检查数组是否包含值，如果没有，则将其添加到arraylist-并发修改异常-Java

Question

I am passing a String name to a method, if that name isn't in the array then it gets added.

I tried this first but got Concurrent Modification Exception

List<String> people = new ArrayList<>();

public void addName(String name) {
    if (people.isEmpty()) {
        people.add(name);
    } else {
        for (String s : people) {
            if (s.equals(name)) {
                System.out.println("dont add");
            } else {
                people.add(name);
            }
        }
    }
}

After reading on forums I learned you have to use iterator to avoid this. I tried it and fixed the Concurrent Modification Exception , but the player gets added even though I stated them not to be added if they exist in the array, I do get this output "name exists" when I pass a name already existing in the list, but then it runs "name added" as well so dont understand why this is happening

if (people.isEmpty()) {
    people.add(name);
} else {
    String name2 = null;
    for (Iterator<String> it = people.iterator(); it.hasNext(); ) {
        String element = it.next();
        if (element.equals(name)) {
            String message = "name exists";
            System.out.println(message);
            name2 = null;
        } else if (!element.equals(name)) {
            System.out.println("Name added");
            name2 = name;
        }
    }
    if (name2 != null) {
        people.add(name2);
    }
}

Answer 1

You can do it simply by:

public void addName(String name) {
    if (!people.contains(name)) {
        people.add(name);
    }
}

Answer 2

It sounds like you are trying to re-invent a set.

Set<String> names = new TreeSet<>(); // Or set of your choice
names.add("Joe"); // Set contents ["Joe"]
names.add("Bob"); // Set contents ["Joe", "Bob"];
names.add("Joe"); // Set contents ["Joe", "Bob"];

If you want to have the printlines

if (names.add(name) {
  System.out.println("Added name " + name);
} else {
  System.out.println("Already added " + name);
}

Answer 3

I reckon you need to break out of your loop once you find that the name already exists. Otherwise name2 might get set to null and then set to a non-null value again.

For example:

           for (Iterator<String> it = people.iterator(); it.hasNext(); ) {
                String element = it.next();
                if (element.equals(name)) {
                    String message = "name exists";
                    System.out.println(message);
                    name2 = null;
                    break;
                } else if(element.equals(name)==false) {
                    System.out.println("Name added");
                    name2 = name;
                }
            }

But do have a look at contains since this is a much much cleaner approach!