Java: Make parameter to a method another method

Question

I want to make a method that does a specific task, but then calls another method with the results of that task. Well easy enough, but the trick is that I want the method (the second one thats getting called by the first one) to be a parameter of the first method.

I probably explained that terribly, so here is what I think it might look like

public static void main(String[] args) {
    double[][] data = {
        {2,6},
        {-32,5}
    }

    loop(data,{System.out.println(row * col)});
}

public static void loop(double[][] data,somemethod(row,col)) {
    for (int row = 0;row < data.length;row++)
        for (int col = 0;col < data[0].length;col++)
            somemethod(row,col);
}

So the loop method does a task, and then runs the code that was passed as a parameter. Can this be done in java? I feel like I have seen it somewhere.

Is there no way whatsoever to do this for java 7? Java 7 is ideal for me — Hurricane Development
– Hurricane Development, Commented Mar 11, 2015 at 20:07
There is no callback in java. In java 7 you can do something "similar" with interface (or abstract class) . — Duffydake
– Duffydake, Commented Mar 11, 2015 at 20:10

Nathan Hughes · Accepted Answer · 2015-03-11 20:42:17Z

5

The pre-Java-8 way to do this was by creating an interface with the method you want called:

interface MyCallback {
    void someMethod(int row, int col);
}

You create an object with the implementation you want:

class MyCallbackImpl implements MyCallback {

    public void somemethod(int row, int col) {
        System.out.println(row * col);
    }
}

and have the method take a parameter implementing that interface:

public static void loop(double[][] data, MyCallback mycallback) {
    for (int row = 0;row < data.length;row++)
        for (int col = 0;col < data[0].length;col++)
            mycallback.somemethod(row,col);
}

If you'd rather not create a separate named class for this you have the alternative of creating an object using an anonymous inner class, calling your loop method like:

loop(data, new MyCallback() {
    public void somemethod(int row, int col) {
        System.out.println(row * col);
    }});

edited Mar 11, 2015 at 20:42

answered Mar 11, 2015 at 20:10

Nathan Hughes

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1 Comment

Timmos Over a year ago

Maybe you can include a lambda example as well?

muasif80 · Accepted Answer · 2015-03-11 20:06:28Z

0

In Java 8 the Lambda expressions make that possible. It was not possible before. This is what I know.

answered Mar 11, 2015 at 20:06

muasif80

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4 Comments

Neeraj Jain Over a year ago

Better to Post your answer with a Working Example

Hurricane Development Over a year ago

Yes, an example would be much appreciated!

Adam Over a year ago

It was technically possible via custom interface

muasif80 Over a year ago

Ok will add one soon.

Laerte · Accepted Answer · 2015-03-11 20:14:01Z

0

You can use Reflection to do this trick!

First, you find the method like this:

java.lang.reflect.Method method;
try {
  method = obj.getClass().getMethod(methodName, param1.class, param2.class, ..);
} catch (SecurityException e) {
  // ...
} catch (NoSuchMethodException e) {
  // ...
}

In your case, you will send methodName and the parameters of methodName as parameters of loop class. After finding the method, you execute it as:

try {
  method.invoke(param1, param1,...);
} catch (IllegalArgumentException e) {
} catch (IllegalAccessException e) {
} catch (InvocationTargetException e) {

Hope it helps...

answered Mar 11, 2015 at 20:14

Laerte

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