How to protect against mutation through getter (returning a copy of the list)?

To protect against mutation of the internal list through a getter, return not the list itself, but its copy. This prevents external modification of the original list.

Example in Java:

public class MyClass {
    private final List<String> items = new ArrayList<>();

    public List<String> getItems() {
        return new ArrayList<>(items); // return a copy
    }

    // methods to modify items inside the class
}

Thus, if someone calls getItems() and tries to modify the returned list, the original items will remain unchanged.

Alternatively, you can return an unmodifiable list:

import java.util.Collections;

public List<String> getItems() {
    return Collections.unmodifiableList(items);
}

This allows reading the data, but any modification attempt will throw an exception.