How can I override the toString method of an ArrayList in Java?

You can't override the toString method of ArrayList¹. Instead, you can use a utility class that converts an ArrayList to a String the way you want/need. Another alternative would be using Arrays.deepToString(yourList.toArray()).

Using Java 8, this can be done very easily:

//List<String> list
System.out.println(String.join(",", list));

Or if you have a List<Whatever>:

System.out.println(
    list.stream()
        .map(Whatever::getFieldX())
        .collect(Collectors.joining(", "))
);

I'm still against extending ArrayList or similar, is technically possible but I don't see it as a good option.

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¹ you could extend ArrayList and override the toString but generally that's not a great idea. Further info:

• Extending a java ArrayList
• Can you extend ArrayList in Java?
• To extend ArrayList, or to not extend ArrayList

You should do something like

public static String listToString(List<?> list) {
    String result = "+";
    for (int i = 0; i < list.size(); i++) {
        result += " " + list.get(i);
    }
    return result;
}

and pass the list in as an argument of listToString(). You can technically extend ArrayList (either with an anonymous class or a concrete one) and implement toString yourself, but that seems unnecessary here.

ArrayList<String> list = new ArrayList<String>()
{
    private static final long serialVersionUID = 1L;

    @Override public String toString()
    {
        return super.toString();
    }
};