Shawn schrieb:

No, Ingo didn't forget the public. A interface only have public methods
so the public keyword may be omitted.

Bye
Michael

Hi,

I do not see any difference!

A class can implement an interface and therefore must implement the
method defined in this interface. So, the class is a subtype of the
interface (->inheritance). Now, if you want to access the method, you
only need a reference to some Object which has the type/subtype of the
interface, therefore, the interface is a place holder (if you want to
call it so)...

Ciao,
Ingo

Don't overlook the use of Interface in conjunction with other forms of
inheritance at the same time:

class hickey extends dohickey implements blah, Mapper {
...
}

Now a class that does a lot of things (like things you need for your
design) can also be used for a Mapper object.  This particular pattern
happens a lot in Swing, for example, especially for action listeners, so
be on the look out for it.

Also, there is another way of doing what you ask.  The Method object
corresponds much more closely to a C function pointer:

class myInvoker( Method func_ptr )
{
    funct_ptr.invoke();
}

static main (args[])
{
    Object o = new Integer();
    Class[] parameterTypes = new Class[] {String.class};
    Method m = = c.getMethod("concat", parameterTypes);

    myInvoker( m );
}

Except that there's a lot more needed than that (I omitted big try ...
catch blocks, for starters).  However, the Method object can be used to
sling methods around something like C functions.

What this is useful for is when you can't use Interfaces.  For example,
you are passed an Object, and you'd like to be able to access methods by
name.  A method called "getName" which returns a string, and a method
called "setName(String)" could be used to set a property called Name in
the Object, and there's no need for any Interface or inheritance at all.
 This provides a lot of flexibility for your programs.

You can also look at an Object, and find what Interfaces it implements,
and even pick and choose which Interfaces you want.  For example, you
might want Mapper2_0, but you'll settle for Mapper1_0.  Again, lots of
flexibility for your programs.

The whole subject is called Reflection, for more go to:

Finding Interface:
http://java.sun.com/docs/books/tutorial/reflect/class/getInterfaces.html

Invoking methods, the real example:
http://java.sun.com/docs/books/tutorial/reflect/object/invoke.html

More reflection, the whole she-bang:
http://java.sun.com/docs/books/tutorial/reflect/index.html

Sorry. I try to do it myself. If anything wrong, please point it out for
me. Thank  you.

interface Mapper {
  void map(int[] d);
}

class Test {

 void doSomethingToArray(Mapper m, int[] aArray) {
    m.map(aArray);
  } //end of method doSomethingToArray

Mapper squareArray = new Mapper()
{
    void map(int[] a)
    {
        for (int i=0; i<a.length; i++)
        {
            a[i] *= a[i];
        }
    }
}

Mapper printArray = new Mapper()
{
    void map(int[] a)
    {
        for (int i=0; i<a.length; i++)
        {
            System.out.println(a[i]);
        }
    }
}

 void test() {
    int[] a={2, 4, 6, 8};
    doSomethingToArray(squareArray, a);
    doSomethingToArray(printArray, a);
 }

} //end of class Test

I tried the following file Test.java. It didn't work.

/*
For testing passing a method as a method parameter
*/

interface Mapper {
  void map(int[] d);
}

class Test {

        void doSomethingToArray(Mapper m, int[] aArray) {
                m.map(aArray);
        } //end of method doSomethingToArray

        Mapper squareArray = new Mapper() {
                void map(int[] a)
                {
                        for (int i=0; i<a.length; i++)
                        {
                                a[i] *= a[i];
                        }
                }
        }

        Mapper printArray = new Mapper() {
                void map(int[] a)
                {
                        for (int i=0; i<a.length; i++)
                        {
                                System.out.println(a[i]);
                        }
                }
        }

        static int[] a={2, 4, 6, 8};

        public static void main(String[] args)
        {
                doSomethingToArray(squareArray, a);
                doSomethingToArray(printArray, a);
        }

} //end of class Test

Below is the error message. I cannot solve it. Back to the interface
issue, I am not allowed to instantiate an interface even I implement its
method?

Thank you very much for your help.

----------
1. ERROR in Test.java (at line 25)
        Mapper printArray = new Mapper() {
        ^^^^^^
Syntax error on token "Mapper", ";", "," expected
----------
2. ERROR in Test.java (at line 39)
        doSomethingToArray(squareArray, a);
                           ^^^^^^^^^^^
Cannot make a static reference to the non-static field squareArray
----------
3. ERROR in Test.java (at line 40)
        doSomethingToArray(printArray, a);
                           ^^^^^^^^^^
Cannot make a static reference to the non-static field printArray
----------
3 problems (3 errors)