretendedFuncPointer.aFunc();
       ...
   }
}

0

Featured Post

The Ultimate Checklist to Optimize Your Website

Websites are getting bigger and complicated by the day. Video, images, custom fonts are all great for showcasing your product/service. But the price to pay in terms of reduced page load times and ultimately, decreased sales, can lead to some difficult decisions about what to cut.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

INTRODUCTION Working with files is a moderately common task in Java.  For most projects hard coding the file names, using parameters in configuration files, or using command-line arguments is sufficient.   However, when your application has vi…
Java contains several comparison operators (e.g., <, <=, >, >=, ==, !=) that allow you to compare primitive values. However, these operators cannot be used to compare the contents of objects. Interface Comparable is used to allow objects of a cl…
Viewers will learn about basic arrays, how to declare them, and how to use them. Introduction and definition: Declare an array and cover the syntax of declaring them: Initialize every index in the created array: Example/Features of a basic arr…
This tutorial covers a step-by-step guide to install VisualVM launcher in eclipse.

707 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question