How to call a method stored in a HashMap? (Java) [

Question

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Function pointers/delegates in Java? (9 answers)

Closed 5 years ago.

I have a list of commands (i, h, t, etc) that the user will be entering on a command line/terminal Java program. I would like to store a hash of command/method pairs:

'h', showHelp()
't', teleport()

So that I can have code something like:

HashMap cmdList = new HashMap();

cmdList.put('h', showHelp());
if(!cmdList.containsKey('h'))
    System.out.print("No such command.")
else
   cmdList.getValue('h')   // This should run showHelp().

Is this possible? If not, what is an easy way to this?

Answer 1

If you are using JDK 7 you can now use methods by lambda expression just like .net.

If Not the best way is to make a Function Object:

public interface Action {    void performAction(); }

Hashmap<string,Action> cmdList;

if(!cmdList.containsKey('h'))
    System.out.print("No such command.") else    cmdList.getValue('h').performAction();

Answer 2

Though you could store methods through reflection, the usual way to do it is to use anonymous objects that wrap the function, i.e.

  interface IFooBar {
    void callMe();
  }


 'h', new IFooBar(){ void callMe() { showHelp(); } }
 't', new IFooBar(){ void callMe() { teleport(); } }

 HashTable<IFooBar> myHashTable;
 ...
 myHashTable.get('h').callMe();

Answer 3

With Java 8+ and Lambda expressions

With lambdas (available in Java 8+) we can do it as follows:

class Test {

    public static void main(String[] args) throws Exception {
        Map<Character, Runnable> commands = new HashMap<>();

        // Populate commands map
        commands.put('h', () -> System.out.println("Help"));
        commands.put('t', () -> System.out.println("Teleport"));

        // Invoke some command
        char cmd = 't';
        commands.get(cmd).run();   // Prints "Teleport"
    }
}

In this case I was lazy and reused the Runnable interface, but one could just as well use the Command-interface that I invented in the Java 7 version of the answer.

Also, there are alternatives to the () -> { ... } syntax. You could just as well have member functions for help and teleport and use YourClass::help resp. YourClass::teleport instead.

• A great Lambda cheat sheet over at Programming.Guide.

• Oracle tutorial here: The Java Tutorials™ – Lambda Expressions.

---

Java 7 and below

What you really want to do is to create an interface, named for instance Command (or reuse for instance Runnable), and let your map be of the type Map<Character, Command>. Like this:

import java.util.*;

interface Command {
    void runCommand();
}

public class Test {

    public static void main(String[] args) throws Exception {
        Map<Character, Command> methodMap = new HashMap<Character, Command>();

        methodMap.put('h', new Command() {
            public void runCommand() { System.out.println("help"); };
        });

        methodMap.put('t', new Command() {
            public void runCommand() { System.out.println("teleport"); };
        });

        char cmd = 'h';
        methodMap.get(cmd).runCommand();  // prints "Help"

        cmd = 't';
        methodMap.get(cmd).runCommand();  // prints "teleport"

    }
}

Reflection "hack"

With that said, you can actually do what you're asking for (using reflection and the Method class.)

import java.lang.reflect.*;
import java.util.*;

public class Test {

    public static void main(String[] args) throws Exception {
        Map<Character, Method> methodMap = new HashMap<Character, Method>();

        methodMap.put('h', Test.class.getMethod("showHelp"));
        methodMap.put('t', Test.class.getMethod("teleport"));

        char cmd = 'h';
        methodMap.get(cmd).invoke(null);  // prints "Help"

        cmd = 't';
        methodMap.get(cmd).invoke(null);  // prints "teleport"

    }

    public static void showHelp() {
        System.out.println("Help");
    }

    public static void teleport() {
        System.out.println("teleport");
    }
}