Maps : In Java Maps can I assign a function to the value in the pair?

Question

I have a set of string which I will be using as the Keys and for a particular string I want a function to be called. So is it possible to assign a function to the value in the pair?

exampleMap.get("SOME_STRING"); // should call a function abc();

The only way this is possible is implementing your own map. The standard out of box Map implementation in java does not allow anything like this. — Santosh, Jul 12 '13 at 07:50
No and yes, but not like you are trying. You'd have to use reflection and around about then you should be asking what has happened to your design — MadProgrammer, Jul 12 '13 at 07:51
Why not use a strategy pattern and store it as the value in the hashmap. — Narendra Pathai, Jul 12 '13 at 07:53

score 7 · Accepted Answer · answered Jul 12 '13 at 07:53

7

Encapsulate your function in a Java interface:

public interface Task {
    void doSomething();
}

and then pouplate the map with instances of this interface:

map.put("someString", new Task() {
    @Override
    public void doSomething() {
        System.out.println("foo");
    }
});
map.put("someOtherString", new Task() {
    @Override
    public void doSomething() {
        System.out.println("bar");
    }
});

Then to call the function (task) associated with a given string s:

map.get(s).doSomething();

answered Jul 12 '13 at 07:53

JB Nizet

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you beat me to it by 19 seconds! – vidit Jul 12 '13 at 07:53
But that only works if you can also control the code that uses the Map (because that has to call `doSomething` now, which it did not do before). – Thilo Jul 12 '13 at 07:56
Maybe I misunderstood the question. I answered to "is it possible to assign a function to the value in the pair". If the OP wants some function to be called each time `get()` is called, then it wouldn't be a Map anymore. – JB Nizet Jul 12 '13 at 08:35
How do I do this with a class that already exists? i don't want to make a new one, just execute an existing method. – markthegrea Mar 13 '19 at 14:46

score 3 · Answer 2 · answered Jul 12 '13 at 07:49

3

No, java does not support any method objects yet. You could use reflection or try to replace your strings with enum constants and call the method on them.

answered Jul 12 '13 at 07:49

morpheus05

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score 2 · Answer 3 · answered Jul 12 '13 at 07:53

Java doesn't support it directly, but you can do it indirectly using interfaces.

public interface Funct {
  public void apply();
}

and then when you want to use it as value, do something like..

Map<String, Funct> aMap = new HashMap<String, Funct>();
aMap.put("foo", new Funct() { public void apply() { System.out.println("bar"); } });

score 1 · Answer 4 · answered Jul 12 '13 at 07:55

1

If you want this to be transparent to the user of the Map, you have to implement your own Map.

answered Jul 12 '13 at 07:55

Thilo

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score 1 · Answer 5 · edited Apr 05 '18 at 21:10

Yes. Although, it's the java version of a "function": an anonymous class.

Best illustrated with an example:

interface MyFunction {
    public int calculate(int a, int b);
}

Map<String, MyFunction> map = new HashMap<String, MyFunction>();

map.put("add", new MyFunction() {
    public int calculate(int a, int b) {
        return a + b;
    });

map.put("multiply", new MyFunction() {
    public int calculate(int a, int b) {
        return a * b;
    });

score 1 · Answer 6 · edited Apr 05 '18 at 19:34

You may do it by using reflection. Consider the following code.

Map<String, Method> map = new HashMap<String, Method>();
SomeClass obj = new Someclass();
Method method = null;
try {
    method = obj.getClass().getMethod("someMethod", param1.class, param2.class, ...);
} catch (SecurityException e) {
} catch (NoSuchMethodException e) {
}   
map.put("someMethod", method);

try {
    (map.get("someMethod")).invoke(obj, arg1, arg2, ...);
} catch (IllegalArgumentException e) {
} catch (IllegalAccessException e) {
} catch (InvocationTargetException e) {
}

The method to be called inside SomeClass

public class SomeClass    {
// Other class contents
    public void someMethod()    {
    // Target method contents
    }
}

A sample code using the above logic:

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.HashMap;
import java.util.Map;

public class Temp3 {
    public static void main(String[] args) {
        Map<String, Method> map = new HashMap<String, Method>();
        Temp3 obj = new Temp3();
        Method method = null;
        try {
            method = obj.getClass().getMethod("someMethod");
        } catch (SecurityException e) {
        } catch (NoSuchMethodException e) {
        }

        map.put("someMethod", method);

        try {
            (map.get("someMethod")).invoke(obj);
        } catch (IllegalArgumentException e) {
        } catch (IllegalAccessException e) {
        } catch (InvocationTargetException e) {
        }
    }

    public void someMethod()    {
        System.out.println("**************************************");
    } 
}

score 1 · Answer 7 · answered Jul 12 '13 at 08:11

If you want that behavior you should define your own class.

The class should implement, at least, the following interface:

interface ApplicativeMap<K,V>{
  void put(K key, Function<V> producer);
  V get(K key);

where Function<V> should be an interface exposing the an V apply() method.

Your implementation should look something similar to this:

class ApplicativeMapImplementation implements ApplicativeMap{ private Map> functions;

 public void put(K key, Function<V> producer){
   functions.put(key, producer);
 }

 public V get(K key){
   if(functions.containsKey(key)){
     return functions.get(key).apply();
   } else{
     throw new NoSuchElementException();
   }
 }

Maps : In Java Maps can I assign a function to the value in the pair?

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