Java - How to execute user input as a method call?

To solve your problem you can use some regex to get the values inside reverse("get this") :

s = s.replaceAll("reverse\\(\"(.*?)\"\\)", "$1");

second instead of that loop you can just use StringBuilder::reverse to reverse your String :

public static void reverse(String s) {
    s = s.replaceAll("reverse\\(\"(.*?)\"\\)", "$1");
    System.out.println(s);
    System.out.println(new StringBuilder(s).reverse().toString());
}

Input

reverse("a b c d")

Output

d c b a

Edit

Based on your comment :

When I run the application. It will ask the user for an input. If the user enters: reverse("I am an apple") It will then output: apple an am I

In this case you have to check the name of the method, so if the String start with name reverse then call reverse method for example :

public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    String sentence = sc.nextLine();

    if (sentence.startsWith("reverse")) {
        reverse(sentence);
    }
}

public static void reverse(String s) {
    s = s.replaceAll("reverse\\(\"(.*?)\"\\)", "$1");
    List<String> stringArray = Arrays.asList(s.split("\\s+"));
    Collections.reverse(stringArray);
    System.out.println(stringArray);
}

If you like to use the java reflection API try something like:

package Test;

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.Scanner;

public class Example {
    public static void main(String[] args) throws InvocationTargetException, ClassNotFoundException, NoSuchMethodException, IllegalAccessException {
        Scanner sc = new Scanner(System.in);
        String sentence = sc.nextLine();
        Class<?> c = Class.forName("Test.Example");//your package and class
                                              //get the method to invoke
        Method  method = c.getDeclaredMethod (sentence.substring(0, sentence.indexOf("(")), String.class);
                           //get the string param
        method.invoke (c, sentence.split("\"")[1]);               
    }
    public static void reverse(String s){
        String [] stringArray;
        stringArray = s.split(" ");
        int counter = stringArray.length;
        for (String word : stringArray) {
            counter -=1;
            System.out.print(stringArray[counter]+" ");
        }
    }
    public static void doNotReverse(String s){
        System.out.print(s);
    }
}

and you are able to type reverse("a b c d") or doNotReverse("a b c d") into console