如何防止添加重复的arraylist对象值？

ArrayList<String> arrayList = new ArrayList<>();
// 添加元素到ArrayList
Set<String> set = new HashSet<>(arrayList);
arrayList.clear();
arrayList.addAll(set);

ArrayList<String> arrayList = new ArrayList<>();
String element = "value";
if (!arrayList.contains(element)) {
    arrayList.add(element);
}

class CustomObject {
    private String value;

    // 省略构造函数和其他方法

    @Override
    public boolean equals(Object obj) {
        if (this == obj) {
            return true;
        }
        if (obj == null || getClass() != obj.getClass()) {
            return false;
        }
        CustomObject other = (CustomObject) obj;
        return value.equals(other.value);
    }
}

ArrayList<CustomObject> arrayList = new ArrayList<>();
CustomObject element = new CustomObject("value");
if (!arrayList.contains(element)) {
    arrayList.add(element);
}