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

Question

This question already has answers here:

Answer 1

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.

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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");
    }
}