Getting all arguments in single String in Java

I would recommend using a StringBuilder. It's efficient, especially if you create the instance with the total length of your strings (check in a loop) as a constructor parameter.
For more info check: http://docs.oracle.com/javase/tutorial/java/data/buffers.html

Arrays of Strings can be converted to single String as given below.

    StringBuilder builder = new StringBuilder();
    for (String str: args)
    {
        builder.append(str);
    }
    System.out.println(builder.toString());

Are you for some reason splitting strings by their commas then putting them into an ArrayList? Any how, to answer the question specifically without trying to guess all the bad things going on...

You can probably do it taking out one replace but... meh.

public static void main(String[] args) {
        ArrayList<String> myArray = new new ArrayList<String>();
        myArray.add("hello");
        myArray.add("I");
        myArray.add("Am");
        myArray.add("Ben");
        myArray.add("!");
        System.out.println(myArray.toString().replaceAll("\\[(.*)\\]", "$1").replaceAll(",", ""));
    }

You could

1. Get the person using the program to quote the string so that it is passed to your program as one argument
2. Modify the way the command line works so that it doesn't interpret spaces as delimiters for arguments
3. Modify main(), specifically how it takes arguments (I'm not sure this is possible or feasible)
4. Write a new array subclass with a .toString() that behaves differently
5. Write a new array subclass with a method that concatenates the array elements to a string
6. Write a static method that concatenates array elements to a string and pass the argument array to it

The last is probably the least work, especially if you don't plan on using it much.